Keithley 177 Flashing 0000 on all AC functions

[J-R]

I have a Keithley 177 that I am trying to repair.  The biggest issue is that whenever AC is selected, all the digits just blink 0000 continuously in both voltage and current modes (and all ranges).  Also, all resistance, DCV and DCA functions work perfectly and are completely spot on, EXCEPT for the 20mV and 200mV settings which both read a bit high and there is not enough adjustment range to correct it.

Following the manual (http://web.mit.edu/8.13/8.13d/manuals/keithly-177-dmm.pdf), under the AC Converter Checks (page 5-9), I got to step 5 and the output was bouncing around 8V instead of being 1V.

Has anyone seen this behavior or have any suggestions on what the problem could be?

[bdunham7]

I've no time at the moment for more than a quick look at the schematic and an offhand guess, but it looks to me like your U103 (AD636 TRMS converter) is bad.  Astonishingly (to me) Mouser has them in stock in the TO-100 package!

[J-R]

Thanks for the suggestion, but unfortunately that part is more money than I paid for the entire unit! So even if it is bad I probably wouldn't replace it.  But I suppose I could consider removing the part and at least testing it.

[bdunham7]

Thanks for the suggestion, but unfortunately that part is more money than I paid for the entire unit!

You're new at this, right? 

Seriously, there are some US sellers on eBay that have it, maybe have a look.

[J-R]

Thanks for the suggestion, but unfortunately that part is more money than I paid for the entire unit!

You're new at this, right? 

Seriously, there are some US sellers on eBay that have it, maybe have a look.

Eh, yes and no, have a few resuscitations under my belt so far: 2 out of 3 Fluke 8050A, 8840A, HP3466A, Keithley 197A and now this 177.

Tinkered some more.  Turns out the DC modes actually were all working perfectly after I did a complete calibration, so odds were good for just the True RMS-DC converter being bad.

TP1 was 8V, but pin 6 on U103 (AD536AJH) was always -0.9V no matter the input or external adjustments.  I tested all the components connected to it and they were fine.  Found the data sheet on the AD536A series so was able to get all the pinouts.  Pulled it out and it was for sure a goner.  No chance getting this back to life with spare parts from the bin, though...

I was able to locate the similar AD536AKH from a reputable US seller for $20 delivered.  Got it installed and ran once more through a complete calibration and all is well.

Cute 19,999 count relic for my "collection".