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Old Fluke Multimeters

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GregDunn:

--- Quote from: med6753 on November 21, 2018, 01:28:38 am ---
That drift could also been caused by this........

https://www.eevblog.com/forum/testgear/test-equipment-anonymous-(tea)-group-therapy-thread/msg1974341/#msg1974341

Took me quite a while to track it down but it is now absolutely rock stable.

--- End quote ---

OK, coming back around to this again; I suspect I have one or more defective relays just as you did, as one of my 8600A units takes a while to warm up on certain ranges.  I have done some more sleuthing on the relays, measuring coil resistance and looking at the NSN specs, and here is what I think.  The U6-P on the input divider is clearly a good match for that Littelfuse HE3351A0500 125Ω unit.  The UF40070 and UF40069 relays seem to differ only in the contact ratings; they are both 600Ω units.  The 40069 is used in the AC Converter, so will see high AC voltages - but it is only rated for DC, while the 40070 is rated for AC/DC operation.  Perhaps the shielding on the 40069 was different, to be able to operate in the presence of higher AC voltages?  Just a guess; that's the only spec which differs materially from the 40070.

So, unless someone sees a problem, I'd think the Littelfuse HE3321A0400 is a suitable replacement for both the UF40069 and UF40070 relays.

med6753:
Agreed, go after the relays first. If you still have a long term drift then consider changing the crystal.

GregDunn:
Not what I'd call a long term drift; it pretty much settles to the last dp after half an hour of warmup, even from cold.  Before that, though, it's off by plenty of mV (10.0023 standard reading 9.987 initially).  The clue that it's a relay, for me, is that switching ranges away from 20V and then back (or selecting auto) causes the error to jump up again until it settles.  A crystal ought not to do that.

Now that I have what I believe to be a list of acceptable replacement relays, I suppose I'll figure out which one is the 10VDC culprit (maybe K2?) and swap it.  I only ordered one of each type until I get a chance to actually test it in-circuit and confirm it works.

med6753:
You can determine real quick if it's the relays. Hang your high impedance 8800A on R52 (TP11) and watch the voltage. If it drifts....it's the relays. No drift, it's something else. Next step would be to monitor the reference voltage on CR14.

GregDunn:
Brilliant - I'll try that after the family gatherings end on Sunday.

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