Date   

Re: Tek 7904 help!

Bill Riches
 

Hi Chck and gang,
I have a later? horizontal board - no tant - 2 small can type caps.  My manual shows the tant caps.  Did check the caps and they were ok.  
Some more measurements.
1. PS disconnected completely2. Resistance measurements taken at Z axis board with Fluke DVM
    VOLTAGE TP    NORMAL    NO PLUGINS    WITH PLUGINS

    +130                    6.6K                9.5K                9.5K
    +50                        2K                 4.6K                4.2K
    +15                        90                  92                    27
     +5                         65                  49                    19
     -15                       110                216                   31
     -50                       2K                   55K                 55K
     +LAMP 5              INF                  INF                 55k
(red underlines from spell check)
3. Highlighted readings look strange.  Could I have 4 plugins with loading.  Inserting each module reduced resistance by a similar amount from 216 ohms to 31 for -15 volt source.  
4. Checked power supply voltages out of mainframe with 120 VAC.  Power about 10 watts or so.130 = 136+5 =   4.8-50= 12+50 = 25-15 = 2+15 = 10
I measured the crt anode voltage out of the PS using and old Pomona tv hv meter and the meter would kick a bit - maybe a few hundred volts..

I guess that low voltages would be low due to burst mode not allowing output filters to charge to normal value.
I am thinking PS is ok or it is shutting itself down.  Just curious why the different resistance readings when plug-ins are installed.  We had quite an electrical storm on Tuesday Evening. Wednesday morning the scope was not working.  As you can see from my QRZ.com page I have a bunch of toys and they are all working - so far.
So - 2 questions - why do plug-ins resistance read low and are my voltages close when the ps is not installed?
Just remembered I do have some old plug ins and will plug them in and see if I get the same resistance drop.
I really appreciate the help.
73
Bill, WA2DVUCape May

On Thursday, July 9, 2020, 12:04:05 AM EDT, Chuck Harris <cfharris@...> wrote:

There are two tantalum capacitors that every 7904 I have had
has blown.  One is on the +50V supply, and doesn't show up in
my schematics, and the other is on the -15V supply, if I recall
correctly.  They both are rated at too low of a voltage.
They are on the Horizontal board, which is on the left side of
chassis, as I recall.  One has a choke feeding it, and the other
a 10 ohm 1/4w resistor.

You should be suspicious of any tantalum capacitor that is rated
less than 2x the applied voltage.  6.3V caps on 5V, and 16V caps
on 15V are especially bad.  Anything on a 50V supply is a problem
because the maximum voltage a dry slug tantalum can be is 60V.

Be warned, there is a blue harmonica connector on that board
that turns to dust when you touch it.


The power supply itself is usually pretty solid.  Look
at the rest of the scope for a blown tantalum.

-Chuck Harris

Bill Riches via groups.io wrote:
  Thank you Raymond and Harvey.  Will try your suggestions and see what happens. Checked all tants in the ps for shorts - no discolored resistors or smell!  Will check them in the nether regions!
73,
Bill
    On Wednesday, July 8, 2020, 08:53:08 PM EDT, Harvey White <madyn@...> wrote:

  When I used a variac, I looked at the current draw.  If it turned out to
be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran
it up to the 80 to 90 volt range if the current draw was excessive.  I
used the variac to check if the main bulk supply was shorted.  When it
wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies
draw excessive current because the switch doesn't come on (and off)
until a minimum voltage, that's the reason for being sneaky in this
case.  You try to avoid that overcurrent draw due to the switching
circuit not working.

More of a turn it up and see what it draws, turn it right back down,
then figure out why.

think that'll work?

Harvey


On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul  9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond









Re: Another 7D20 attenuator question

Dallas Smith
 

Thanks Dennis,

I do not think the problem is the relays but the substrata PCB contact area.
I get all the relay test errors which make me think the problem may be elsewhere?

I noticed that the 'A12 & A13 substrate PCB has tarnished relay
contacts. there are a dull gray (lead?). I thought Tek gold plated PCB
contacts for non corrosion and good contact resistance?
Since I have two 7D20's I think I'll have enough if any relay is actually bad.

When the 7D20 is booted up and the attenuators are tested, could the reference voltage being measured be in error because of channel level calibration error?


Re: Pallet of scopes for sale

Richard in Edenton NC
 

You can always buy them and sell some. Then the rest you get for free


Re: Purpose of Q32 in S-52

Albert Otten
 

You are right Ed. I overlooked that the negative supply at R72 would also be missing.

I don't remember that I have seen such a protection with an active component (so not counting clamping diodes and fuse-like resistors) elsewhere in Tek equipment, except perhaps in SMPS.

Albert

On Wed, Jul 8, 2020 at 11:26 PM, Ed Breya wrote:


Albert,

When the -12.4 V is lost, it's essentially zero V, so the emitters of Q72 and
Q74 are effectively grounded through R72, and the long-tail pair cannot
provide feedback. Assuming they're Si transistors, the TD voltage can't be
high enough to turn Q74 on (unless the actual TD current is very high up its
diode curve, and in danger anyway). The -12.4 V emitter feed via R72 makes it
able to operate around zero, and without it, it's open-loop.

Ed


Re: Pallet of scopes for sale

Dave Seiter
 

I'm an auction bottom feeder too, mostly because I hate getting stuck with worthless junk, but also because I resell a lot in order to finance my acquisitions.  I've noticed that getting deals has been getting harder in the past four years, and really tough in the past 6 months (unless you're looking for machinery that requires pros to move, then it's practically given away for scrap value).
Some auctions allow only bank transfers as payment.  This appears to be an effort to weed out casual bidders; and I saw a machinery auction last year in SoCal where they actually stated they would only sell to "industry professionals".  That auction only sold about 30% of it's items- I would have bid on two larger items myself until I read the requirements, too many hoops to jump through! I don't know what they were trying to avoid, but it seemed to have backfired.  I chatted with another guy who had wanted to bid also- he suspected that something funny was going on, like they had to go through the motions of the auction, but wanted to retain a good portion of the equipment (or sell it cheap to another party.)
My Tek "problem" started about 13 years ago when I got two pallets of gear (4'+ tall) from a series of Telogy auctions.  I got them for $50 each- no one else bid because they had put the obviously broken items on top (like a 7104 that looked practically new, except where the side of the CRT face had been hit by something like a forklift.  Along with some working 7104s, I ended up with about 7 (R)7844s, along with piles of other gear.  I had a "scope henge" in the garage for a while.   
-Dave

On Wednesday, July 8, 2020, 06:33:44 PM PDT, Paul Amaranth <paul@...> wrote:
Anyway, I'm a bottom feeder at auctions.  In the olden days (with
5% buyer's premium, the local auctioneer used to call me "Mr Dollar"
Part of the fun for me is getting things *really* cheap.


Re: GOULD SCOPE SUPPLIES #file-notice NOT TEK RELATED GOULD DSO RELATED FOR: Dan Brasier #file-notice

Garth Daddy
 

I seem to remember replying to this request regarding the pens, quite a long time ago!
I never had a response. The pens are a technical drawing pen I don't know about the graph paper, I have a Gould 4 channel scope which worked OK first time around second time it smoked the power supply suspect switch mode power supply cap but have not had the time to do a serious repair.


Re: R7704+7A18+7B53A in Newport Beach, CA to part with

ChrisBeee
 

...since Abc Xyz did not respond and I am a bit reluctant to deal with a ghost: the scope is still available for non-ghosts ;-)
Chris

On Wed, Jul 8, 2020 at 11:10 AM, ChrisBeee wrote:


Hi ABC XYZ (what's your real name?),
please contact me offline for details.
Thanks!
Chris

On Tue, Jul 7, 2020 at 10:12 PM, Abc Xyz wrote:


Chris...i could come get it.

On Tue, Jul 7, 2020, 10:50 AM ChrisBeee via groups.io <chrisbeee=
yahoo.com@groups.io> wrote:

I have a leftover from my time in Southern California I want to part with:
a nice R7704 including a 7A18 and a 7B53A plug in. It has been stored in a
dry garage lab after I bought it from Gov surplus somewhere in 2008 or
2009
(ex-NASA unit). This unit has not been run for quite a while, so please be
prepared to revive the PS carefully! No warranty for meeting
manufacturer's
specs of course ;-)
I would like to give it in good hands because shipping across the pond
does not really make sense.
Price: what you feel it is worth for you, first come first serve.
The only caveat: this unit is for local pick up only (location: San
Joaquin Hills area, Newport Beach, CA.).
Please contact me off-line, thanks!
Chris




Re: Tektronix 2230

Saroj Pradhan
 

By the way, I got MUR460 and replaced Cr907. It is quite big. For the
moment I had soldered it in the pcb directly.

On Thu, Jul 9, 2020, 10:25 AM Saroj Pradhan via groups.io <sarojman.Pradhan=
gmail.com@groups.io> wrote:

Hi leo

For the fan I did mistake, I soldered the wire in reverse order. I soldered
the wire in correct order and it worked with external supply.
As Bert suggested the problem might be associated with high voltage
transformer and he was pretty sure about this. I decided to dismantle T906
which was the only component that was not tested in preregulator
section.There were many exposed copper spots in the primary winding.
Rewound the transformer, put in the circuit and power on the scope(with
limiter bulb). Same situation. This time the bulb flashes little dimmer
than it was before and the fan spins for the moment when the light is on.
Power on led and 'A trigger' led flash.
I am quite reluctant to power on the scope without limiter bulb.
By the way how can I measure inrush current. Any idea?
Tkanks.


On Sat, Jul 4, 2020, 10:23 AM Saroj Pradhan via groups.io
<sarojman.Pradhan=
gmail.com@groups.io> wrote:

Hi Bert
OK it means in my case t906.
Do you have the schematic for 2230 so that we can confirm the
transformer.
Thanks.


On Sat, Jul 4, 2020, 9:12 AM Bert Haskins <bhaskins@...>
wrote:


On 7/3/2020 9:50 PM, Saroj Pradhan wrote:
Hi Bert
Thanks for your concern and suggestion.

But it operates normally with external power supply (in my case I
used
38v
dc across tp950 and tp940).

Saroj
No, I mean the HV supply, the one that runs off the input AC and
supplies the +40V.

Shorted turns make it draw too much current.




On Sat, Jul 4, 2020, 1:28 AM Bert Haskins <bhaskins@...>
wrote:

On 7/2/2020 8:51 PM, Saroj Pradhan wrote:
Hi Leo
No worry. It happens.
Sure I will check the component.
I confirmed that the scope is still working with external supply.
But before going further I want share with you my experience during
the
testing of scope with limiter bulb.
1. That day you suggested to put on the scope without limiter bulb.
And
you know the outcome.
2.It seems that even with shorted q9070
I could hear the same high pitch(with limiter bulb lighting)
3 changed q9070 and put on the scope(with limiter bulb). This time
I
had
flashing light and there was power led at front panel also flashing.
And I
could hear the pitch at the same frequency as of bulb or led
flashing.

Put off the scope. Again put on the scope with same result. Third
time
when I put on the scope q9070 damaged again. Any logic behind this?
Thanks.
Saroj
First, let me say that I have several 223x family scopes and had
very
few problems with them.

I did have a 2215A that uses a similar power supply that gave me
fits
until I finally figured out that the high voltage transformer had
shorted windings.

I proved this to myself by doing a ringing test after removing the
transformer.

Good luck,

Bert













Re: Tektronix 2230

Saroj Pradhan
 

Hi leo

For the fan I did mistake, I soldered the wire in reverse order. I soldered
the wire in correct order and it worked with external supply.
As Bert suggested the problem might be associated with high voltage
transformer and he was pretty sure about this. I decided to dismantle T906
which was the only component that was not tested in preregulator
section.There were many exposed copper spots in the primary winding.
Rewound the transformer, put in the circuit and power on the scope(with
limiter bulb). Same situation. This time the bulb flashes little dimmer
than it was before and the fan spins for the moment when the light is on.
Power on led and 'A trigger' led flash.
I am quite reluctant to power on the scope without limiter bulb.
By the way how can I measure inrush current. Any idea?
Tkanks.


On Sat, Jul 4, 2020, 10:23 AM Saroj Pradhan via groups.io <sarojman.Pradhan=
gmail.com@groups.io> wrote:

Hi Bert
OK it means in my case t906.
Do you have the schematic for 2230 so that we can confirm the transformer.
Thanks.


On Sat, Jul 4, 2020, 9:12 AM Bert Haskins <bhaskins@...> wrote:


On 7/3/2020 9:50 PM, Saroj Pradhan wrote:
Hi Bert
Thanks for your concern and suggestion.

But it operates normally with external power supply (in my case I
used
38v
dc across tp950 and tp940).

Saroj
No, I mean the HV supply, the one that runs off the input AC and
supplies the +40V.

Shorted turns make it draw too much current.




On Sat, Jul 4, 2020, 1:28 AM Bert Haskins <bhaskins@...>
wrote:

On 7/2/2020 8:51 PM, Saroj Pradhan wrote:
Hi Leo
No worry. It happens.
Sure I will check the component.
I confirmed that the scope is still working with external supply.
But before going further I want share with you my experience during
the
testing of scope with limiter bulb.
1. That day you suggested to put on the scope without limiter bulb.
And
you know the outcome.
2.It seems that even with shorted q9070
I could hear the same high pitch(with limiter bulb lighting)
3 changed q9070 and put on the scope(with limiter bulb). This time I
had
flashing light and there was power led at front panel also flashing.
And I
could hear the pitch at the same frequency as of bulb or led flashing.

Put off the scope. Again put on the scope with same result. Third
time
when I put on the scope q9070 damaged again. Any logic behind this?
Thanks.
Saroj
First, let me say that I have several 223x family scopes and had very
few problems with them.

I did have a 2215A that uses a similar power supply that gave me fits
until I finally figured out that the high voltage transformer had
shorted windings.

I proved this to myself by doing a ringing test after removing the
transformer.

Good luck,

Bert











Re: Another 7D20 attenuator question

 

Hi Dallas,
Tek began making their own parts out of necessity in the late 1960s. Some parts such as timing capacitors had to have exceptional characteristics for the instruments they went into to meet specifications and to hold calibration over long periods of time and environmental conditions. Timing capacitors that were part of sweep circuits are a good example of a component Tek made themselves. Tek also had to make the ICs that the on-screen readout of the 7000 series used. Later they were making their own high frequency analog ICs.
I do not know what it was about other manufacturer's relays that Tek did not like but at one point they decided to make their own relays. The "relay expert" they put in charge wasn't an expert after all. The Tek made relays that went into the 7000 series instruments have been a huge problem ever since.
I have removed lots of them from plugins I reduced to parts. I must have had about 100 of them in several configurations. I manually tested a few of them before giving up on that strategy because it took so long. Instead I built a little test fixture to test them and went through them all much faster. I must have tossed about half of them as not working or not reliable.
If you send me the part numbers of the ones you need I'll see if I have any of them.
Send it to me off list at dennis at ridesoft dot com.
Dennis Tillman W7pF

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Dallas Smith
Sent: Tuesday, July 07, 2020 10:33 AM
To: TekScopes@groups.io
Subject: [TekScopes] Another 7D20 attenuator question

My second 7D20 attenuators started acting up. Cleaned relays and substrate to no avail. I noticed that the 'A12 & A13 substrate PCB has tarnished relay contacts. there are a dull gray (lead?). I thought Tek gold plated PCB contacts for non corrosion and good contact resistance? Is there a way to gold plate the contact areas. Were they ever gold? I think this is the root problem with these plugins, cleaning seems to have no effect. Reluctant to buy another unit with this factory design error.

What do you say?





--
Dennis Tillman W7pF
TekScopes Moderator


Re: Tek 7904 help!

Chuck Harris
 

There are two tantalum capacitors that every 7904 I have had
has blown. One is on the +50V supply, and doesn't show up in
my schematics, and the other is on the -15V supply, if I recall
correctly. They both are rated at too low of a voltage.
They are on the Horizontal board, which is on the left side of
chassis, as I recall. One has a choke feeding it, and the other
a 10 ohm 1/4w resistor.

You should be suspicious of any tantalum capacitor that is rated
less than 2x the applied voltage. 6.3V caps on 5V, and 16V caps
on 15V are especially bad. Anything on a 50V supply is a problem
because the maximum voltage a dry slug tantalum can be is 60V.

Be warned, there is a blue harmonica connector on that board
that turns to dust when you touch it.


The power supply itself is usually pretty solid. Look
at the rest of the scope for a blown tantalum.

-Chuck Harris

Bill Riches via groups.io wrote:

Thank you Raymond and Harvey.  Will try your suggestions and see what happens. Checked all tants in the ps for shorts - no discolored resistors or smell!  Will check them in the nether regions!
73,
Bill
On Wednesday, July 8, 2020, 08:53:08 PM EDT, Harvey White <madyn@...> wrote:

When I used a variac, I looked at the current draw.  If it turned out to
be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran
it up to the 80 to 90 volt range if the current draw was excessive.  I
used the variac to check if the main bulk supply was shorted.  When it
wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies
draw excessive current because the switch doesn't come on (and off)
until a minimum voltage, that's the reason for being sneaky in this
case.  You try to avoid that overcurrent draw due to the switching
circuit not working.

More of a turn it up and see what it draws, turn it right back down,
then figure out why.

think that'll work?

Harvey


On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul  9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond









Re: GOULD SCOPE SUPPLIES #file-notice NOT TEK RELATED GOULD DSO RELATED FOR: Dan Brasier #file-notice

ken chalfant
 

Hi Dan,

I have responded to your general posting twice using the “service@... <mailto:service@...>" regarding pens and paper for a Gould DSO 400 series.

Have you received my emails?

Group: Thank you for the bandwidth and your patient understanding.

Regards,

Ken

On 6Jul, 2020, at 1:16 PM, MyTubeAudio <@MyTubeAudio> wrote:

I have some NOS color plotter pen kits and NOS paper for Gould 400-series DSO's and I've recently posted them to EBAY.

Member Ken Chalfant on this message board was looking for Gould supplies and I think these are correct.

Ken - please send me a message at @MyTubeAudio and we'll sort this out if you are still in need.

- Dan Brasier, Holland, Michigan



Re: Tek 7904 help!

Bill Riches
 

Thank you Raymond and Harvey.  Will try your suggestions and see what happens. Checked all tants in the ps for shorts - no discolored resistors or smell!  Will check them in the nether regions!
73,
Bill

On Wednesday, July 8, 2020, 08:53:08 PM EDT, Harvey White <madyn@...> wrote:

When I used a variac, I looked at the current draw.  If it turned out to
be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran
it up to the 80 to 90 volt range if the current draw was excessive.  I
used the variac to check if the main bulk supply was shorted.  When it
wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies
draw excessive current because the switch doesn't come on (and off)
until a minimum voltage, that's the reason for being sneaky in this
case.  You try to avoid that overcurrent draw due to the switching
circuit not working.

More of a turn it up and see what it draws, turn it right back down,
then figure out why.

think that'll work?

Harvey


On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul  9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond




Re: Pallet of scopes for sale

Paul Amaranth
 

Hey Dave, with that SA in there you did OK. I only did not consider
it a bargain because I'm up to me ears in stuff and most of that
would have been duplicates :-)

Anyway, I'm a bottom feeder at auctions. In the olden days (with
5% buyer's premium, the local auctioneer used to call me "Mr Dollar"
Part of the fun for me is getting things *really* cheap.

Paul

On Wed, Jul 08, 2020 at 02:19:20PM -0400, David Kuhn wrote:
" While not out of line for a 576 + 2 575s plus misc other stuff it
was not what I would consider a bargain."

I'm the idiot that won the auction. I had a nice vacation out of getting
the equipment and visited the two largest flea markets in the Cincinnati
area, when I was out there over this last weekend.

The was TWO (575s) on the pallot (I thought only one) in addition to the
576. There was a worthless HP Logic Analyser. There was however a nice HP
10K-1.5GHZ Spectrum analyzer and it appears to work and has a great CRT.
Also on the pallot, all those 576 plugin, three TDS420A (I think) with
GPIB ports and Thermal Printers and scope probes. The one I tested in the
hotel room seems noisy on the test square wave probe calibrator, but had a
really good and bright trace trace. It also seemed to go to some
ridiculously high digitation rate for a scope like that. The thermal
printer worked. There also were two no-name brand analog scopes and a Tek
2245 Analog scope that works fine and has a great trace. There is more
stuff in the boxes too. Seemed like foam with a bunch of transistors in it
with the gain documented and a few other things.

I am not sure what I am going to do with all that stuff. For now, it is
getting unloaded into my one shed. Next month, a friend will help clear
out the garage and the industrial shelves and move all the gear to there.
I don't really have a use for it, but if it works, the 576 will sit in the
corner of my large bench and look impressive (if I keep it). I don't know
because of their weight, if I will try to sell the 575's or part them out.
Having them packed and then shipped would probably cost $50 for
professional packing and another $100+ shipping, which no one will want to
pay. So confirming that they work and then parting them out, may be the
best option, but not until I play with them for nostalgia's sake. When I
went to Electronic School in 79-81, they had them and they were old back
then. So I want to admire them inside and out and then figure out what to
do with them. The other scopes should be okay for sale if I want. Heck I
think I paid $400 for NOS for a GPIB interface for a TDS420A that I already
had two/three years ago.

I really don't care if I lose money on these units. We had a blast back
home over the weekend and the excitement of searching through all the stuff
on the pallot.

Plus at the flea markets (with masks of course), my kid got a couple of DS
Games and a sword. Then at Microcenter Mall in Cincinnati, my kid also
picked up a professional microphone and accessories for his future "game
podcasting career" (heh heh heh) and we got a 3D printer to play with for
my wife's crafting.

All, in all, it was a good 4 days. Not a bargain or not, I am happy.

Dave

On Sat, Jun 27, 2020 at 4:08 PM Paul Amaranth <paul@...> wrote:

For those who are interested, the bid closed at $655 + $50.44 buyers
premium + $65.50 tax for a total of $770.94

While not out of line for a 576 + 2 575s plus misc other stuff it
was not what I would consider a bargain.

Paul

On Fri, Jun 19, 2020 at 12:33:59PM -0400, David Kuhn wrote:
" My only hope is that somebody bids it up so it's past my bargain
threshold."

And what do you think that would be? It would be an excuse to take a
trip
to Cincy again (about 8 hours from here). I lived there from 1985-2000
and
miss the place.

Dave

On Thu, Jun 18, 2020 at 2:48 PM Paul Amaranth <paul@...>
wrote:

I don't need them; I have all the scopes and curve tracers I could want
and more
projects than I can get to. But Cinci is within driving range and I
hear
the
auction calling ...

My only hope is that somebody bids it up so it's past my bargain
threshold.

Must resist ....

Paul

On Thu, Jun 18, 2020 at 02:33:20PM -0400, Roy Morgan wrote:

On Jun 18, 2020, at 10:14 AM, Bill Higdon via groups.io
<willard561=
aol.com@groups.io> wrote:

https://www.govdeals.com/index.cfm?fa=Main.Item&itemid=1779&acctid=8907

If you're in the area of the auction it might be interesting I see
2
curve tracers

Ok ok, “THE AREA” seems to be Cincinnati Ohio.

Roy
Who will not go there, as such as he’d like to own that stuff!!!!

Roy Morgan
K1LKY since 1958
k1lky68@...










--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@... | Unix/Linux - We don't do windows








--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@... | Unix/Linux - We don't do windows







!DSPAM:5f060e5851551091416528!
--
Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@... | Unix/Linux - We don't do windows


Re: Tek 7904 help!

Harvey White
 

When I used a variac, I looked at the current draw.  If it turned out to be excessive, I looked for a problem with the primary supply.

I don't remember that I spent too much time at lower voltages, and ran it up to the 80 to 90 volt range if the current draw was excessive.  I used the variac to check if the main bulk supply was shorted.  When it wasn't, I ran it up higher.

It seemed to work without stressing the supply too much.  Some supplies draw excessive current because the switch doesn't come on (and off) until a minimum voltage, that's the reason for being sneaky in this case.  You try to avoid that overcurrent draw due to the switching circuit not working.

More of a turn it up and see what it draws, turn it right back down, then figure out why.

think that'll work?

Harvey

On 7/8/2020 8:39 PM, Raymond Domp Frank wrote:
On Thu, Jul 9, 2020 at 02:10 AM, Harvey White wrote:

Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond



Re: Tek 7904 help!

 

On Thu, Jul 9, 2020 at 02:10 AM, Harvey White wrote:


Since you have a variac, you might want to run the voltage up from zero
- The 7904 has a primary switching power supply (PSU), so *no variac* allowed: You may overload (and damage) circuits while varying the input voltage.
- If the PSU is in "tick mode" (it makes a ticking sound several times per second) chances are you won't blow up anything important while testing under power.
- The PSU needs a minimum load to operate, not too small, preferably on several outputs. Sense lines are connected to loads and you at least have to connect those for regulation to occur.

Some things to do if the PSU is in tick mode:
- Check dipped tantalums on the backplane for shorts. Several (at the sides) are easy to reach and the 'scope will be OK while you test without them. Just temporarily lift one leg.
- Check individual parts of the backplane by temporarily disconnecting part of the supply from it. Look in the Service Manual to find their connecting sockets, on the PSU or on the backplane.
- Don't forget: The PSU contains (dipped) tantalum caps as well.
- Be aware that in tick mode, the HV supply may still reach appreciable/noticeable/risky voltages, unless it's not working at all. I've seen it deliver several hundred volts in a ticking 'scope.

Raymond


Re: Tek 7904 help!

Harvey White
 

Comments interspaced.  While I have a 7904, I don't remember these particular problems, but these ideas might help.

On 7/8/2020 7:30 PM, Bill Riches via groups.io wrote:
My 7904 just stopped working.  I have had it for about 5 years - no problems - used several times a week.
1. No blown fuses
No blown fuses says that the bulk supply, rectifiers, and filter capictors are likely ok, but does not say anything about the bulk capacitors not being open.  Check them by looking at ripple on the bulk supply if you have another scope.
.2.  Powered with metered Sencore variac -  it draws about 10 watts after initial kick and meter pulses slightly.  I assure it is in pulse mode.
It would say that, and it makes sense.  Been there, think I've done that.

3.  High voltage warning light is on and when power is removed it goes out after a few min.
Not sure that this is significant, depending on where the HV supply is derived.  Things at this point suggest that you start disconnecting power supply and see if it works.  Note that supply may need a minimum load on one supply to regulate.  Not sure about this.
3. Removed all plugins and it still would not power up.
Plugins therefore are not the only problem.  If using a plugin in a working scope brings the supply down, then it's a plugin.

    4. Removed ps and checked all caps and diodes for shorts.

Then the PS, unless it needs a minimum load, should be ok for shorts.  If it will work without a load, look at the supply that has an adjustment.  It will be a reference supply.  Tektronix liked to use a negative supply.  That must be very close for the other supplies to be within tolerance.


5. Checked resistance readings of supply voltages with ps connected and unconnected - they are close.

I think here you need to define "close".

The supply generally ticks (in startup mode) when there's too much current drawn.  Something's got to be causing that.


I have not checked out ps with 20 volts input and using test scope yet.
Go for the test scope.  See if any supply actually gets to the desired voltage before the scope PS shuts down.  Storage scope would be good, but slow sweep and dim light can give you an idea.


I would like to rule out ps problems and wonder if any of our group would have a working ps that I could purchase.
Nope, not here but it's diagnosable... and fixable, generally.

Question is:  what supply is shutting the scope down.  IIRC, there's a resistor from each supply to a summing junction, and then that junction has to be right for the main supply (not bulk) to be running.

So your problems can be twofold, one is that there is a supply demanding too much current, another is that there's a supply overvoltage.

Since you have a variac, you might want to run the voltage up from zero, looking at the outputs of a supply and the inputs to that regulator.  Until you get to about 80 or 90 volts, you can expect the supplies to be a little under.  If any supply overshoots sufficiently, then you're messing with the overvoltage protection.  SImilarly, look at the overcurrent protection on any supply, it's generally a transistor BE junction across a sense resistor, and that sense resistor in the output of the supply just after the regulator.

Watching that voltage (either one) as the supply ticks should show you a lot.

Harvey


Any help would be appreciated.
73,
Bill, WA2DVU - (good on QRZ)Cape May, NJ609 425 8651



Tek 7904 help!

Bill Riches
 

My 7904 just stopped working.  I have had it for about 5 years - no problems - used several times a week.
1. No blown fuses.2.  Powered with metered Sencore variac -  it draws about 10 watts after initial kick and meter pulses slightly.  I assure it is in pulse mode.3.  High voltage warning light is on and when power is removed it goes out after a few min. 3. Removed all plugins and it still would not power up.4. Removed ps and checked all caps and diodes for shorts.5. Checked resistance readings of supply voltages with ps connected and unconnected - they are close.
I have not checked out ps with 20 volts input and using test scope yet.
I would like to rule out ps problems and wonder if any of our group would have a working ps that I could purchase.
Any help would be appreciated.
73,
Bill, WA2DVU - (good on QRZ)Cape May, NJ609 425 8651


Re: Purpose of Q32 in S-52

 

On Wed, Jul 8, 2020 at 09:57 PM, Ed Breya wrote:


The question then is what happens past the valley - how much current and
voltage are available to continue up the diode curve, where power dissipation
increases a lot. It depends on the specifics of the circuitry, which could be
figured out if necessary.
This is the situation I had in mind in my post, though written down rather clumsily - incorrectly, actually.
There's also the situation where the -12V is - for a short or long period - between say -6V and -12V. The digilog stuff still (mostly) works but the -12V is off by a lot. Can't think the consequences through ATM.

Raymond


Re: Purpose of Q32 in S-52

Ed Breya
 

Albert,

When the -12.4 V is lost, it's essentially zero V, so the emitters of Q72 and Q74 are effectively grounded through R72, and the long-tail pair cannot provide feedback. Assuming they're Si transistors, the TD voltage can't be high enough to turn Q74 on (unless the actual TD current is very high up its diode curve, and in danger anyway). The -12.4 V emitter feed via R72 makes it able to operate around zero, and without it, it's open-loop.

Ed