Author Topic: Fluke 8800A/AF (power supply fixed - now logic issues)  (Read 644 times)

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Online GregDunn

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Fluke 8800A/AF (power supply fixed - now logic issues)
« on: December 15, 2018, 04:20:32 pm »
Yes, I snagged another 8800A.  I figured for $20 worst case I'd have some extra parts lying around - but the first goal is to try to resuscitate it.  The unit is very clean inside and out - no cosmetic damage or broken pieces.  It was listed as "not working" and indeed when plugged in, nothing happened.  Line fuse was OK (checked it first) so likely nothing dead shorted in the PSU.  Inside, nothing looked obviously damaged, though clearly someone had been in and replaced filter caps. I don't like the age of them, so they will be getting swapped anyway.

All the PSU voltages are very low, but the pre-regulator voltage levels are fine - so the caps are probably working for now, but I suspect U17/U18 may be toasted.  I'm going to start running through the troubleshooting steps tomorrow since I have some free time.

But as I usually do, I'm taking a little time to check out possible replacement parts.  There's nothing too crazy in this unit except for the two voltage reference devices.  One is CR37, a MPD200 stabistor.  Replacements for these seem to be very hard to locate, and even the recommended cross-reference devices don't show up in any of the usual places (best options seem to be a 1N4156 or CMXSTB200, essentially a high precision dual-diode).  There's also U9, some kind of stable voltage reference, which is not listed except in a Fluke part number matched set.   >:(  It is shown in the schematic as an NPN transistor in series with a zener.

If anyone has a clue what might be readily available replacements for these, I'd be interested.  Hopefully the problem is just one of the regulator or op amp chips, but since I have to order any or all of them regardless, I might as well get everything the first time, right?
« Last Edit: December 16, 2018, 06:06:24 pm by GregDunn »
 

Offline coromonadalix

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Re: Fluke 8800A/AF power supply parts
« Reply #1 on: December 16, 2018, 12:59:33 am »
I would remove or at least check the changed capacitors for any leaks / imdedance  ...  if one of them is/was backward installed ??

Remove them and check impedances on the board again ..

You have a thread here :
https://www.eevblog.com/forum/repair/fluke-8800a-repair/

Maybe you've seen it ?
 

Offline Kleinstein

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Re: Fluke 8800A/AF power supply parts
« Reply #2 on: December 16, 2018, 02:45:39 am »
The special reference made out of an zener and NPN could be a rather good one, like Motorola SZA263 / LTFLU or similar. They are hard to get and usually rather expensive. It's possibly the most valuable part of the DMM and likely still OK. The other stabilizers should be much less critical.

For such a repair one should not be in a hurry - its more like measure first and only than buy replacement parts. The only exception can be large electrolytic caps or some tantalum caps.
 

Online GregDunn

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Re: Fluke 8800A/AF power supply parts
« Reply #3 on: December 16, 2018, 05:27:12 am »
Thanks, I actually hadn't seen that other thread - some useful info there which may apply to mine when I get a little further along.

TP7 = 5.5V;  TP8 = 6-7V;  TP9=2-3V  so the 5VDC (TP9-TP8) is in the neighborhood of 4V actual.  I'm getting ready to run the troubleshooting test on the regulators in a little bit, but since U17 is the main reference that's my suspect for now; if the +18 is bad, nothing else will be right.

I agree, the references are probably OK - I measured across the stabistor and it looks like the voltage is OK (around 1.2V) until I bring the variac up to about half the mains voltage, and then is affected by the supply voltages jumping around (the +18 regulator or associated bits).  It looks like someone has also replaced Q11-12 and Q63 with whatever they had in their parts box, so no guarantees they are good either.  You can't see it, but Q63's leads are all squashed into an "S" shape and the heat sinks are perilously close to other metal bits - accidents just waiting to happen.

I am definitely replacing the filter caps - but visually and electrically the voltages don't raise any red flags (C18-19 each have about 34-38V, C20-21 about 25V and C22 about 9V) so I don't think they're the primary causes of the issue.  They're high enough for the regulators to work, so hopefully I can at least isolate the bad actor while I'm waiting for parts.
 

Offline Kleinstein

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Re: Fluke 8800A/AF power supply parts
« Reply #4 on: December 16, 2018, 06:27:47 am »
U17 should be an xx723 voltage regulator - so something one could still get.  Chances are CR37 is just a double diode like shown in the drawing - so two diodes in series could be a suitable replacement if needed. It looks like the 723 internal reference is used for start up (reduced with the 2 diodes) and later the more accurate 7 V reference is used to control the supply.  So it is kind of normal that later the voltage over CR37 could go close to zero later, when the input voltage is sufficient.

I would not be to worried about the exact type for Q11,Q12 and Q63. These are not critical transistors in the supply - so lots of choices.
 

Online GregDunn

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Re: Fluke 8800A/AF power supply parts
« Reply #5 on: December 16, 2018, 09:46:52 am »
Yes, the references are the only items that aren't readily available but something could be done to replace them if they turn out to be faulty; the 10V zener is easy, and there is a MPD200 equivalent in SMD (CMXSTB200) that could be bodged into the circuit without too much trouble.  The ICs are a 7805, 723 and a LM301, no problem.

I ran the power supply check (short pins 3-4 on the 723 to force it to use its internal reference) and the ±7VDC supplies looked decent but the ±18VDC supplies were far out of tolerance though symmetrical.  This says to me that either U17 or U18 is not working properly.  I do note that there are some lovely tantalums on the output of both chips, so I think I'll pull those and test them too (every time I get a new Fluke, I order more tantalums).   :-DD

I assume I'm not seeing anything on the display because the ±18V is not there to power up the controller chip...
 

Online GregDunn

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Re: Fluke 8800A/AF power supply parts
« Reply #6 on: December 16, 2018, 12:41:30 pm »
Well, that was a good guess.  C25, the tant sitting on the output of the 723 VR, was bad.  Replacing it (and the other tants in the PSU for good measure) allowed the power supply voltages to come good.  They're now fully within spec, and stable.   :-+

Unfortunately, the display still won't light up, except for the "200mV" annunciator and a slight flicker of the 7 segment units when I power off.  So I moved on to the next step, testing the ADC.  Putting 1VDC in, the signal on TP13 is supposed to also be 1V.  It measures exactly 0.1VDC.   :P  That seems to be a problem with the input divider.  The relay doesn't actuate no matter what I do, so I'm guessing the control signals are not being generated.  The ADC signals on TP13 are missing, too.  So something in the control circuitry is broken.  I sure hope it isn't the main control chip.

Edit: Clock going into pin 6 of U11 is fine, but all the other pins on the chip are static - no activity and they sit at -13V or -18V no matter what.  There are no strobe signals to the display, either, which is probably why it's blank.  I fear that this is a dead U11, and I really don't want to pull the working one out of my other 8800A to verify it.   >:D
« Last Edit: December 16, 2018, 03:28:09 pm by GregDunn »
 

Offline Kleinstein

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Re: Fluke 8800A/AF (power supply fixed - now logic issues)
« Reply #7 on: December 17, 2018, 03:50:16 am »
The is still a small chance the U11 is waiting for an external signal of some kind. One possible signal would be the CM signal from the analog part of the ADC. Depending on the details of the implementation U11 might just hang and wait for a change that never happens.
 

Online GregDunn

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Re: Fluke 8800A/AF (power supply fixed - now logic issues)
« Reply #8 on: December 17, 2018, 09:00:25 am »
If I read the description correctly, the control chip should initiate a conversion cycle CM high stops the count of the dual-slope integrator when the voltage reaches a set level.  Unfortunately, for whatever reason, CM is low all the time - which should cause the counter to keep running.  But the counter never even starts because the timing signals are not present.  I'd expect the INT signal to be issued periodically regardless of the value coming back from the ADC, because otherwise the meter would halt on an overrange condition waiting for the second phase of the ADC cycle to complete instead of giving an overrange display and starting on the next cycle... but INT is stuck low.

It's also apparently not resetting properly on power-up because the range relay K5 and the input divider are stuck in a state where selecting 2VDC (or in fact any manual range) attenuates the input signal by 10x.  Even so, putting a signal in should allow the ADC to measure something, and it isn't completing a cycle.  It keeps coming back to the control signals out of U11, and at least one of them should be showing a repetitive waveform of some kind, I think.  Not looking good.

I'm not even sure if I can swap chips with the other 8800 because Fluke uses two different types of U11 (PMOS and CMOS) and the board jumpers are soldered differently for each one.
 

Online GregDunn

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Re: Fluke 8800A/AF (power supply fixed - now logic issues)
« Reply #9 on: December 18, 2018, 02:13:18 pm »
Just as a last-ditch effort, I decided to pull the AC and Ω boards to make sure something on one of them wasn't interfering with operation of the timing/display circuitry.  No difference.  There are no further Ta caps which might be affecting the control chip operation either.  Unless I have a brainstorm, I'm going to have to declare U11 the cause, with no easy way to confirm other than swapping out a chip from a working meter and risking damage to it as well.  Maybe I'll find a gutted 8800A eventually with a working U11; until then, I'm going to put this one back in the long term queue.

At least I got a working 8800A in the mail today for a very reasonable $35, so no complaints overall!  Just sadness that I haven't been able to make everything work.  Time to put mains-fed power supplies in my otherwise functional 8600A units and get them out of the queue.
 

Online GregDunn

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Re: Fluke 8800A/AF (power supply fixed - now logic issues)
« Reply #10 on: January 04, 2019, 01:35:41 pm »
Since I got the other 8800A out of the queue today (working fine), I decided to make certain that the U11 chip was the real cause of the problem in this one.  So I carefully removed the chip from the working one and replaced it with the unknown.  Result: U11 definitely bad.  Restored the good U11 to the working one and of course it came up just fine.  I didn't want to risk putting the good U11 in this one, but it would likely be OK since the power supply voltages are good (now) and I have checked the passive components around it with no apparent issues.

So I seem to have one remaining 8800A which is likely good except for the U11 controller.  All power supply voltages are fine, and the AC / Ohms boards work OK in another unit.  Unless I can score a spare U11 somewhere, this one will unfortunately become a parts unit.
 

Offline Kleinstein

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Re: Fluke 8800A/AF (power supply fixed - now logic issues)
« Reply #11 on: January 04, 2019, 09:22:12 pm »
Having the 8800 as a parts unit is not that bad. The reference is sometimes relatively high valued. With an external temperature stabilization the AZS263 can be a good reference - AFAIAK used in the old FLUKE 732A.
 

Online GregDunn

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Re: Fluke 8800A/AF (power supply fixed - now logic issues)
« Reply #12 on: January 05, 2019, 06:01:30 am »
I guess having 3 working units isn't so bad.  :)  Since I got a parts delivery yesterday, time to work on the 8600A unit that's out of cal now.
 


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