Date   
Re: 7904 problem

Craig Sawyers <c.sawyers@...>
 

At power up, there is a very bright unfocused spot on the screen with absolutely no reaction at
any
front panel command.
I only saw this once on my 7904. I had set up a sampling set up (7T11, 7S11 and either an S4 or
S6). Fortunately I was watching the screen at the time, when suddenly bam! A maximum brightness
spot bang in the middle of the screen. I hit the power switch FAST.

After trying other plugins, and confirming that it wasn't the 7904, I eventually found a shorted
tubular tantalum (it was a small black one with a taper on one end) in the sampling head. I forget
which line it was taking down, but whichever it was failed to trip the protect circuits in the power
supply.

Craig

Re: 7904 problem

 

Hello Pascal,

It is always wise to read the Theory of Operation in the Service Manual
before trying to troubleshoot a Tek product. It sounds like you have done
that.

Next I would check all the power supply voltages against what the manual
says they should be.

I would pay particular attention to the beam (Z-Axis) blanking and to the
HV.

Two things you can do without any test equipment are
1) Try turning off the lights in the room so it is completely dark. Then
press the beam finder to see if you see any sign of brightness on the CRT.
2) If there is none then try to see inside the neck of the CRT back by the
socket to determine if the filament is glowing. If there is no filament glow
then the CRT might be bad.

Assuming that you will eventually get this scope working I would like to
suggest you buy a timebase that is a better match to the 500+MHz capability
of your 7904. The 7B92A would be ideal. The 7B71 is a very old plugin and
not great at triggering or capable of triggering at the higher frequencies
the 7904 can display.

Dennis Tillman W7PF

-----Original Message-----
Sent: Friday, September 16, 2016 10:52 AM
Subject: Re: [TekScopes] 7904 problem

The intensity knob have no effect on brightness.

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Posted by: fcbtt3.nina@...
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Re: 7854 ROMS

bobh@joba.com
 

I am not sure about this and I am interested myself. I know some 7854 bin files are available on TekWiki.
Bob.


From: mailto:TekScopes@...
Sent: Thursday, September 15, 2016 3:49 PM
To: TekScopes@...
Subject: [TekScopes] 7854 ROMS


Does anyone know the highest revision level of the ROMS in a 7854?

[Non-text portions of this message have been removed]

Re: FBSOA or RBSOA?

Dave Wise
 

An update. I discovered another criterion. Quasi-saturation. Since the part is employed as a non-saturating switch, if the on voltage and current are in the quasi-saturation region, the part will be too slow. The only place to check this beforehand in the datasheet is the common emitter output characteristics, the Ic vs Vce family for a variety of base currents.

A good example is the 2SC4614. At 300mA, you have to push Vce over 5V before the transistor moves out of quasi-saturation region into the active region. At the normal on voltage of 1.5V, storage time is over half a microsecond. This will be true for most parts rated over 100V. So I have worry not only about my basement, but my ceiling!

Dave Wise
________________________________________

Re: Hello from newcomer Fabio Trevisan - My first Tek Scope 464 + DM44

Fabio Trevisan
 

Hi David, Just to give a feedback:
It's a long message and OF COURSE don't feel obliged to either to read it or answer... telling it to someone is my way to organize the ideas and - maybe - the outcome can be disclosed later and help someone.


On the DM44 issues, I installed the missing R1142, R1143 and R1147 on the A&B Timing Switch board, and now it can measure time on all horizontal scales.


Oh... And there's a TEK trick there...


On the 464 service manual "A&B Timing Switch Diagram 7", it shows the 3 resistors above as 2K, 1K and 1K respectively... and I naively installed them exactly as such... just to find out that the measurements were wrong in scale. (but at least, this time it was measuring in all horizontal time-bases, and not just on the 2s multiples).
Went to the Parts list of the 464, and it stated: Check DM44 manual for Resistor Values...!!!
Then, went to DM44 manual and, next to the DM44's parts list, there's an additional section describing all the values of all components that need to be added, removed, or changed values, for each scope model that the DM44 fits in...
Guess what, the correct values are 500R, 250R and 250R..!!! And not 2k, 1k and 1k as it was drawn on 464's diagram.


DM 44 is still sometimes crazy (less now than it was at the beginning), but I assume it's some bad contact somewhere which I will eventually find out.


Now, regarding the HV supply... I did some progress in making various checks but still didn't find the root cause for the drop of HV after the scope heats-up...


1. When the HV drops, cathode voltage drops to around 1000V, and then, in 20~30 seconds, it drops to 920V. when I shut it off.


2. Checked the Unregulated +15 V and it reads about 24V (just a tad higher than what shows in the schematics). The ripple is about 1Vpp while in normal operation and, after the HV drops, the ripple increased to almost 2Vpp until when I shut it off...
2.b. Still at the Unreg +15V, as the manual doesn't state how much is the acceptable ripple, I did the following... I added a brand new 6800uF cap in parallel (to the original which is nominal 5500uF +100% -10%)... and the ripple under normal condition dropped from 1Vpp to 0.5Vpp... I didn't care to look after the HV drops because it doesn't add anything to the conclusion.
So, IMHO, the original cap is performing roughly the same as the brand new 6800uF cap... In other words... C1487 is GOOD.


3. I don't have a temperature probe, but using my finger (with the scope off, and after discharging!), and while the HV is still normal... both the driving transistor and the HV transformer are hot, but not too hot... I can keep my finger on them, so they're about or below 70 Deg Celsius.
But when HV drops and I leave it on for about 20~30 secs which is as far as I went, both the transistor and the HV transformer becomes VERY Hot, to the point that I cannot keep my finger on one of the pins from the HV transformer (I can keep the finger on its plastic body though)


4. I don't have a reference stated on the manual, but the Post Acceleration anode is - while in normal operation - at 7.22kV. I didn't measure it yet after HV is down. I suppose that it will be down by the same amount (33% below the 7.22kV)...


4b. I measured the 5 resistors of 3M Ohms in the HV multiplier and they're all up by 10%, like 3.3M each.
It's not ideal, but I don't think it's the cause for the cathode HV drop, which is the main issue.


4c. I managed to find some ESJA53-12 diodes, which are 12KV, 5mA fast recovery diodes (actually better than the originals, at 100ns, while the originals are 250ns, only the forward voltage is worse, at 37V while the originals are about 25V and this doesn't seem to be a problem here).
I already tested them reverse biased at the first stage of the HV multiplier (3.61kV) and they're quite promising at 1.1nA (an indirect measure... I actually measured how much voltage it allowed to leak to a 10Mohm voltmeter, and it read 11mV!!!)
I will try to replace - first only CR1503 at the cathode - and see if it helps.
Specially this one, which is mounted right under the transformer and soldered very closely to its pins, if it's leaking too much, as it gets heated by the transformer, it may be causing the whole mess.


If replacing the diode doesn't help, however, I'm afraid I will need to start looking for an HV transformer, as I`m not seeing much more that can be ruled out...
The HV caps seem to be in rather good condition, and they're HUGE (I imagine they're very robust)...


Rgrds,


Fabio

Re: Tek 2465 attenuator rebuild

Siggi
 

On Fri, 16 Sep 2016 at 14:13 dkolbach@... [TekScopes] <
TekScopes@...> wrote:



I will give it a try, I have not even open it. My scope will not calibrate
the second channel, I was advised that the attenuator needed to be replaced
as there is no way to fix them.
What seems to be the problem? When you "will not calibrate" what does that
mean?
Note that you can diagnose some attenuator problems with a DMM, by simply
measuring the resistance from the center of the BNC connector to the
shield, as you run the vertical gain through the range. The resistance
should stay smack-on 1MOhm throughout.
When you then click in the 50Ohm termination, you should get near as 50Ohm
as makes no difference.

Re: 7904 problem

 

The high voltage side of the anode connector is fully enclosed so
assuming that the CRT is discharged, it is safe to leave disconnected.
I might put some tape over the end to keep contamination from getting
inside.

The CRT side can be discharged by touching the exposed metal pin to
the chassis.

On Sat, 17 Sep 2016 00:09:09 +0200, you wrote:

What I did was to disconnect the HV cable to the CRT. With extreme care and
thick rubber gloves.

Göran

Re: 7904 problem

Göran Krusell
 

What I did was to disconnect the HV cable to the CRT. With extreme care and
thick rubber gloves.
Göran


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Re: 7904 problem

 

The +130 volt supply comes directly from the off-line power supply
which uses it for regulation and I see no fuse in the schematics
although there is a 10 ohm series resistor before the CRT circuits. If
the +130 volt output was completely missing, then the switching power
supply would shut down.

On 16 Sep 2016 13:22:58 -0700, you wrote:

From the discussion so far, it seems to me that the +130V supply may be gone. As I recall, there's a small fuse protecting it on the power supply regulator board. If plug-ins are installed and timebase set up for a proper free-running sweep, that should spread the beam out to a workable non-damaging level. If the sweep doesn't happen on-screen even though it should, then the 130V is probably out. In that case, I'd take David's suggestion and unplug the CRT socket or the anode connection to protect the CRT while servicing.

Ed

Re: 7904 problem

simoniep
 

With CRT disconnected, a tic tic is now audible and 2 neon lamps flash on the Z axis board.

Re: 7904 problem

simoniep
 

No action possible on the spot with vertical and horizontal controls.

Re: 7904 problem

David DiGiacomo
 

On Fri, Sep 16, 2016 at 2:52 PM, <fcbtt3.nina@...> wrote:

It seems to be very easy to disconnect the CRT. 2 plugs on the Z axis board and 1 plug on the +21KV lead.
It's better to leave the CRT anode lead connected.

You didn't mention whether the horizontal and vertical position
controls allow you to move the spot.

If they do, you have the common HV DC restorer failure.

If not, you probably have that plus some other problem.

Re: 7904 problem

simoniep
 

It seems to be very easy to disconnect the CRT. 2 plugs on the Z axis board and 1 plug on the +21KV lead.

Re: 7904 problem

Ed Breya
 

From the discussion so far, it seems to me that the +130V supply may be gone. As I recall, there's a small fuse protecting it on the power supply regulator board. If plug-ins are installed and timebase set up for a proper free-running sweep, that should spread the beam out to a workable non-damaging level. If the sweep doesn't happen on-screen even though it should, then the 130V is probably out. In that case, I'd take David's suggestion and unplug the CRT socket or the anode connection to protect the CRT while servicing.

Ed

Re: 7904 problem

Fabio Trevisan
 

Hello Pascal,
I don't know the innards of the 7xxx mainframe to know if what I'm about to
suggest is easy or not to achieve...
In order to protect the CRT and at the same time not changing whatever else
where you will want to troubleshoot, the easiest way is to disconnect the
heater.
Just make sure you disconnect both pins because in many CRT circuits,
usually one of the sides of the heater is also connected to the cathode and
then I'm afraid that cathode voltage may find its way through the heater
into something else. (As I speak I don't even know if what I'm saying makes
any sense, but it's better to be safe then sorry.

With no heater, there's no emission and therefore no damage to the tube.
Since there won't be any HV current flowing, I think that it's expected
that the Post acceleration anode may be higher than spec'd, since it's non
regulated (the highest voltage which, in the case of the 7xxx series is
probably around 16kV) but I don't think that it will cause any damage and
it will not affect any other voltage, since the cathode voltage is
regulated and no matter if current is flowing or not it should maintain the
voltage according to the regulation circuit.
This will allow you to check all the CRT biasing with peace of mind that
you aren't burning your CRT.
Brgrds,
Fabio

On Sep 16, 2016 4:38 PM, "HankC @HankC [TekScopes]" <
TekScopes@...> wrote:



Can you de-focus it ?
HankC, Boston WA1HOS

[Non-text portions of this message have been removed]


Re: 7904 problem

simoniep
 

3 solutions:1 disconnecting the CRT, 2 disconnecting P1710, 3 desoldering R1603 and/or R1887.

I try to deternine the more convenient

Re: 7904 problem

simoniep
 

No, no action possible of the spot.

Re: 7904 problem

 

That is weird that the focus control has no effect. Based on the
schematic, it looks like that could happen if some combination of the
-50 and -130 volt low voltage power supply outputs are missing.

Instead of disabling the high voltages, what about disconnecting the
CRT socket?

On 16 Sep 2016 11:37:44 -0700, you wrote:

No effect of focus control.

I am searching on the schematic how to stop HV degeration. Seems to be difficult because HV and LV are generated thought the same transformer (T1310).

Re: 7CT1N cap selection

 

Definitely class B with the emitters and bases tied together. I have
no idea why Tektronix thought using 25 volt capacitors at 30 volts was
a good or even acceptable idea.

On Fri, 16 Sep 2016 12:52:47 -0500, you wrote:

Hi,

Just received a very nice looking 7CT1N curve tracer module and find it is
in fairly good working order except for some scratchy pots and sticky
switched. An ERC check of the aluminum caps found C32 with a cold
resistance of 10 ohms. Looking at the schematic and the cap shows it to be
a 100uf 25V cap. It is used to decouple a transistor collector that is
driver by a 30V line. I do not know what class of operation is used but it
seems likely to me that both C30 and C32 will see a lot of overvoltage.
Perhaps a 35 or 50V cap would be safer. I have ordered 100uf/35V caps for
replacement.

Jerry

Re: 7904 problem

 

Can you de-focus it ?
 HankC, Boston WA1HOS

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