Date   

Re: 7904 Mainframe damaged

Chuck Harris <cfharris@...>
 

God yes, we all feel that way, almost to a crippling
degree, at first.

What has to happen, if you are ever going to get good
at this stuff, is to gain the perspective that only
making mistakes can give you, and to learn.

You have to make mistakes because you are human... it's
what you do next that is really, really important.

-Chuck Harris

JJ wrote:
...

Really! That's precisely how I felt: "When that thing fried and it was my
fault it was worse than if I had run over somebody's dog or something."


Re: 422 AC/DC Version

 

Got back into it. When it shuts down it is apparently losing voltage from the back board which supplies the 24 volts to the inverter board. There is no voltage to R 1193 when it is down, and that comes directly off the supply except for L 1182.

So the problem is in the back part of the power supply. I have yet to take that apart, err, wait I did have the battery pack out. Access is not that easiest thing in the world but not that hard. I am pretty sure it is all going to be a simple iron power supply with a switch for charging etc. Familiarizing myself with it will be good if I am to put in more advanced cells and modify the charging circuit to match. Hopefully it will just work like a - like a - cellphone, run and charge at the same time. Be "eveready" so to speak.

I also ran the power switch through all its positions and from what I heard the batteries do actually work but with very limited charge life. That means the problem is probably after the switch, so I know what to do now.

For now, I would still really like to get this channel switching issue fixed. It runs long enough for that. At worst the power problem is the power transformer, maybe the other roe will work... ??? If not there are solutions all over the place. it could maybe just be a battery operated scope and run and charge off the wall wart from an older printer. Some of them had some balls. Like 24 volts at 3 amps, that would probably do it.

Unless shutting down becomes a nuisance I plan to concentrate on the channel switching. i have one last check to clear the inverter/convertor circuit and I almost did it. When shut down take a resistance measurement at the DC input to it. It could be shorting out. I'll probably give it one more and take the reading. No matter what I have to set it aside for a short and check out a printer. That is not going to take that long, at the worst it will involve ordering a new print head. But that'll get it off the bench.

The only thermal cutout I see is at the primary of the AC transformer. That means that when I switched it to battery it should have at least had a power light for a second. It did not.

Going back to the other thread now.


Re: 7904 Mainframe damaged

JJ
 

Thanks. I'm plodding along following the the manual. You got me thinking
there could be a point somewhere in a path through the instrument's
grounding system that took the most stress and failed - maybe on the main
interconnect board.

Really! That's precisely how I felt: "When that thing fried and it was my
fault it was worse than if I had run over somebody's dog or something."

On Wed, Apr 11, 2018 at 9:19 PM, Jeff Urban <JURB6006@gmail.com> wrote:

On Wed, Apr 11, 2018 at 11:06 am, JJ wrote:


I was measuring a voltage on a leg of a bridge rectifier (110v) when
there
was a snap - and I realized what I had done.
Don't sweat that, it is nothing. I interpolated two plugs in a 7603 hat
dumped 130 volts on the 5 volt line. Of course it took out the horizontal
preamp output IC, one of the most unobtanium things in it.

Luckily I had a parts donor handy, and much to my surprise I got it
running again. Luckily when one thing shorts it protects the rest of the
parts on the line :-)...usually.

When that thing fried and it was my fault it was worse than if I had run
over somebody's dog or something.





Re: 7904 Mainframe damaged

 

On Wed, Apr 11, 2018 at 11:06 am, JJ wrote:


I was measuring a voltage on a leg of a bridge rectifier (110v) when there
was a snap - and I realized what I had done.
Don't sweat that, it is nothing. I interpolated two plugs in a 7603 hat dumped 130 volts on the 5 volt line. Of course it took out the horizontal preamp output IC, one of the most unobtanium things in it.

Luckily I had a parts donor handy, and much to my surprise I got it running again. Luckily when one thing shorts it protects the rest of the parts on the line :-)...usually.

When that thing fried and it was my fault it was worse than if I had run over somebody's dog or something.


Thanks to everyone who helped with calibration ideas.

Brendan
 

Finally finished my 485 I used a pg506 a afg3012 and a agilent 6634b for gain adjustments. This is as close as i could get. 1 ns at 20mV with the pg506 and 067-0681-01 TD pulser https://groups.io/g/TekScopes/photo/37195/13?p=Name,,,20,1,0,0 every adjustable capacitor on the vertical board for channel one needed replacing along with one in the main vertical transient response set.


Re: TEK475 Scope: Extremely long Stabilization Time?

Redguuz
 

Hi,

Of course, I played with all the trigger settings.

Once warmed up: It triggers OK on setting

Couplng: AC, HF Reject and Best on DC
Source : Normal and Chanel 2 ( I mostly use Channel 2 but Channel 1 has same behaviou)
Trigger mode: AUTO and Normal
A trigger : Slope Level ( tuned for vey low input signal levels (almost negligible).
So my operation of this scope is at should be (in my opinion)

However, if I shut down the scope and the R&S RC generator (set at 1 KHz) and power on the next day I get the secribed odd behaviour of this scope.

The R&S generator (albeit vey old 1960's) is not the culprit (checked with a 1960's Dynamco Nuvistor scope.

AS I said upon warming up, the TEK stabilises again to the condition, it had the day before before shutting down. This can take 15 minutes.
So, I was wondering whether this transient behaviour is normal (simply let the scope idle for 15 minutes) or whether something is starting to break down (I 'm thinking about leaky capacitors in teh power lines, tunnel diodes or whatever.

Should I really be concerned or simply leave it as it is: Strat the scope and have a coffee.
After all the vintage R&S valve equipment also ideally needs some 15 minutes to have fully stabilised.

Everybody, thanks for your suggestions!


Re: 7904 Mainframe damaged

Göran Krusell
 

Okey, you are in good company!

Göran


Re: 7904 Mainframe damaged

John Griessen
 

On 04/11/2018 01:05 PM, JJ wrote:
I was measuring a voltage on a leg of a bridge rectifier (110v) when there
was a snap - and I realized what I had done.
You may not have "Done" anything. Like Chuck said... components dying can make sounds like that.
Popcorn popping sounds. Explosive sounds.


Re: 7904 Mainframe damaged

Chuck Harris <cfharris@...>
 

Hi John,

I am not like that. I have done all of these dumb
things myself (and more). I am quite happy to call
myself dumb, but you will have to decide how harsh
you are willing to be with yourself.

If there is no zap mark on the ground clip, there was
very little current involved. I don't believe that it
caused your main problem.

I have burned ground wires clean off during probe
misadventures on my 7000 series scopes without any
untoward result... Phone switch -48V supply, I'm
looking right at you!... (I was even warned, sigh...)

Anecdotal evidence, I know, but I am very certain that
tektronix tested these scopes for sensitivity to that
accident during their design process.... And way, way
worse! And a heck of a lot of us have repeated the,
uhmmm, testing.

The most I can say that has ever happened when I did
such stunts, was the scope triggered from the RF
generated by the zap.

What I think we have here is two events that although
coincident'ish in time, were totally unrelated.

The favorite time for a 7904 to eat a tantalum is
within a second, or so, of turn on. Indeed, that
is a very common time for any component failure.

I have never had a tantalum fail during operation...
they seem to self repair during operation. The truly
explosive failures happen when they are employed on
high current supplies, without any current limitation.

I have had more than a few 6.3V 100uf tantalums on 5V
logic power busses scare the Bejezus out of me, on
power application... but I digress.

The 7904 power supply is very sensitive to over current.
It will go into tick mode for some combinations of
plugins... for instance, mine cannot power a 7L18.

The 7904 is an early 7000 series scope, and was made
long before any of the current hog plugins were made.

It is fine with any combination of 7A and 7B plugins
that I have tried.

The 50V supplies are favorites for knocking a 7904
supply down into kick mode. They are low current, and
a little 4.7 ohm 1/4 watt resistor can do the job on
power up.... though it would burn up if the short
happened after power is up.

You say it wouldn't respond to any controls. Really?
Any?

-Chuck Harris

JJ wrote:

Chuck,

There are no burn marks on the scope probe, or ground wire, or probe
alligator clip. The probe works fine. No burn marks on the 7A26. I tested
it everything out on my 7834 storage scope - works fine. All plugins work
fine, There are no burn marks inside the plugin area. I have inspected all
boards that are visible with a magnifying glass - no signs of component
failure. I can't inspect the main interface board until I remove it.

I was measuring a voltage on a leg of a bridge rectifier (110v) when there
was a snap - and I realized what I had done. The scope itself didn't go off
- the CRT was still active with a noise signal on the CRT - but the scope
wasn't responding to any actions on the controls. I was hoping against all
hope that if I powered it off and then back on again that I would get
lucky. Or, maybe a fuse when it didn't come on - all fuses were fine.

Please don't tell me how dumb I was - tell me something that I don't
already know. :).

Best,
John

On Wed, Apr 11, 2018 at 1:35 PM, Chuck Harris <cfharris@erols.com> wrote:

The problem is, the current from the probe's ground
lead is directly connected to the chassis at the BNC
connector on the plugin's front panel, and at the
finger stock around the front panel, and at the side
rails along the bottom and top of the plugin slot...

And then the chassis of the scope is directly connected
to the ground pin on the power cord... Which was
connected to the ground pin on the isolation tranny,
which was connected to the building's ground, and at
the service panel to the mains ground lead (neutral)...

So, how much current were we talking here?

Enough to vaporize the ground wire? Not from what we've
heard.

Did it even burn the plating off of the ground clip?

Enough to blow a fuse somewhere? Not from what we've
heard.. AFAICR.

And, exactly what was shorted by the ground wire?

Mains? A filter cap on a switching supply? An 800V
power supply on a monster tube amp?

I'm having more than a little trouble believing the
presumed cause fits the symptoms. Something else is
going on.

A very popular failure in the 7904's 50V power supplies
is a 50V tantalum capacitor on the Horizontal amplifier
board. It doesn't seem to show up on the schematic in
the obvious place, and I don't remember if it is on the
+50, or the -50V supply, but it is there. Usually, when
it shorts, it incinerated a little 10 ohm resistor placed
there as a fuse, and eliminates the horizontal amplifier
from the circuit. If the resistor is not killed, the
resistor takes down the current overload, and shuts down
the supply.

I'd look there.

-Chuck Harris

Robert Hay wrote:
Just to re-cap, your initial message indicated your probe ground lead
contacted a
high voltage point on a device under test with a common ground. This
would raise the
scope vertical input ground and cause high current to find its way thru
your scope.
This apparently caused a failure in the scope.

Your resistance tests don't point to a short on any of the LV supplies.
The author
of the old article used a VOM so I would tend to compare VOM readings to
those given
in the article. If you tested using both Fluke and VOM did you find
much differences
in readings?

The minus 51 V load resistance is much higher than the article
measurement. The plus
5 is lower resistance. So, those might be the places to look. Did you
check the
diodes and caps for the -51 volt supply? If they are good, you may be
looking for
something failed open on the mainframe side since your measurement of
31.6k with the
LVPS disconnected is much higher than the 2k given in the article.

Usually, the tic-tic mode is the power supply letting you know there is
a short on
one of the supplies and saving itself from further damage. As far as
testing it
without load to see if you still get the tic-tic or not it would be good
if one of
the more experienced members with testing the 7904 supply jumps in here.

Bob.

On 4/10/2018 4:37 PM, JJ wrote:
In reading that doc, I'm concerned that the problem is in that inverter
control chip. That's probably impossible to find. I found the schematic
of
what's inside it:
http://w140.com/tekwiki/wiki/File:155-0067-02_block.png

Z-axis Board voltages PSU in Mainframe
+130 8.3k
+50 3.7k
+15 90.1 ohms
-15 205 ohms
-50 4.2k
+5 47 ohms

+5V lamp 2 ohms

Z-axis Board voltages PSU out of Mainframe
+130 9,9k
+50 4.6k
+15 92.3 ohms
-15 216 ohms
-50 31.6k
+5 48 ohms

+5V lamp 23k

On Tue, Apr 10, 2018 at 6:55 PM, Robert Hay <bobh@joba.com> wrote:

Ok, you said the resistances were very high, how much higher than those
suggested by the article?

+130 volts 6k
+50 2k
+15 90 Ohms
-15 100 Ohms
-50 250 Ohms
+5V lamp 800 Ohms

Bob.




On 4/10/2018 1:28 PM, JJ wrote:

Bob, I found that the diodes seem OK when I disconnected one lead and
measured both resistance and diode forward and reverse voltage. I
found
that the short across those two diodes was due to a shorted capacitor
on
the rectifier board - I was actually measuring the winding tap
resistance
through a weird path. I reconnected the PSU back into the mainframe
after
changing the cap. There was no glory - a very low tick coming from the
supply.

I'll need to continue debug by following the procedure in the that
document. I measured the resistances in z-axis board as the procedure
indicates while the PSU was out of the mainframe - they are pretty
much in
line - a couple of resistances were much higher than the table - none
were
lower . I'll measure them now while the power supply is in the
mainframe,
Also, I get different results using a VOM and a DVM. Maybe there's
something wrong with my VOM - it's pretty old. Is a VOM required to
get
the
proper measurements as indicated in the table?

.Best,
John

On Tue, Apr 10, 2018 at 2:41 PM, Robert Hay <bobh@joba.com> wrote:

They say in the article that a low resistance indicates a problem in
the
mainframe which is true if you are looking at something shorting to
ground. But, in your case I would not discount that high current
caused
a
open circuit while trying to get back to the source. I suppose you
already
looked for any signs of burning or smoked components, leads, traces,
etc.

Bob.



On 4/10/2018 7:44 AM, JJ wrote:

Yes the resistances were very high. According to that doc, the issue
should
be on the PSU. I'm going to put the PSU back into the mainframe,
connect
the cables, and make those voltage measurements on the LV regulator
board.
I didn't check for voltages before taking the PSU out because I
didn't
know
at that time that the PSU needs to be under minimum load.

I found a shorted cap C1360 on the rectifier board (+54v filter cap
on
output of Pi filter). I'm going to lift one lead to ensure it's the
problem. I'm also going to validate that the two power diodes that I
found
are shorted - by disconnecting the wires going to those diodes and
measuring. If the diodes are bad, I'll replace the bad 10A diodes
with
two
5A diodes in parallel temporarily - hoping that's OK, I'll then
hook up
the
mainframe's cables to the PS and check the voltages. I think the 3
cables
are long enough for the PSU to hang out the back. If not, I plan on
making
extender cables.

Best,
John

On Tue, Apr 10, 2018 at 10:16 AM, Robert Hay <bobh@joba.com> wrote:

Have you checked resistances as suggested in the Service Scope
article?

And then voltages with the supply outside the scope but still
connected?

Bob.


On 4/10/2018 6:38 AM, JJ wrote:

Hi Tony, So, based on your findings, you have confirmed that the
the
PSU

needs to be under load in order to be functional. I will check
all the
caps
on those 3 boards as well. Thank you for the info - it's very,
very
helpful.
I hope the experts on this forum can help you out with the
remainder
of
your 7904's issues.

Best,
John

On Tue, Apr 10, 2018 at 9:29 AM, Yiu On Tony C via Groups.Io <
tonycheung_hk=yahoo.com@groups.io> wrote:

Hi JJ :

I am from Hong Kong ! I just complete the repair of PSU of
TEK7904 , I
am
lucky , I find a SHORT E-Cap. once I replace it and turn on PSU
alone ,
It
still sound shut-down ! once I connect to main unit , it come
normal
.
I found few E-Cap short on A9 H amp board, A2 Main interface
board
and
A12 Rectifier board
For the power diode , I did check each by de-solder the to wire
only
.
My 7904 still have other issue , H ok but the read-out still
error
in
units , Y are not function -- position knob no response .

RegardTony CheungAPR 10 2018


From: JJ <jajustin@gmail.com>
To: TekScopes@groups.io
Sent: Tuesday, April 10, 2018 7:16 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged

Dave, I went through hundreds of files in the files section of
the
archive
and wasn't able to identify the schematic describing the minimum
loading
requirements for power supplies. Tried searching, browsing, ad
nauseum!
No
glory. Are you sure that Jerry uploaded it?

Best,
John

On Mon, Apr 9, 2018 at 4:54 PM, Dave Daniel <kc0wjn@gmail.com>
wrote:

On some of the 7xxx supplies (SMPS supplies, I believe), one
needs to
put

a minimum load on the power supply output in order for the
supply to
work.
I don't remember if that is true for the 7904 power supply.

Jerry Massengale built one of these loads. I think he uploaded a
copy
of
the schematic to the files section back in maybe2015.

DaveD


On 4/9/2018 12:22 PM, JJ wrote:

I removed the power supply from the mainframe - it's on my
bench.
First,

I'll remove the wire from the diode terminal and check to see
if the
diode
is actually shorted as others have suggested. I was thinking of
replacing
the 10A diodes with two 5A diodes in parallel that I have
available
in
my
parts bin temporarily to see if I get all the other voltages
back -
right
now there are no voltages at the test points of the rectifier
board
and

the
low voltage regulator. I wouldn't think there would be that
much
load
with
the supply removed? That way I can continue to debug. Thoughts?

On Mon, Apr 9, 2018 at 2:06 PM, Jeff Urban <JURB6006@gmail.com
wrote:

Incidentally, that stuff about the mini EMP is not very likely.
Nott

only
does it actually have to happen at a strength to do something,
it
also

needs to be oriented the right way to do something. Also I erred
about
the
vaporized foil on the board, it was about ½ cm., not inch.

Anyway, if you are sure about the diodes I guess you know
what to
do,

if
possible. It might be rough to gt everything hooked back up
for a
live

test
without actually assembling it. Changing the diodes right
away ?
You
could
do that. And of course watch for mounting screws that also
function
as

a
ground, that has tripped me up a couple of times.
Since you have one + and one - diode bad, assuming you don't
have a
short
to ground, I would check see if there is a short between the +
and
-

legs
of that supply. At this time I have no idea what those sources
feed
but

it
could be a series arranged push pull output to something and
while
both
of
the output devices could be shorted, the load isn't low enough

impedance,
or maybe even electrostatic, to read a short to ground. It
happens.














Re: 7904 Mainframe damaged

Adrian
 

Hi Forgive this if it's been covered earlier, I've been following this thread on and off but a) I'm not familiar with this particular power supply and b) I dislike/distrust switchmode PSUs! However I couple of questions keep coming to mind.

Did I dream it or was it said at the outset that following the unfortunate event the 'scope continued to function and the fault only manifested itself after a power cycle? If so there may be a clue there?

Second, if I've followed the thread correctly, there still seems to be doubt as to whether the fault is in the PSU or one of the circuits it powers? If so, is there sense in powering some or all of these circuits from a bench supply to check their function independently?

Just thoughts.....Now I'll get back to fixing a 1950s scope which at least has a PSU I can understand - Transformer + rectifier tube + capacitor = job done!

Adrian

On 4/11/2018 6:35 PM, Chuck Harris wrote:
I'm having more than a little trouble believing the
presumed cause fits the symptoms. Something else is
going on.


Re: Trying to fix up my 2213A

Ted Rook
 

I encourage the OP to go for the Artek Media 2213A pdf manual. I have it and the quality is
superb, color, hi-res, full size fold out schematics, page links.......and you get it immediately.

Ted

On 11 Apr 2018 at 5:58, tom jobe wrote:

That is quite a selection of free manuals someone is offering, but both
of the manuals they have are for the non-A version of the 2213 which is
a much different oscilloscope than the 2213A, especially in the power
supply section where many of the age related problems are found.
The best copy of the manual I have for the 2213A came from Artek Media
for very little money.
Original printed copies of the 2213A Service Manual are somewhat rare
and and usually 'pricey'.
I bought a 're-printed' copy of the 2213A manual years ago on eBay
before I had the capability to print schematics on 11 x 17 paper and I
found that 2213A 're-print' to be useless.
tom jobe...


On 4/11/2018 2:32 AM, Jeff Urban wrote:
It appears a free service manual is available here :

http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/

Looks like 2 there, one is 15 MB and the other is 83 MB, obviously take the bigger one... :-)





Re: 7904 Mainframe damaged

JJ
 

Chuck,

There are no burn marks on the scope probe, or ground wire, or probe
alligator clip. The probe works fine. No burn marks on the 7A26. I tested
it everything out on my 7834 storage scope - works fine. All plugins work
fine, There are no burn marks inside the plugin area. I have inspected all
boards that are visible with a magnifying glass - no signs of component
failure. I can't inspect the main interface board until I remove it.

I was measuring a voltage on a leg of a bridge rectifier (110v) when there
was a snap - and I realized what I had done. The scope itself didn't go off
- the CRT was still active with a noise signal on the CRT - but the scope
wasn't responding to any actions on the controls. I was hoping against all
hope that if I powered it off and then back on again that I would get
lucky. Or, maybe a fuse when it didn't come on - all fuses were fine.

Please don't tell me how dumb I was - tell me something that I don't
already know. :).

Best,
John

On Wed, Apr 11, 2018 at 1:35 PM, Chuck Harris <cfharris@erols.com> wrote:

The problem is, the current from the probe's ground
lead is directly connected to the chassis at the BNC
connector on the plugin's front panel, and at the
finger stock around the front panel, and at the side
rails along the bottom and top of the plugin slot...

And then the chassis of the scope is directly connected
to the ground pin on the power cord... Which was
connected to the ground pin on the isolation tranny,
which was connected to the building's ground, and at
the service panel to the mains ground lead (neutral)...

So, how much current were we talking here?

Enough to vaporize the ground wire? Not from what we've
heard.

Did it even burn the plating off of the ground clip?

Enough to blow a fuse somewhere? Not from what we've
heard.. AFAICR.

And, exactly what was shorted by the ground wire?

Mains? A filter cap on a switching supply? An 800V
power supply on a monster tube amp?

I'm having more than a little trouble believing the
presumed cause fits the symptoms. Something else is
going on.

A very popular failure in the 7904's 50V power supplies
is a 50V tantalum capacitor on the Horizontal amplifier
board. It doesn't seem to show up on the schematic in
the obvious place, and I don't remember if it is on the
+50, or the -50V supply, but it is there. Usually, when
it shorts, it incinerated a little 10 ohm resistor placed
there as a fuse, and eliminates the horizontal amplifier
from the circuit. If the resistor is not killed, the
resistor takes down the current overload, and shuts down
the supply.

I'd look there.

-Chuck Harris

Robert Hay wrote:
Just to re-cap, your initial message indicated your probe ground lead
contacted a
high voltage point on a device under test with a common ground. This
would raise the
scope vertical input ground and cause high current to find its way thru
your scope.
This apparently caused a failure in the scope.

Your resistance tests don't point to a short on any of the LV supplies.
The author
of the old article used a VOM so I would tend to compare VOM readings to
those given
in the article. If you tested using both Fluke and VOM did you find
much differences
in readings?

The minus 51 V load resistance is much higher than the article
measurement. The plus
5 is lower resistance. So, those might be the places to look. Did you
check the
diodes and caps for the -51 volt supply? If they are good, you may be
looking for
something failed open on the mainframe side since your measurement of
31.6k with the
LVPS disconnected is much higher than the 2k given in the article.

Usually, the tic-tic mode is the power supply letting you know there is
a short on
one of the supplies and saving itself from further damage. As far as
testing it
without load to see if you still get the tic-tic or not it would be good
if one of
the more experienced members with testing the 7904 supply jumps in here.

Bob.

On 4/10/2018 4:37 PM, JJ wrote:
In reading that doc, I'm concerned that the problem is in that inverter
control chip. That's probably impossible to find. I found the schematic
of
what's inside it:
http://w140.com/tekwiki/wiki/File:155-0067-02_block.png

Z-axis Board voltages PSU in Mainframe
+130 8.3k
+50 3.7k
+15 90.1 ohms
-15 205 ohms
-50 4.2k
+5 47 ohms

+5V lamp 2 ohms

Z-axis Board voltages PSU out of Mainframe
+130 9,9k
+50 4.6k
+15 92.3 ohms
-15 216 ohms
-50 31.6k
+5 48 ohms

+5V lamp 23k

On Tue, Apr 10, 2018 at 6:55 PM, Robert Hay <bobh@joba.com> wrote:

Ok, you said the resistances were very high, how much higher than those
suggested by the article?

+130 volts 6k
+50 2k
+15 90 Ohms
-15 100 Ohms
-50 250 Ohms
+5V lamp 800 Ohms

Bob.




On 4/10/2018 1:28 PM, JJ wrote:

Bob, I found that the diodes seem OK when I disconnected one lead and
measured both resistance and diode forward and reverse voltage. I
found
that the short across those two diodes was due to a shorted capacitor
on
the rectifier board - I was actually measuring the winding tap
resistance
through a weird path. I reconnected the PSU back into the mainframe
after
changing the cap. There was no glory - a very low tick coming from the
supply.

I'll need to continue debug by following the procedure in the that
document. I measured the resistances in z-axis board as the procedure
indicates while the PSU was out of the mainframe - they are pretty
much in
line - a couple of resistances were much higher than the table - none
were
lower . I'll measure them now while the power supply is in the
mainframe,
Also, I get different results using a VOM and a DVM. Maybe there's
something wrong with my VOM - it's pretty old. Is a VOM required to
get
the
proper measurements as indicated in the table?

.Best,
John

On Tue, Apr 10, 2018 at 2:41 PM, Robert Hay <bobh@joba.com> wrote:

They say in the article that a low resistance indicates a problem in
the
mainframe which is true if you are looking at something shorting to
ground. But, in your case I would not discount that high current
caused
a
open circuit while trying to get back to the source. I suppose you
already
looked for any signs of burning or smoked components, leads, traces,
etc.

Bob.



On 4/10/2018 7:44 AM, JJ wrote:

Yes the resistances were very high. According to that doc, the issue
should
be on the PSU. I'm going to put the PSU back into the mainframe,
connect
the cables, and make those voltage measurements on the LV regulator
board.
I didn't check for voltages before taking the PSU out because I
didn't
know
at that time that the PSU needs to be under minimum load.

I found a shorted cap C1360 on the rectifier board (+54v filter cap
on
output of Pi filter). I'm going to lift one lead to ensure it's the
problem. I'm also going to validate that the two power diodes that I
found
are shorted - by disconnecting the wires going to those diodes and
measuring. If the diodes are bad, I'll replace the bad 10A diodes
with
two
5A diodes in parallel temporarily - hoping that's OK, I'll then
hook up
the
mainframe's cables to the PS and check the voltages. I think the 3
cables
are long enough for the PSU to hang out the back. If not, I plan on
making
extender cables.

Best,
John

On Tue, Apr 10, 2018 at 10:16 AM, Robert Hay <bobh@joba.com> wrote:

Have you checked resistances as suggested in the Service Scope
article?

And then voltages with the supply outside the scope but still
connected?

Bob.


On 4/10/2018 6:38 AM, JJ wrote:

Hi Tony, So, based on your findings, you have confirmed that the
the
PSU

needs to be under load in order to be functional. I will check
all the
caps
on those 3 boards as well. Thank you for the info - it's very,
very
helpful.
I hope the experts on this forum can help you out with the
remainder
of
your 7904's issues.

Best,
John

On Tue, Apr 10, 2018 at 9:29 AM, Yiu On Tony C via Groups.Io <
tonycheung_hk=yahoo.com@groups.io> wrote:

Hi JJ :

I am from Hong Kong ! I just complete the repair of PSU of
TEK7904 , I
am
lucky , I find a SHORT E-Cap. once I replace it and turn on PSU
alone ,
It
still sound shut-down ! once I connect to main unit , it come
normal
.
I found few E-Cap short on A9 H amp board, A2 Main interface
board
and
A12 Rectifier board
For the power diode , I did check each by de-solder the to wire
only
.
My 7904 still have other issue , H ok but the read-out still
error
in
units , Y are not function -- position knob no response .

RegardTony CheungAPR 10 2018


From: JJ <jajustin@gmail.com>
To: TekScopes@groups.io
Sent: Tuesday, April 10, 2018 7:16 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged

Dave, I went through hundreds of files in the files section of
the
archive
and wasn't able to identify the schematic describing the minimum
loading
requirements for power supplies. Tried searching, browsing, ad
nauseum!
No
glory. Are you sure that Jerry uploaded it?

Best,
John

On Mon, Apr 9, 2018 at 4:54 PM, Dave Daniel <kc0wjn@gmail.com>
wrote:

On some of the 7xxx supplies (SMPS supplies, I believe), one
needs to
put

a minimum load on the power supply output in order for the
supply to
work.
I don't remember if that is true for the 7904 power supply.

Jerry Massengale built one of these loads. I think he uploaded a
copy
of
the schematic to the files section back in maybe2015.

DaveD


On 4/9/2018 12:22 PM, JJ wrote:

I removed the power supply from the mainframe - it's on my
bench.
First,

I'll remove the wire from the diode terminal and check to see
if the
diode
is actually shorted as others have suggested. I was thinking of
replacing
the 10A diodes with two 5A diodes in parallel that I have
available
in
my
parts bin temporarily to see if I get all the other voltages
back -
right
now there are no voltages at the test points of the rectifier
board
and

the
low voltage regulator. I wouldn't think there would be that
much
load
with
the supply removed? That way I can continue to debug. Thoughts?

On Mon, Apr 9, 2018 at 2:06 PM, Jeff Urban <JURB6006@gmail.com
wrote:

Incidentally, that stuff about the mini EMP is not very likely.
Nott

only
does it actually have to happen at a strength to do something,
it
also

needs to be oriented the right way to do something. Also I erred
about
the
vaporized foil on the board, it was about ½ cm., not inch.

Anyway, if you are sure about the diodes I guess you know
what to
do,

if
possible. It might be rough to gt everything hooked back up
for a
live

test
without actually assembling it. Changing the diodes right
away ?
You
could
do that. And of course watch for mounting screws that also
function
as

a
ground, that has tripped me up a couple of times.
Since you have one + and one - diode bad, assuming you don't
have a
short
to ground, I would check see if there is a short between the +
and
-

legs
of that supply. At this time I have no idea what those sources
feed
but

it
could be a series arranged push pull output to something and
while
both
of
the output devices could be shorted, the load isn't low enough

impedance,
or maybe even electrostatic, to read a short to ground. It
happens.












Re: 7904 Mainframe damaged

Chuck Harris <cfharris@...>
 

The problem is, the current from the probe's ground
lead is directly connected to the chassis at the BNC
connector on the plugin's front panel, and at the
finger stock around the front panel, and at the side
rails along the bottom and top of the plugin slot...

And then the chassis of the scope is directly connected
to the ground pin on the power cord... Which was
connected to the ground pin on the isolation tranny,
which was connected to the building's ground, and at
the service panel to the mains ground lead (neutral)...

So, how much current were we talking here?

Enough to vaporize the ground wire? Not from what we've
heard.

Did it even burn the plating off of the ground clip?

Enough to blow a fuse somewhere? Not from what we've
heard.. AFAICR.

And, exactly what was shorted by the ground wire?

Mains? A filter cap on a switching supply? An 800V
power supply on a monster tube amp?

I'm having more than a little trouble believing the
presumed cause fits the symptoms. Something else is
going on.

A very popular failure in the 7904's 50V power supplies
is a 50V tantalum capacitor on the Horizontal amplifier
board. It doesn't seem to show up on the schematic in
the obvious place, and I don't remember if it is on the
+50, or the -50V supply, but it is there. Usually, when
it shorts, it incinerated a little 10 ohm resistor placed
there as a fuse, and eliminates the horizontal amplifier
from the circuit. If the resistor is not killed, the
resistor takes down the current overload, and shuts down
the supply.

I'd look there.

-Chuck Harris

Robert Hay wrote:

Just to re-cap, your initial message indicated your probe ground lead contacted a
high voltage point on a device under test with a common ground. This would raise the
scope vertical input ground and cause high current to find its way thru your scope.
This apparently caused a failure in the scope.

Your resistance tests don't point to a short on any of the LV supplies. The author
of the old article used a VOM so I would tend to compare VOM readings to those given
in the article. If you tested using both Fluke and VOM did you find much differences
in readings?

The minus 51 V load resistance is much higher than the article measurement. The plus
5 is lower resistance. So, those might be the places to look. Did you check the
diodes and caps for the -51 volt supply? If they are good, you may be looking for
something failed open on the mainframe side since your measurement of 31.6k with the
LVPS disconnected is much higher than the 2k given in the article.

Usually, the tic-tic mode is the power supply letting you know there is a short on
one of the supplies and saving itself from further damage. As far as testing it
without load to see if you still get the tic-tic or not it would be good if one of
the more experienced members with testing the 7904 supply jumps in here.

Bob.

On 4/10/2018 4:37 PM, JJ wrote:
In reading that doc, I'm concerned that the problem is in that inverter
control chip. That's probably impossible to find. I found the schematic of
what's inside it:
http://w140.com/tekwiki/wiki/File:155-0067-02_block.png

Z-axis Board voltages PSU in Mainframe
+130 8.3k
+50 3.7k
+15 90.1 ohms
-15 205 ohms
-50 4.2k
+5 47 ohms

+5V lamp 2 ohms

Z-axis Board voltages PSU out of Mainframe
+130 9,9k
+50 4.6k
+15 92.3 ohms
-15 216 ohms
-50 31.6k
+5 48 ohms

+5V lamp 23k

On Tue, Apr 10, 2018 at 6:55 PM, Robert Hay <bobh@joba.com> wrote:

Ok, you said the resistances were very high, how much higher than those
suggested by the article?

+130 volts 6k
+50 2k
+15 90 Ohms
-15 100 Ohms
-50 250 Ohms
+5V lamp 800 Ohms

Bob.




On 4/10/2018 1:28 PM, JJ wrote:

Bob, I found that the diodes seem OK when I disconnected one lead and
measured both resistance and diode forward and reverse voltage. I found
that the short across those two diodes was due to a shorted capacitor on
the rectifier board - I was actually measuring the winding tap resistance
through a weird path. I reconnected the PSU back into the mainframe after
changing the cap. There was no glory - a very low tick coming from the
supply.

I'll need to continue debug by following the procedure in the that
document. I measured the resistances in z-axis board as the procedure
indicates while the PSU was out of the mainframe - they are pretty much in
line - a couple of resistances were much higher than the table - none were
lower . I'll measure them now while the power supply is in the mainframe,
Also, I get different results using a VOM and a DVM. Maybe there's
something wrong with my VOM - it's pretty old. Is a VOM required to get
the
proper measurements as indicated in the table?

.Best,
John

On Tue, Apr 10, 2018 at 2:41 PM, Robert Hay <bobh@joba.com> wrote:

They say in the article that a low resistance indicates a problem in the
mainframe which is true if you are looking at something shorting to
ground. But, in your case I would not discount that high current caused
a
open circuit while trying to get back to the source. I suppose you
already
looked for any signs of burning or smoked components, leads, traces, etc.

Bob.



On 4/10/2018 7:44 AM, JJ wrote:

Yes the resistances were very high. According to that doc, the issue
should
be on the PSU. I'm going to put the PSU back into the mainframe, connect
the cables, and make those voltage measurements on the LV regulator
board.
I didn't check for voltages before taking the PSU out because I didn't
know
at that time that the PSU needs to be under minimum load.

I found a shorted cap C1360 on the rectifier board (+54v filter cap on
output of Pi filter). I'm going to lift one lead to ensure it's the
problem. I'm also going to validate that the two power diodes that I
found
are shorted - by disconnecting the wires going to those diodes and
measuring. If the diodes are bad, I'll replace the bad 10A diodes with
two
5A diodes in parallel temporarily - hoping that's OK, I'll then hook up
the
mainframe's cables to the PS and check the voltages. I think the 3
cables
are long enough for the PSU to hang out the back. If not, I plan on
making
extender cables.

Best,
John

On Tue, Apr 10, 2018 at 10:16 AM, Robert Hay <bobh@joba.com> wrote:

Have you checked resistances as suggested in the Service Scope article?

And then voltages with the supply outside the scope but still
connected?

Bob.


On 4/10/2018 6:38 AM, JJ wrote:

Hi Tony, So, based on your findings, you have confirmed that the the
PSU

needs to be under load in order to be functional. I will check all the
caps
on those 3 boards as well. Thank you for the info - it's very, very
helpful.
I hope the experts on this forum can help you out with the remainder
of
your 7904's issues.

Best,
John

On Tue, Apr 10, 2018 at 9:29 AM, Yiu On Tony C via Groups.Io <
tonycheung_hk=yahoo.com@groups.io> wrote:

Hi JJ :

I am from Hong Kong ! I just complete the repair of PSU of TEK7904 , I
am
lucky , I find a SHORT E-Cap. once I replace it and turn on PSU
alone ,
It
still sound shut-down ! once I connect to main unit , it come normal
.
I found few E-Cap short on A9 H amp board, A2 Main interface board
and
A12 Rectifier board
For the power diode , I did check each by de-solder the to wire only
.
My 7904 still have other issue , H ok but the read-out still error
in
units , Y are not function -- position knob no response .

RegardTony CheungAPR 10 2018


From: JJ <jajustin@gmail.com>
To: TekScopes@groups.io
Sent: Tuesday, April 10, 2018 7:16 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged

Dave, I went through hundreds of files in the files section of the
archive
and wasn't able to identify the schematic describing the minimum
loading
requirements for power supplies. Tried searching, browsing, ad
nauseum!
No
glory. Are you sure that Jerry uploaded it?

Best,
John

On Mon, Apr 9, 2018 at 4:54 PM, Dave Daniel <kc0wjn@gmail.com>
wrote:

On some of the 7xxx supplies (SMPS supplies, I believe), one needs to
put

a minimum load on the power supply output in order for the supply to
work.
I don't remember if that is true for the 7904 power supply.

Jerry Massengale built one of these loads. I think he uploaded a
copy
of
the schematic to the files section back in maybe2015.

DaveD


On 4/9/2018 12:22 PM, JJ wrote:

I removed the power supply from the mainframe - it's on my bench.
First,

I'll remove the wire from the diode terminal and check to see if the
diode
is actually shorted as others have suggested. I was thinking of
replacing
the 10A diodes with two 5A diodes in parallel that I have available
in
my
parts bin temporarily to see if I get all the other voltages back -
right
now there are no voltages at the test points of the rectifier board
and

the
low voltage regulator. I wouldn't think there would be that much
load
with
the supply removed? That way I can continue to debug. Thoughts?

On Mon, Apr 9, 2018 at 2:06 PM, Jeff Urban <JURB6006@gmail.com>
wrote:

Incidentally, that stuff about the mini EMP is not very likely.
Nott

only
does it actually have to happen at a strength to do something, it
also

needs to be oriented the right way to do something. Also I erred
about
the
vaporized foil on the board, it was about ½ cm., not inch.

Anyway, if you are sure about the diodes I guess you know what to
do,

if
possible. It might be rough to gt everything hooked back up for a
live

test
without actually assembling it. Changing the diodes right away ?
You
could
do that. And of course watch for mounting screws that also
function
as

a
ground, that has tripped me up a couple of times.
Since you have one + and one - diode bad, assuming you don't have a
short
to ground, I would check see if there is a short between the + and
-

legs
of that supply. At this time I have no idea what those sources feed
but

it
could be a series arranged push pull output to something and while
both
of
the output devices could be shorted, the load isn't low enough

impedance,
or maybe even electrostatic, to read a short to ground. It happens.









Re: 7904 Mainframe damaged

JJ
 

The resistance values I gave were using a Fluke. The readings weren't far
off from my old VOM. I read last night that the 5v lamp voltage could cut
off the inverter through the controller IC on the rectifier board. In
essence, I'm following the manual's procedure. You may be right with a
problem on main interface board. Tony from Hong Kong found a shorted cap on
that board.

I'll check for component problems for 5v and 51v on regulator and rectifier
boards.

Thx
John

On Wednesday, April 11, 2018, Robert Hay <bobh@joba.com> wrote:

I just re-read the 7904 service manual section on testing the LVPS and the
resistance given there for the +5 V point is 50 Ohms which compares well to
your 47 Ohms.

Following the procedure in the SM for a LVPS in tic-tic mode should help
isolate the trouble.

Bob.


On 4/11/2018 8:10 AM, Robert Hay wrote:

Just to re-cap, your initial message indicated your probe ground lead
contacted a high voltage point on a device under test with a common
ground. This would raise the scope vertical input ground and cause high
current to find its way thru your scope. This apparently caused a failure
in the scope.

Your resistance tests don't point to a short on any of the LV supplies.
The author of the old article used a VOM so I would tend to compare VOM
readings to those given in the article. If you tested using both Fluke and
VOM did you find much differences in readings?

The minus 51 V load resistance is much higher than the article
measurement. The plus 5 is lower resistance. So, those might be the
places to look. Did you check the diodes and caps for the -51 volt
supply? If they are good, you may be looking for something failed open on
the mainframe side since your measurement of 31.6k with the LVPS
disconnected is much higher than the 2k given in the article.

Usually, the tic-tic mode is the power supply letting you know there is a
short on one of the supplies and saving itself from further damage. As far
as testing it without load to see if you still get the tic-tic or not it
would be good if one of the more experienced members with testing the 7904
supply jumps in here.

Bob.

On 4/10/2018 4:37 PM, JJ wrote:

In reading that doc, I'm concerned that the problem is in that inverter
control chip. That's probably impossible to find. I found the schematic
of
what's inside it:
http://w140.com/tekwiki/wiki/File:155-0067-02_block.png

Z-axis Board voltages PSU in Mainframe
+130 8.3k
+50 3.7k
+15 90.1 ohms
-15 205 ohms
-50 4.2k
+5 47 ohms

+5V lamp 2 ohms

Z-axis Board voltages PSU out of Mainframe
+130 9,9k
+50 4.6k
+15 92.3 ohms
-15 216 ohms
-50 31.6k
+5 48 ohms

+5V lamp 23k

On Tue, Apr 10, 2018 at 6:55 PM, Robert Hay <bobh@joba.com> wrote:

Ok, you said the resistances were very high, how much higher than those
suggested by the article?

+130 volts 6k
+50 2k
+15 90 Ohms
-15 100 Ohms
-50 250 Ohms
+5V lamp 800 Ohms

Bob.




On 4/10/2018 1:28 PM, JJ wrote:

Bob, I found that the diodes seem OK when I disconnected one lead and
measured both resistance and diode forward and reverse voltage. I found
that the short across those two diodes was due to a shorted capacitor
on
the rectifier board - I was actually measuring the winding tap
resistance
through a weird path. I reconnected the PSU back into the mainframe
after
changing the cap. There was no glory - a very low tick coming from the
supply.

I'll need to continue debug by following the procedure in the that
document. I measured the resistances in z-axis board as the procedure
indicates while the PSU was out of the mainframe - they are pretty
much in
line - a couple of resistances were much higher than the table - none
were
lower . I'll measure them now while the power supply is in the
mainframe,
Also, I get different results using a VOM and a DVM. Maybe there's
something wrong with my VOM - it's pretty old. Is a VOM required to get
the
proper measurements as indicated in the table?

.Best,
John

On Tue, Apr 10, 2018 at 2:41 PM, Robert Hay <bobh@joba.com> wrote:

They say in the article that a low resistance indicates a problem in
the

mainframe which is true if you are looking at something shorting to
ground. But, in your case I would not discount that high current
caused
a
open circuit while trying to get back to the source. I suppose you
already
looked for any signs of burning or smoked components, leads, traces,
etc.

Bob.



On 4/10/2018 7:44 AM, JJ wrote:

Yes the resistances were very high. According to that doc, the issue

should
be on the PSU. I'm going to put the PSU back into the mainframe,
connect
the cables, and make those voltage measurements on the LV regulator
board.
I didn't check for voltages before taking the PSU out because I
didn't
know
at that time that the PSU needs to be under minimum load.

I found a shorted cap C1360 on the rectifier board (+54v filter cap
on
output of Pi filter). I'm going to lift one lead to ensure it's the
problem. I'm also going to validate that the two power diodes that I
found
are shorted - by disconnecting the wires going to those diodes and
measuring. If the diodes are bad, I'll replace the bad 10A diodes
with
two
5A diodes in parallel temporarily - hoping that's OK, I'll then hook
up
the
mainframe's cables to the PS and check the voltages. I think the 3
cables
are long enough for the PSU to hang out the back. If not, I plan on
making
extender cables.

Best,
John

On Tue, Apr 10, 2018 at 10:16 AM, Robert Hay <bobh@joba.com> wrote:

Have you checked resistances as suggested in the Service Scope
article?

And then voltages with the supply outside the scope but still
connected?

Bob.


On 4/10/2018 6:38 AM, JJ wrote:

Hi Tony, So, based on your findings, you have confirmed that the the
PSU

needs to be under load in order to be functional. I will check all
the
caps
on those 3 boards as well. Thank you for the info - it's very, very
helpful.
I hope the experts on this forum can help you out with the
remainder
of
your 7904's issues.

Best,
John

On Tue, Apr 10, 2018 at 9:29 AM, Yiu On Tony C via Groups.Io <
tonycheung_hk=yahoo.com@groups.io> wrote:

Hi JJ :

I am from Hong Kong ! I just complete the repair of PSU of TEK7904
, I

am
lucky , I find a SHORT E-Cap. once I replace it and turn on PSU
alone ,
It
still sound shut-down ! once I connect to main unit , it come
normal
.
I found few E-Cap short on A9 H amp board, A2 Main interface board
and
A12 Rectifier board
For the power diode , I did check each by de-solder the to wire
only
.
My 7904 still have other issue , H ok but the read-out still
error
in
units , Y are not function -- position knob no response .

RegardTony CheungAPR 10 2018


From: JJ <jajustin@gmail.com>
To: TekScopes@groups.io
Sent: Tuesday, April 10, 2018 7:16 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged

Dave, I went through hundreds of files in the files section of the
archive
and wasn't able to identify the schematic describing the minimum
loading
requirements for power supplies. Tried searching, browsing, ad
nauseum!
No
glory. Are you sure that Jerry uploaded it?

Best,
John

On Mon, Apr 9, 2018 at 4:54 PM, Dave Daniel <kc0wjn@gmail.com>
wrote:

On some of the 7xxx supplies (SMPS supplies, I believe), one
needs to
put

a minimum load on the power supply output in order for the supply
to

work.

I don't remember if that is true for the 7904 power supply.
Jerry Massengale built one of these loads. I think he uploaded a
copy
of
the schematic to the files section back in maybe2015.

DaveD


On 4/9/2018 12:22 PM, JJ wrote:

I removed the power supply from the mainframe - it's on my bench.
First,

I'll remove the wire from the diode terminal and check to see if
the

diode

is actually shorted as others have suggested. I was thinking of
replacing
the 10A diodes with two 5A diodes in parallel that I have
available
in
my
parts bin temporarily to see if I get all the other voltages
back -
right
now there are no voltages at the test points of the rectifier
board
and

the

low voltage regulator. I wouldn't think there would be that much
load
with
the supply removed? That way I can continue to debug. Thoughts?

On Mon, Apr 9, 2018 at 2:06 PM, Jeff Urban <JURB6006@gmail.com>
wrote:

Incidentally, that stuff about the mini EMP is not very likely.
Nott

only

does it actually have to happen at a strength to do something,
it
also

needs to be oriented the right way to do something. Also I erred

about
the
vaporized foil on the board, it was about ½ cm., not inch.

Anyway, if you are sure about the diodes I guess you know what
to
do,

if

possible. It might be rough to gt everything hooked back up
for a
live

test

without actually assembling it. Changing the diodes right away ?
You
could
do that. And of course watch for mounting screws that also
function
as

a

ground, that has tripped me up a couple of times.
Since you have one + and one - diode bad, assuming you don't
have a

short

to ground, I would check see if there is a short between the +
and
-

legs

of that supply. At this time I have no idea what those sources
feed
but

it

could be a series arranged push pull output to something and
while
both
of
the output devices could be shorted, the load isn't low enough

impedance,

or maybe even electrostatic, to read a short to ground. It
happens.











Re: 7B53A broken switch (mixed mode switch)

Fabio Trevisan
 

Hi Phillip,
I`m glad enough that the message came through before Max will actually need to crimp.
Since the switch went off (on your side) without mishaps... you're all still happy.
I wish I knew it was bakelite also. It was - to say the least - a thrill, when mine snapped under my pliers.
Fortunately it was a tiny piece at the end and didn't compromise too much, structurally speaking.
If being bakelite, on one hand is bad because it's fragile, on the other hand, it's good it wasn't plastics, because at least it's glueable.
I was kind of disappointed with Tek on this switch approach... A sliding switch with such a short throw, that tends to rock when you move it... and they didn't even care about placing the actuator closer to the switch, so to minimize the rocking forces.
It's definitely not the kind of elegant design I would expect from Tek.
Rgrds,
Fabio


Re: 7904 Mainframe damaged

bobh@joba.com
 

I just re-read the 7904 service manual section on testing the LVPS and the resistance given there for the +5 V point is 50 Ohms which compares well to your 47 Ohms.

Following the procedure in the SM for a LVPS in tic-tic mode should help isolate the trouble.

Bob.

On 4/11/2018 8:10 AM, Robert Hay wrote:
Just to re-cap, your initial message indicated your probe ground lead contacted a high voltage point on a device under test with a common ground.  This would raise the scope vertical input ground and cause high current to find its way thru your scope.  This apparently caused a failure in the scope.

Your resistance tests don't point to a short on any of the LV supplies.  The author of the old article used a VOM so I would tend to compare VOM readings to those given in the article.  If you tested using both Fluke and VOM did you find much differences in readings?

The minus 51 V load resistance is much higher than the article measurement.  The plus 5 is lower resistance.  So, those might be the places to look.  Did you check the diodes and caps for the -51 volt supply?  If they are good, you may be looking for something failed open on the mainframe side since your measurement of 31.6k with the LVPS disconnected is much higher than the 2k given in the article.

Usually, the tic-tic mode is the power supply letting you know there is a short on one of the supplies and saving itself from further damage.  As far as testing it without load to see if you still get the tic-tic or not it would be good if one of the more experienced members with testing the 7904 supply jumps in here.

Bob.

On 4/10/2018 4:37 PM, JJ wrote:
In reading that doc, I'm concerned that the problem is in that inverter
control chip. That's probably impossible to find. I found the schematic of
what's inside it:
http://w140.com/tekwiki/wiki/File:155-0067-02_block.png

Z-axis Board voltages PSU in Mainframe
+130  8.3k
+50    3.7k
+15    90.1 ohms
-15   205  ohms
-50   4.2k
+5    47 ohms

+5V lamp  2 ohms

Z-axis Board voltages PSU out of Mainframe
+130  9,9k
+50    4.6k
+15    92.3 ohms
-15   216  ohms
-50   31.6k
+5   48 ohms

+5V lamp  23k

On Tue, Apr 10, 2018 at 6:55 PM, Robert Hay <bobh@joba.com> wrote:

Ok, you said the resistances were very high, how much higher than those
suggested by the article?

+130 volts  6k
+50  2k
+15 90 Ohms
-15 100 Ohms
-50  250 Ohms
+5V lamp 800 Ohms

Bob.




On 4/10/2018 1:28 PM, JJ wrote:

Bob, I found that the diodes seem OK when I disconnected one lead and
measured both resistance and diode forward and reverse voltage. I found
that the short across those two diodes was due to a shorted capacitor on
the rectifier board - I was actually measuring the winding tap resistance
through a weird path. I reconnected the PSU back into the mainframe after
changing the cap. There was no glory - a very low tick coming from the
supply.

I'll need to continue debug by following the procedure in the that
document. I measured the resistances in z-axis board as the procedure
indicates while the PSU was out of the mainframe - they are pretty much in
line - a couple of resistances were much higher than the table - none were
lower . I'll measure them now while the power supply is in the mainframe,
Also, I get different results using a VOM and a DVM. Maybe there's
something wrong with my VOM - it's pretty old. Is a VOM required to get
the
proper measurements as indicated in the table?

.Best,
John

On Tue, Apr 10, 2018 at 2:41 PM, Robert Hay <bobh@joba.com> wrote:

They say in the article that a low resistance indicates a problem in the
mainframe which is true if you are looking at something shorting to
ground.  But, in your case I would not discount that high current caused
a
open circuit while trying to get back to the source.  I suppose you
already
looked for any signs of burning or smoked components, leads, traces, etc.

Bob.



On 4/10/2018 7:44 AM, JJ wrote:

Yes the resistances were very high. According to that doc, the issue
should
be on the PSU. I'm going to put the PSU back into the mainframe, connect
the cables, and make those voltage measurements on the LV regulator
board.
I didn't check for voltages before taking the PSU out because I didn't
know
at that time that the PSU needs to be under minimum load.

I found a shorted cap C1360 on the rectifier board (+54v filter cap on
output of Pi filter). I'm going to lift one lead to ensure it's the
problem. I'm also going to validate that the two power diodes that I
found
are shorted - by disconnecting the wires going to those diodes and
measuring. If the diodes are bad, I'll replace the bad 10A diodes with
two
5A diodes in parallel temporarily - hoping that's OK, I'll then hook up
the
mainframe's cables to the PS and check the voltages. I think the 3
cables
are long enough for the PSU to hang out the back. If not, I plan on
making
extender cables.

Best,
John

On Tue, Apr 10, 2018 at 10:16 AM, Robert Hay <bobh@joba.com> wrote:

Have you checked resistances as suggested in the Service Scope article?

And then voltages with the supply outside the scope but still
connected?

Bob.


On 4/10/2018 6:38 AM, JJ wrote:

Hi Tony, So, based on your findings, you have confirmed that the the
PSU

needs to be under load in order to be functional. I will check all the
caps
on those 3 boards as well. Thank you for the info - it's very, very
helpful.
I hope the experts on this forum can help you out with the remainder
of
your 7904's issues.

Best,
John

On Tue, Apr 10, 2018 at 9:29 AM, Yiu On Tony C via Groups.Io <
tonycheung_hk=yahoo.com@groups.io> wrote:

Hi JJ :

I am from Hong Kong ! I just complete the repair of PSU of TEK7904 , I
am
lucky , I find a SHORT E-Cap. once I replace it and turn on PSU
alone ,
It
still sound shut-down ! once I connect to main unit , it come normal
.
I found few E-Cap short on A9 H amp board, A2 Main interface board
and
A12 Rectifier board
For the power diode , I did check each by de-solder the to wire only
.
My 7904 still have other issue , H  ok but the read-out still error
in
units , Y  are not function -- position knob no response .

RegardTony CheungAPR 10 2018


          From: JJ <jajustin@gmail.com>
     To: TekScopes@groups.io
     Sent: Tuesday, April 10, 2018 7:16 PM
     Subject: Re: [TekScopes] 7904 Mainframe damaged

Dave, I went through hundreds of files in the files section of the
archive
and wasn't able to identify the schematic describing the minimum
loading
requirements for power supplies. Tried searching, browsing, ad
nauseum!
No
glory. Are you sure that Jerry uploaded it?

Best,
John

On Mon, Apr 9, 2018 at 4:54 PM, Dave Daniel <kc0wjn@gmail.com>
wrote:

On some of the 7xxx supplies (SMPS supplies, I believe), one needs to
put

a minimum load on the power supply output in order for the supply to
work.
I don't remember if that is true for the 7904 power supply.

Jerry Massengale built one of these loads. I think he uploaded a
copy
of
the schematic to the files section back in maybe2015.

DaveD


On 4/9/2018 12:22 PM, JJ wrote:

I removed the power supply from the mainframe - it's on my bench.
First,

I'll remove the wire from the diode terminal and check to see if the
diode
is actually shorted as others have suggested. I was thinking of
replacing
the 10A diodes with two 5A diodes in parallel that I have available
in
my
parts bin temporarily to see if I get all the other voltages back -
right
now there are no voltages at the test points of the rectifier board
and

the
low voltage regulator. I wouldn't think there would be that much
load
with
the supply removed? That way I can continue to debug. Thoughts?

On Mon, Apr 9, 2018 at 2:06 PM, Jeff Urban <JURB6006@gmail.com>
wrote:

Incidentally, that stuff about the mini EMP is not very likely.
Nott

only
does it actually have to happen at a strength to do something, it
also

needs to be oriented the right way to do something. Also I erred
about
the
vaporized foil on the board, it was about ½ cm., not inch.

Anyway, if you are sure about the diodes I guess you know what to
do,

if
possible. It might be rough to gt everything hooked back up for a
live

test
without actually assembling it. Changing the diodes right away ?
You
could
do that. And of course watch for mounting screws that also
function
as

a
ground, that has tripped me up a couple of times.
Since you have one + and one - diode bad, assuming you don't have a
short
to ground, I would check see if there is a short between the + and
-

legs
of that supply. At this time I have no idea what those sources feed
but

it
could be a series arranged push pull output to something and while
both
of
the output devices could be shorted, the load isn't low enough

impedance,
or maybe even electrostatic, to read a short to ground. It happens.







Re: 7904 Mainframe damaged

bobh@joba.com
 

Just to re-cap, your initial message indicated your probe ground lead contacted a high voltage point on a device under test with a common ground.  This would raise the scope vertical input ground and cause high current to find its way thru your scope.  This apparently caused a failure in the scope.

Your resistance tests don't point to a short on any of the LV supplies.  The author of the old article used a VOM so I would tend to compare VOM readings to those given in the article.  If you tested using both Fluke and VOM did you find much differences in readings?

The minus 51 V load resistance is much higher than the article measurement.  The plus 5 is lower resistance.  So, those might be the places to look.  Did you check the diodes and caps for the -51 volt supply?  If they are good, you may be looking for something failed open on the mainframe side since your measurement of 31.6k with the LVPS disconnected is much higher than the 2k given in the article.

Usually, the tic-tic mode is the power supply letting you know there is a short on one of the supplies and saving itself from further damage.  As far as testing it without load to see if you still get the tic-tic or not it would be good if one of the more experienced members with testing the 7904 supply jumps in here.

Bob.

On 4/10/2018 4:37 PM, JJ wrote:
In reading that doc, I'm concerned that the problem is in that inverter
control chip. That's probably impossible to find. I found the schematic of
what's inside it:
http://w140.com/tekwiki/wiki/File:155-0067-02_block.png

Z-axis Board voltages PSU in Mainframe
+130 8.3k
+50 3.7k
+15 90.1 ohms
-15 205 ohms
-50 4.2k
+5 47 ohms

+5V lamp 2 ohms

Z-axis Board voltages PSU out of Mainframe
+130 9,9k
+50 4.6k
+15 92.3 ohms
-15 216 ohms
-50 31.6k
+5 48 ohms

+5V lamp 23k

On Tue, Apr 10, 2018 at 6:55 PM, Robert Hay <bobh@joba.com> wrote:

Ok, you said the resistances were very high, how much higher than those
suggested by the article?

+130 volts 6k
+50 2k
+15 90 Ohms
-15 100 Ohms
-50 250 Ohms
+5V lamp 800 Ohms

Bob.




On 4/10/2018 1:28 PM, JJ wrote:

Bob, I found that the diodes seem OK when I disconnected one lead and
measured both resistance and diode forward and reverse voltage. I found
that the short across those two diodes was due to a shorted capacitor on
the rectifier board - I was actually measuring the winding tap resistance
through a weird path. I reconnected the PSU back into the mainframe after
changing the cap. There was no glory - a very low tick coming from the
supply.

I'll need to continue debug by following the procedure in the that
document. I measured the resistances in z-axis board as the procedure
indicates while the PSU was out of the mainframe - they are pretty much in
line - a couple of resistances were much higher than the table - none were
lower . I'll measure them now while the power supply is in the mainframe,
Also, I get different results using a VOM and a DVM. Maybe there's
something wrong with my VOM - it's pretty old. Is a VOM required to get
the
proper measurements as indicated in the table?

.Best,
John

On Tue, Apr 10, 2018 at 2:41 PM, Robert Hay <bobh@joba.com> wrote:

They say in the article that a low resistance indicates a problem in the
mainframe which is true if you are looking at something shorting to
ground. But, in your case I would not discount that high current caused
a
open circuit while trying to get back to the source. I suppose you
already
looked for any signs of burning or smoked components, leads, traces, etc.

Bob.



On 4/10/2018 7:44 AM, JJ wrote:

Yes the resistances were very high. According to that doc, the issue
should
be on the PSU. I'm going to put the PSU back into the mainframe, connect
the cables, and make those voltage measurements on the LV regulator
board.
I didn't check for voltages before taking the PSU out because I didn't
know
at that time that the PSU needs to be under minimum load.

I found a shorted cap C1360 on the rectifier board (+54v filter cap on
output of Pi filter). I'm going to lift one lead to ensure it's the
problem. I'm also going to validate that the two power diodes that I
found
are shorted - by disconnecting the wires going to those diodes and
measuring. If the diodes are bad, I'll replace the bad 10A diodes with
two
5A diodes in parallel temporarily - hoping that's OK, I'll then hook up
the
mainframe's cables to the PS and check the voltages. I think the 3
cables
are long enough for the PSU to hang out the back. If not, I plan on
making
extender cables.

Best,
John

On Tue, Apr 10, 2018 at 10:16 AM, Robert Hay <bobh@joba.com> wrote:

Have you checked resistances as suggested in the Service Scope article?

And then voltages with the supply outside the scope but still
connected?

Bob.


On 4/10/2018 6:38 AM, JJ wrote:

Hi Tony, So, based on your findings, you have confirmed that the the
PSU

needs to be under load in order to be functional. I will check all the
caps
on those 3 boards as well. Thank you for the info - it's very, very
helpful.
I hope the experts on this forum can help you out with the remainder
of
your 7904's issues.

Best,
John

On Tue, Apr 10, 2018 at 9:29 AM, Yiu On Tony C via Groups.Io <
tonycheung_hk=yahoo.com@groups.io> wrote:

Hi JJ :

I am from Hong Kong ! I just complete the repair of PSU of TEK7904 , I
am
lucky , I find a SHORT E-Cap. once I replace it and turn on PSU
alone ,
It
still sound shut-down ! once I connect to main unit , it come normal
.
I found few E-Cap short on A9 H amp board, A2 Main interface board
and
A12 Rectifier board
For the power diode , I did check each by de-solder the to wire only
.
My 7904 still have other issue , H ok but the read-out still error
in
units , Y are not function -- position knob no response .

RegardTony CheungAPR 10 2018


From: JJ <jajustin@gmail.com>
To: TekScopes@groups.io
Sent: Tuesday, April 10, 2018 7:16 PM
Subject: Re: [TekScopes] 7904 Mainframe damaged

Dave, I went through hundreds of files in the files section of the
archive
and wasn't able to identify the schematic describing the minimum
loading
requirements for power supplies. Tried searching, browsing, ad
nauseum!
No
glory. Are you sure that Jerry uploaded it?

Best,
John

On Mon, Apr 9, 2018 at 4:54 PM, Dave Daniel <kc0wjn@gmail.com>
wrote:

On some of the 7xxx supplies (SMPS supplies, I believe), one needs to
put

a minimum load on the power supply output in order for the supply to
work.
I don't remember if that is true for the 7904 power supply.

Jerry Massengale built one of these loads. I think he uploaded a
copy
of
the schematic to the files section back in maybe2015.

DaveD


On 4/9/2018 12:22 PM, JJ wrote:

I removed the power supply from the mainframe - it's on my bench.
First,

I'll remove the wire from the diode terminal and check to see if the
diode
is actually shorted as others have suggested. I was thinking of
replacing
the 10A diodes with two 5A diodes in parallel that I have available
in
my
parts bin temporarily to see if I get all the other voltages back -
right
now there are no voltages at the test points of the rectifier board
and

the
low voltage regulator. I wouldn't think there would be that much
load
with
the supply removed? That way I can continue to debug. Thoughts?

On Mon, Apr 9, 2018 at 2:06 PM, Jeff Urban <JURB6006@gmail.com>
wrote:

Incidentally, that stuff about the mini EMP is not very likely.
Nott

only
does it actually have to happen at a strength to do something, it
also

needs to be oriented the right way to do something. Also I erred
about
the
vaporized foil on the board, it was about ½ cm., not inch.

Anyway, if you are sure about the diodes I guess you know what to
do,

if
possible. It might be rough to gt everything hooked back up for a
live

test
without actually assembling it. Changing the diodes right away ?
You
could
do that. And of course watch for mounting screws that also
function
as

a
ground, that has tripped me up a couple of times.
Since you have one + and one - diode bad, assuming you don't have a
short
to ground, I would check see if there is a short between the + and
-

legs
of that supply. At this time I have no idea what those sources feed
but

it
could be a series arranged push pull output to something and while
both
of
the output devices could be shorted, the load isn't low enough

impedance,
or maybe even electrostatic, to read a short to ground. It happens.






Re: 7904 Mainframe damaged

JJ
 

Good idea for a load. I was wondering I have some ribbon cable (like this:
https://www.ebay.com/itm/273079315705) that I could use to extend the
cables further out the back of the unit - do you see any problems with
doing that?

On Wed, Apr 11, 2018 at 8:30 AM, Jeff Urban <JURB6006@gmail.com> wrote:

If they all read high in resistance, then a likely reason is that they're
cold. If it was one or two...but all of them ?

There is still probably something wrong right in that power supply. Not
sure exactly what to check next right now, I have other things on my mind.
(court case in about an hour) Later on I will review all the information
and try to come up with something helpful.

Remember test lights ? An incandescent that you wire in lieu of a blown
fuse. It limits damage and lights up when there is a short. You start
unplugging things until the light goes out and there's the short. Well
there are other uses. you could use it as a dummy load for the power
supply. Any source up to about 130 volts can be loaded and a 100 watt will
pull about 1 amp. That should keep it down so it regulates. I have one of
those emergency lights electricians use but you can just take a couple of
test leads to the plug on a table lamp or something, The only thing is that
is must be an incandescent and they are getting a bit scarce, in California
especially. But a CFL or LED won't work right for this purpose. And using a
transformer and a car headlight or something like that won't work for DC. A
simple old fashioned 75 or even 60 watt bulb will do the trick.

Then the power supply should run without being connected to the rest of
it, which I am sure will make any live tests easier. (or possible at all)




Re: Trying to fix up my 2213A

tom jobe <tomjobe@...>
 

That is quite a selection of free manuals someone is offering, but both of the manuals they have are for the non-A version of the 2213 which is a much different oscilloscope than the 2213A, especially in the power supply section where many of the age related problems are found.
The best copy of the manual I have for the 2213A came from Artek Media for very little money.
Original printed copies of the 2213A Service Manual are somewhat rare and and usually 'pricey'.
I bought a 're-printed' copy of the 2213A manual years ago on eBay before I had the capability to print schematics on 11 x 17 paper and I found that 2213A 're-print' to be useless.
tom jobe...

On 4/11/2018 2:32 AM, Jeff Urban wrote:
It appears a free service manual is available here :

http://bee.mif.pg.gda.pl/ciasteczkowypotwor/Tek/

Looks like 2 there, one is 15 MB and the other is 83 MB, obviously take the bigger one... :-)




Re: 7904 Mainframe damaged

 

If they all read high in resistance, then a likely reason is that they're cold. If it was one or two...but all of them ?

There is still probably something wrong right in that power supply. Not sure exactly what to check next right now, I have other things on my mind. (court case in about an hour) Later on I will review all the information and try to come up with something helpful.

Remember test lights ? An incandescent that you wire in lieu of a blown fuse. It limits damage and lights up when there is a short. You start unplugging things until the light goes out and there's the short. Well there are other uses. you could use it as a dummy load for the power supply. Any source up to about 130 volts can be loaded and a 100 watt will pull about 1 amp. That should keep it down so it regulates. I have one of those emergency lights electricians use but you can just take a couple of test leads to the plug on a table lamp or something, The only thing is that is must be an incandescent and they are getting a bit scarce, in California especially. But a CFL or LED won't work right for this purpose. And using a transformer and a car headlight or something like that won't work for DC. A simple old fashioned 75 or even 60 watt bulb will do the trick.

Then the power supply should run without being connected to the rest of it, which I am sure will make any live tests easier. (or possible at all)

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