SOLVED - Fluke Combiscope PM3380B Power Supply probs

[QQE06-40]

Hi guys,
about 8 weeks ago i found a Fluke PM3380B (2ch 100MHz Combiscope) in the dumpster. The front panel was a bit

damaged (e.g. the mains switch and it´s associated mechanics) but it was operational on channel 1, but had

some problems with ch2. It took me some time to repair the front panel damages, but couldn`t find the reason

for the problems with ch2. Didn`t mind about that, because i rarely need a second channel. So far the good

story.

6 weeks later i found another PM3380B in the dumpster. The case and the plastics from the chassis were

severely broken and in a horrible condition, but all the electronics and even the crt seemed to be ok.

Putting it on the mains the power supply  gave a loud squeeling sound. No other sign of life. This led me to the decision to use

this one as a donor unit and try to fix the problems on the first scope.

First things i wanted to replace were the mains switch, the front panel and the input board. I opened the

case and for what reason ever i connected the good scope to the mains and switched it on. Guess what: the

same squeeling sound from the power supply board! That happened before changing anything! I disconnected

every board one after the other, but the sound appeared every time i switch the power on. So i´m pretty sure i

have at least one defective power supply board. I disconnected the high voltage tripler with no change. On

first and second sight i can`t find any bad components on the board and without an expander board it is

rather difficult to measure.

I would very much appreciate any help from You. Is there anybody who has experiences with these scopes and

could give me some hints? Can i test the power supply board on the bench disconnected from the rest of

the cicuit (obviously leaving the tripler connection open...)?

Thanks in advance for any help!

Wolf

[tautech]

Welcome to the forum.

So we are all on the same page, here's a link to the Service manual:
http://elektrotanya.com/fluke_pm3370b_pm3380b_pm3390b_pm3384b_pm3394b_oscilloscope.pdf/download.html
50.5 MB

Wait until "processing" changes to "Get Manual".

[QQE06-40]

Thank You tautech for the link, but i already have the service manual with the schematics. They do help a lot when testing the board out of the unit. Chapter 8.8 describes some maintenance routines for the power supply, but several dummy load resistors are needed. And i have my doubts this procedure will bring me any further. Anyway, are there any known components that tend to fail on these boards?
Greetz,
Wolf

[HighVoltage]

I have many of these scopes and even had many more over the years and they are great scopes, even based on today's standard. They are pretty stable but once in a while they fail. The high pitch noise from the power supply is one of these failures. The problem is usually found with a cold spray for the components around the large transformer. Usually a failure of a resistor and transistor combination.

I have not been successful in getting the board to run on the bench and I do not have an extender either. But probing around with the board installed eventually led to the problem component and a solution.

At one time I put the full scope in a freezer and had it cooled down and the power supply worked again. This gave me time to analyze the test points and finally isolate the problem.

If you ever find an extender in your dumpster, let me know.

[grumpydoc]

If you ever find an extender in your dumpster, let me know.

I had some luck with stacked  DIN 41612 connectors for the digital card. You could probably do the same with the PSU or CPU card (watch the fact that the latter uses all three rows of pins).

[QQE06-40]

What a big fool i am!!  Thinking before acting!
Spent hours to find the fault in the power supply. With two boards on hand i thought it would be a good idea to make some comparisons. The second board from the donor unit was in a worse condition than the original. Some areas of the pcb were looking pretty toasted, they obviously suffered from too much heat. Reworked some bad solder joints, found an electrolytic cap with loss of capacitance and high ESR. Replaced but with no success. Then i stopped acting and begun thinking. Not having an extender board i measured the resistance between ground and the +5V line on the connector board without all the other boards. Viola: short circuit. Looking for the reason for that short circuit i remembered that i replaced the signal board before switching the scope on. Ahhh, the screw in the middle of the board fixing it to the chassis was too long and reached the +5V-rail on the bottom of the connector board.
Yes, you are right, i am an idiot! Nevertheless thanks to everyone who tried to help me!
@HighVoltage: if i ever find an extender board in the dumpster i will surely let you know