From memory of that far back.

Those Tantalex caps, I believe are "Wet" tantalum types. Personally, I've not known one to fail, unless grossly over-voltaged!? But there is always a first time.

The black extenders to the push switches can usually be gently persuaded off the switches themselves with a little persuasion from a flat screwdriver blade to spread the gap in the black part, then pulling away from the switch.? Be gentle, at their age, the black plastic might have become brittle.

"Have Fun"...

??? Dave ex Tek UK of that era.

--
Created on and sent from a Unix like PC running and using free and open source software:

Dear Chuck,
Thank you very much for your support.
After spending hours trying to figure out why the scope had all sort of "strange behaviors and displays", I decided to replace the HV board.
I have one spare since there are unobtainium components there.
After replacing the HV board the dots disappeared as well as the "strange behaviors and displays".

Can you explain why a high grid bias interfere with the workings of the A5 board?

I could conclude CAL08, normally, but the top message line is positioned almost out of the CRT.
What is causing that?

I noticed that I have to checked the high volatges after replacing the boards; maybe that is causing the proble.
I will search the service manual for the djustments, in case there are some.

Regards,
Roger

Hi all,
(please note supporting pictures for the below at?/g/TekScopes2/album?id=278771)

I am starting work on this SG502, so its condition is virtually unknown, but during the visual inspection, I noticed the right leg (as per picture) of C160 is detached. Measuring it returns pretty high (over 450uF) and a good amount of ESR, so this being in signal path, I decided to replace it with a high quality, low-ESR cap. More on that specifically at #2 below.?

A couple of issue with that.

• To access the trace side of this (namely the pads to unsolder this cap), I need to remove the long fingers of the MULTIPLIER switches (see pics). Can anyone please remind me (I have a feeling I did this before but can't think of how I did it) how to take apart those switches? Namely, remove the extending part so I can properly solder a cap there.?
• The cap itself has markings indicating it may be a tantalum, but is that really so? A 350uF/15V value seems really high for that, and aside from that, it rather looks like a bipolar electrolytic. Thoughts on a correct replacement?

Thank you,
Radu.?

Radu Bogdan Dicher <vondicher@...> added the album SG 502 Repair

There are a few variants of the power supply, some have less pins on one of the output connectors (So each power connector is the same size) depending on scope model, and there are other versions with differences in the standby power part of the supply, among other things.

My tester allows you to check voltages, test the on/off function and it will apply a small load to each voltage rail to check that they don't immediately collapse when turned on.
IIRC, a few of the voltage rails are monitored for current draw so if you don't apply a load, the PSU won't work properly (hence my tester)

The C17 capacitor replacement is mandatory (if fitted, some variants don't have it) as they are almost always high ESR, so check that and replace with a new good quality low ESR cap.

Rogerio,

So, are you saying that when you adjust the GRID BIAS pot on the
HV board, you cannot make the spot go away? If that is what you
are saying, start there.

As to the voltage on pin 7 of U1956B, OPAMPS are about one of the
simplest components in existence to test... right up there with
diodes.

"B" is just a non inverting buffer, so, whatever voltage is on
the non-inverting input (pin 5) should appear exactly on the
output (pin 7). If it does, that stage is probably good.

The combination of "B" and "A" creates an amplifier with a gain
of -1/5th. (Av = -Rin x Rfeedback) Rin is 10K (R1944), and
Rfeedback is 2K (R1971), and the output is the (+1.4V) bubble.

Probe around, the math is easy.

Don't get fussed about probing around on the HV board. The only
voltage there that would raise your eyebrows if you contacted it
accidentally with your finger is the -300V. The anode voltage is
well contained in the tripler module, and is not dangerous anyway.
I have been zapped numerous times over my life by such supplies,
and it is like getting zapped on a doorknob from walking across a
carpet on a dry winter's day. Surprising, but thus far not lethal.

The danger is in the current and the Joules of energy available on
the board, not the voltage. There are no big storage capacitors
in the HV section to make up the dangerous 10 or so Joules needed
to kill a human, and the sustainable current is only around a
milliamp.

The only thing HV does is reach out and touch you if you get too
close. Without the requisite current, or joules, it is just
annoying.

The board and your test equipment, has way more to worry about
from your probe slips than you do about the board biting you.

-Chuck Harris

On Sat, 24 Sep 2022 03:57:29 -0700 "Rogerio O" <rodd414@...>
wrote:

Thank you for your reply Chuck.
I will check the A5 board, although, as I said it has NOT any SMD
capacitor. Also, would you please comment on the voltage at U1956B?
I guess now the scope has two problems: one in A5 board and one in HV
board. ***quote
But when I was checking the voltages in the HV board to try to
understand why I could not lower the brightness of the spot, I
noticed the volatge on pin1 of U1956B was -0.87V. According to the
schematics, it should read +1.4V.

Thank you for your reply Chuck.
I will check the A5 board, although, as I said it has NOT any SMD capacitor.
Also, would you please comment on the voltage at U1956B?
I guess now the scope has two problems: one in A5 board and one in HV board.
***quote
But when I was checking the voltages in the HV board to try to understand why I could not lower the brightness of the spot, I noticed the volatge on pin1 of U1956B was -0.87V.
According to the schematics, it should read +1.4V.

The 2465B is entirely fly by wire... That is, the controls on the
front panel are read by an ADC, processed by the CPU on the A5 board,
and used to program a sample and hold gate with the DC value that
is used to control the <<whatever>>.

It is not uncommon for the sample and hold gate to lose an opamp,
or a hold capacitor, and freeze a function. All of that stuff is on
the A5 board. If the A5 board hasn't had its SMD electrolytic
capacitors replaced, it can cause lots of strange problems.

You can easily tell if the capacitors need replacing, look at their
solder joints. If they are shiny, they are probably fine. If they
look milky white, they are bad. If you touch a soldering iron to
the joint, and it smells like a cross between rotting fish and a
radiator boiling over, they are bad.

-Chuck Harris


On Fri, 23 Sep 2022 12:49:11 -0700 "Rogerio O" <rodd414@...>
wrote:

Another observation:
The scope is capable of displaying a signal that is aplied to any of
the channels. The gain and sweep time seems to be working.
However, I can not change the position of teh waveform that is
displayed. Is this an indication that the problem is in the A5 board?
Roger

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Hi,

you can check video of Jared repairing such power supply. As he mentioned TDS500/600/700 have similar power supplies. It also includes information about external dummy load construction with all needed information.

-vf-

Another observation:
The scope is capable of displaying a signal that is aplied to any of the channels.
The gain and sweep time seems to be working.
However, I can not change the position of teh waveform that is displayed.
Is this an indication that the problem is in the A5 board?
Roger

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On the subject of absorbing moisture, only in relation to CTVs, my experience of Grundig colour TVs from the early all solid state era was thus.

The line output transformers NEVER failed, but the (so called) triplers did.I originally lived in the "Potteries" area (Stoke on Trent) in the UK.

Every August, the whole industry shut down for two weeks, many people locking their homes and holidaying elsewhere. On the Monday after they returned to work, I could be certain of at least two, sometimes six phone calls saying TV is dead! I always had a stock of triplers at that time. Obviously moisture absorbtion. Summer time, high humidity, a very dry component left to absorb, and it did.

Later sets moved to "Diode split line output transformers", both parts combined in one unit.

They also failed, but being a sealed unit, I don't know if windings or capacitors/diodes.

My only suggestion is to either replace, or first get somebody with a known good one to measure their resitances.

Les.

Les,
Excellent!? You are right .? My use of? the term "Short" is not the best choice of words with regards to this symptom.? Since the winding has higher resistance than expected, it certainly is not a "short", in the conventional sense.?

In regards to the "ringing" test, I read a comment or discussion about that subject some time ago in another discussion.? I have never done such a test, but it sounds simple enough.?

Moisture absorption is certainly a possibility.? The coil appears to be molded and encapsulated with a black epoxy or plastic material.? Not saying it cannot have been affected by moisture but it is a later design and one that they seem to have reduced moisture absorption.? Scope is stored and operated in a low humidity and controlled environment.?

This one is not one of those infamous "brown epoxy" molded transformers, like those in the early Type 576, That being said, some other failure of the molded epoxy encapsulation could be possible.? Whatever the failure was, it was sudden and total, not like those old brown Type 576 transformers that would run for a few minutes and then fail.? This instrument was running fine one day and totally blank the next time it was turned on. ??

I would suppose that there must be a failure in the insulation, arc over or carbon track that is causing the symptoms noted? ?

Appreciate your comment and advice. ?? ??

Thank you for your reply Chuck.

As I have mentioned in my post, all the voltages present in J119 are spot on, according to table 5-1 and my A5 board does not have the smd leaky capacitors.

But when I was checking the voltages in the HV board to try to understand why I could not lower the brightness of the spot, I noticed the volatge on pin1 of U1956B was -0.87V.
According to the schematics, it should read +1.4V.

Unfortunately the components on this board are soldered, so I would not like to mess around unless I have to.
Do you think the IC is damaged?

By the way, the reason I was performing CAL08 was to try to fix a display issue.
There are four horizontal lines of dots showing on the screen.
I will upload a photo of what was happening before.?
/g/TekScopes2/album?id=278726

Rogerio O <rodd414@...> added the album 2465B strange display: The horizontal dotted liea are not affected by INTENSITY, but the do respond to REDOUT INTENSITY

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You think you have a shorted turn, yet also think resistance is too high. Unless serious internal burning, the two suggestions are incompatible.

I don't know this particular transformer, but HV transformers can easily absorb moisture if left off for a while, followed by their immediate demise.

The only test for a shorted transformer I know of involves "Ringing" it and observing the result on a scope.

I did this a few times back in my CTV repair days. I used a little multivibrator to create the pulses, and the difference between a good and bad transformer was immediately obvious.

Read up on the subject and try it.

Les.

There is no visible sign of damage.? But there was only about 31VDC at the HV Test point.? This power supply is buried and not at all easy to test.? I checked every component on the board and found 2 suspect electrolytic caps.? Everything else looked and tested good, with the exception of the Transformer.? I'm thinking that 1200 turns of 37 gauge must have some considerable resistance. I do not know how many feet of wire 1200 turns is, but 100 feet would have about 55 ohms.? So this winding seems to already have too much resistance. 250 Ohms would require almost 500 feet of wire @ 532.1 m Ohm per foot.? Seems like too much resistance.? A direct comparison can be made with the 600V secondary winding that is 37 gauge wire, only 222 Turns and it shows about 23.7 Ohms of resistance.? Using that winding as a guide, I would say that based on that 600V winding the 1200 turn winding should have about 128 ~ 150 Ohms end to end.? Thoughts?? ?? Many Thanks for the replies and the opinions, they are appreciated.? Trying to learn something here. ?
? ?

The power supply in my TDS684A has decided to go into hiccup mode. Researching the topic, I keep finding schematics and advice only for the TDS520 and similar. Does anyone have or know where I can find a schematic for the actual TDS684A power supply? Or for that matter, experience of the most common failure modes/device failures? And, it seems folks are capable of running these supplies stand alone on the bench - which connector/pins need to be shorted/or grounded as would be provided by the front panel power button to get the supply to turn on?

Thanks!
Reinhard Metz
n49ex@...

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Thinking about how this could happen, is there a short in the winding, and that would be between the center tap (beginning to centertap ok) and the end of the winding?

Harvey