Repair of a Fluke PM3394B

[Norbert67]

Hello
Recently I bought a used PM3394B unit which is fully functional the only problem is with it that I cannot focus accurately the trace and the text. Its blurry on the digital channels too. Tried to measure the components close to grids and final Z amp but they look OK. Unfortunately I cant measure the high voltages because I dont own high voltage probes or test leads. But I think the problem wont be there, since the intensity and focus works and the 3kV ceramic cap are OK -  I measure them on RLC.

Any suggestion are welcome.

If I´ll put together the final amp board I upload some pictures.

[BreakingOhmsLaw]

The High voltages are derived from

Unfortunately I cant measure the high voltages because I dont own high voltage probes or test leads. But I think the problem wont be there, since the intensity and focus works and the 3kV ceramic cap are OK

The list of things that can cause focus problems is very short and all of them relate to voltages.
Start by measuring the 58V supply for the EHT converter. Then check the voltage on test point MV11, you should see around 200V peak-to-peak. I am assuming you have an oscilloscope available.

. If it is not ok, common faults are:
 - V1103 & 1104 dead
 - C1112 leaky/dull (remove and check)

You can get old school DC HT probes for around 30$ on ebay. The ones that look like a parrying dagger. These usually require a multimeter that has exactly 10MOhm internal resistance as they use the meter as the dividing resistor.
If you have bad luck, the BG2000 voltage multiplier on EHT board is dead. These have become increasingly hard to find in the right pin configuration.

Another (albeit theoretical) fault is a too high filament voltage heater voltage. Should be 6,3V. When it's too high, the EHT can collapse as it not able to provide the increased current caused by a too hot filament. This is very unlikely in this unit, as the filament voltage is tapped from the EHT transformer.

Pictures of the screen would be helpful.

[HighVoltage]

On these PM3394B it is possible that someone changed the calibration settings.
In the maintenance menu, behind the pass-code wall, there are settings for focus on special areas on the screen.

[EHT]

On mine it had similar symptoms due to the high voltage connector on the tube being slightly loose. Reseat it and make sure the skirt seals well. It looked and felt fine because the skirt was in place, but the conductor was not mated properly. I guess the previous owner had interfered with it and not put it back together properly.
Correcting this plus calibrating it sorted it out.

[Norbert67]

Thanks for all the useful suggestions.
Yes  I have a second oscilloscope PM3092.
I measured V1103 and V1104. they ok. C1112 is in order too. But I´ve found  a 27uf/100V which had a D factor, the phase degree and ESR slighty off -  I will change that next week when I get new components.
The EHT converters supply is 60V, so thats ok too.
The voltage at MV11 is precisely at 200Vpp. The manual states that its sinusoidal, but on the screen there are some kinks in the signal. I upload a picture.

[Norbert67]

I uploaded some pictures about the problematic oscilloscope and the same signal on my 2nd scope.

[Norbert67]

Another thing I noticed when measuring the smd components is this transistor V1101. Originally is supposed to be like this?Because the schematic doesnt show any resistor connected in series with the collector to the supply.

[EHT]

The manual states that its sinusoidal, but on the screen there are some kinks in the signal. I upload a picture.

Actually in the circuit description on p 5.6-3 it does show more detail and why there is a kink there, but nonetheless yours isn't quite as shown

Originally is supposed to be like this?

Mine has many such hardware mods.

As you're seeing 200V AC at point MV11 then I suppose it must be on the high voltage side. I presume you exhausted the easy option of reseating the lead/cap and recalibrating and I suppose you can't see or hear any arcing, so aside from the multiplier there are only a few other components that it could be.

However, I'm not sure how the focus control is implemented. Perhaps possible problem there?

[BreakingOhmsLaw]

From what I see from the pictures, the anode high voltage is okay. You can tell because this will "zoom" or unzoom the picture. But it's more or less identical to your other scope, so I'm going to go ahead and say the EHT is okay. That leaves the focus voltage.

As a next step, please check if connecting a signal to the Z-Input will blank the signal completely. Hook up a signal gen and feed a square wave signal into it. Low level 0V, peak 2.4V.
The trace should blank completely during 2.4V. This will tell us if the Focus/Z-blank circuit is not completely dead. Also, does the focus change at all when you adjust it?

[Norbert67]

Sorry for the late reply.
Yes, I tried to reseat the anode cap. And after reading your answer I found the diagram in the manual, and You have right, the EHT is OK I guess.

To answer Your question, the focus is working in order, just the end result is not sharp enough. On my second Fluke there is margin around 1 whole rotation of the encoder when picture sharp and the sharpness is not getting worse. At this even if I set the best I can its still blurry (but when I turn it to left or right it starts to distort the picture vertically or horizontally- so its working in order, just to me it looks like there´s a noise which is coupled to the final amp, which distorts the picture-but thats just a hunch).

So my the second guess would be the TEXT signal path, or the final Y amp.
From the digital board the signal coming out is OK. I will measure the continuing path to thefinal amp too.

[HighVoltage]

From what you write, this should all be adjustable in the maintenance menu.
You can set the sharpness for different areas of the screen.
I had a problem like this before on a PM3394B and it was not a hardware problem.

[Norbert67]

Yes, You´re right.
The problem was the misalignment of the astig parameter in screen calibration menu.
In my previous attempt to calibrate, this parameter was misaligned by so much that changing the value just a bit I didnt notice any change. I needed turn all the way down to zero. I was negligent because the seal for calibration was in place, I wouldn´t thought that any value would be that far away from the optimum.
The trace is still a bit thicker than that of my other scope´s, but this is a minor problem and I can live with that.
Thank You all for guiding me to the solution!

[HighVoltage]

I am glad you solved the problem.

[shakalnokturn]

but when I turn it to left or right it starts to distort the picture vertically or horizontally

That was a big pointer towards astigmatism!

[Norbert67]

Well, it looks like I´ve got another problem on my hands.
I noticed before, that in DIGITAL mode the 1st and 3rd channel was offsetted down almost 1 division but after 20-30 minutes went back to normal. But now it starts from the center and slowly fall down until it disappears from the display. The autocal no longer completes its procedure.
Here is what I´ve done:
All the tests passed.
Took out the digital board and measured the electr. caps.
Took out the dsp and cleaned the pins of the IC and the socket.
The N8001 and N8002´s heatsink was bent and the thermal paste was hard. So I replaced the paste and reseated the cooler to fit the surface of the IC.
Unfortunately, non of these fixed the issue.

[HighVoltage]

This is a hardware problem with a broken hybrid chip made by Philips that is no longer available.
It always effects CH1/CH3 or CH2/CH4

[Norbert67]

Strangely enough, because I couldn´t get the autocal to complete its task, I switched off the unit that day.
The second day I switched on and now the autocal is working and I get a about half the division offset out cold, after 5 minutes its quarter of 1 division and when its warm (around 30 minutes) its spot on.
I did noticed how temperature compensated the unit is. I would have never imagined that it only takes a 1 or 2 blows of air to the digital board to offset the channels. 

[HighVoltage]

You are very lucky then!

[Norbert67]

Yes I am indeed.There are days that the trace is a bit off but the autocal is working so not a big problem.
Interesting that the 33xx series dont have the comp trigger function that the previous 30xx series had.

[sarahMCML]

Hi,

I have the same problem as Norbert67 had, on channels 1/3, except my PM3394B hasn't recovered.
It fails Self Calibration after about a minute, and comes up with a "Calibration 101" error message at the bottom of the screen.
I have 2 questions for the experts here:
1) Did Philips/Fluke ever produce a separate unit Fault Finding manual.
2) Which chip produces the fault, because it's not too obvious to me going through the maintenance manual, probing the board hasn't revealed anything, and the in-built test routines of the DAC's that I've looked at seem to be fine!

Many thanks,
Sarah.