Fluke/Phillips PM3394 "Delta-T" repair (Help with circuit interpretation)

[EHT]

Firstly, thank you again GrumpyDoc and everyone else who has contributed on this thread, as it's my board!

I can now confirm that RUNDT/RUNDTN are present and the voltages correspond to those in the schematic "1" to "0" swing

  Excellent news! It lives!

This is all excellent work but I'm having trouble keeping track of the measurements. GrumpyDoc - have you tried checking for short circuits on the opamps & FETs & diode test on V8207 & V8124? This might also reveal if there is a mechanically caused short/open on the PCB. I don't know if this is one of the areas with rework but it all looked very fragile.

I wonder why they changed V8127 from the FET & diode to bipolar? Must be some reason, which perhaps could have been poor reliability somehow? The diode looks like a protection which limits V8127 G to approx 5.5v.

[grumpydoc]

Sorry for the lack of updates but I got diverted by the presence of new toys

Armadillo - sorry didn't quite do the measurements you suggested. I did measure the voltage across R8491 and got the expected 3.08V so the charge current source was running. I then had another look at the voltages around V8124 and V8126 - this time with a DMM rather than the 'scope to get accurate numbers rather than a general impression and they looked a bit odd - in particular the BE voltage on V8124 was too low.

Removed the two transistors and sure enough V8124 is shorted. Replaced them with ones taken from a dead digitiser board and bingo! no more "Delta-T" timeout messages and the trace actually looks OK in equivalent time mode. A bit noisy but I'm hoping that will go away with an autocal so the 'scope has the correct constants to use for time measurements.

Looks like it might be fixed!! 

I'll run it through an autocal tomorrow and see if it passes.

[Armadillo]

Great you done it. Congrates, good to know that you nailed it.

 

All the best tomorrow.

[EHT]

 

Thanks GrumpyDoc and those who contributed to the thread!

I'm so glad it wasn't one of those ASICs!

[grumpydoc]

Thanks GrumpyDoc and those who contributed to the thread!

Glad to have helped - I learned some bits, which is always worthwhile.

I'm so glad it wasn't one of those ASICs!

Indeed.

I am pleased to say that the 'scope passes its autocal procedure with the repaired digitiser card in situ so there shouldn't be any other faults on the board. Oh, I got the two transistors mixed up - it was V8126 which was shorted. Not sure how I got the 8V measurement though across the timing capacitor, I think I mentally applied the 10x for the probe when I had already set the 'scope for a 10x probe attenuation (thus multiplying by 100, not by 10). It lead me astray a bit in looking at the discharge current source first but I got there in the end

To add a few photos to the thread this is what the trace looked like originally when the 'scope was trying to use the equivalent time sampling mode to display a 2MHz sine wave. 



The same input now looks like this     



Here is the sawtooth waveform on the timing capacitor (C8271) measured on another digitiser board. The duration seems to be about 7us so the original pulse must have been about 14ns. Sorry about the wobbly camera work!



Finally I couldn't let the board go without some photos of the re-work that has been done. This was actually done by Phillips and is documented in the service manual but I had not seen it on a board before. In fact I have not seen this degree of rework on any commercial board ever and there is much more of it than shown in just these two shots. There are whole rows of transistors tagged on top of other transistors (a few can be seen above and to the right of the green wire in the first shot.

[tautech]

Good work grumpydoc, how long has this scope tormented you.....a couple of years at least IIRC.

Will it go down the road now you have that 1 GHz LeCroy ?

[The Soulman]

Good news and thanks for reporting back! 

It look likes you use a camera with a 1.8 (ish) lens? 

[grumpydoc]

Good work grumpydoc, how long has this scope tormented you.....a couple of years at least IIRC.

Thankfully it hasn't all been the same 'scope!

I started with a PM3382, quickly decided that I like them as oscilloscopes but wanted a 3394B - which I sort of gradually acquired in bits of mostly non-working ones.

I have 5 working out of 7 purchases which isn't that bad

I really need to sell the spare ones on eBay though.

In fact this repair isn't one of mine - I offered to help another forum member out after he bought one on eBay which  the vendor was convinced had a "small PSU fault". With these things it is *much* easier if you have some working exemplars from which to take notes and measurements.

Will it go down the road now you have that 1 GHz LeCroy ?

Probably not - I usually turn the Phillips on first and most of the time it is all I need. The fans on the LeCroy are a bit loud for my taste but it will be useful when I need something with deep sample memories, high bandwidth and/or the more versatile triggering options. It should be easier to do screen shots on the new 'scope as well so I think the WavePro will see more use than the 9354 did.

What I will definitely now sell is my "spare" 9354 (I bought two for £125 a few years ago) and I really ought to get at least some of the spare PM33xx's calibrated and on eBay.

Good news and thanks for reporting back! 

It look likes you use a camera with a 1.8 (ish) lens? 

I have a couple of lenses - A Tamron 10-24mm and a Canon 18-200mm, I'd quite like something as fast as a f1.8 as neither of the two are better than f3.5 and the workshop¹ does not have much natural daylight so I usually tend to be using ISO1600-3200 and trying to hand hold 1/20th of a second. Image stabilisation helps a lot of course but I get quite a few shots which look like the trace on the LeCroy above.

The shots of the PCB were done with the 18-200 through a x3 macro lens, hence the shallow depth of field.

EDIT:
[1] Well, I say "workshop". In reality it is a bench along one wall of the garage amidst ever higher piles of stuff which I need to either fix or admit I can't and get rid of. As someone said in another thread "hoarding is part of the hobby"