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Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)

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joeqsmith:
Both states wouldn't drive it.   I increased both tants and it now will change states but I don't like that bootstrap circuit.   I may limit the base drive of Q3 while I'm in there.   

I haven't done anything with the alignment yet and will wait until I get the board back in its clam shell.   Looking at the 10K in my test box.

***
Added a small resistor and ran it through the ranges before buttoning it up.  Sadly, there appears to be at least one more problem.   Looks like the current source.   Fluke has a house number 261578.  Appears to be a Siliconix logo, J2137.   So far no luck finding a data sheet for it.  The 1986 small signal databook I found did not list it.   

SilverSolder:

The 10K resistor looks pretty good?  What's wrong with the current source - noisy?

dietert1:
Used our two old Fluke 8502A recently. One of them shifts 15 ppm in 10 V DC range when connecting Guard to Low, the other one doesn't. Any ideas?

Had a similar issue with the relays K1 and K2 on one DC input conditioner but it went after exercising them via GPIB for 100 or 200 cycles. The relays sound completely different between both units. I replaced all those orange tantals and small electrolytics by film caps, MLCCs and new electrolytics. All of them were still ok, though. Distributed about ten MLCCs onto each ADC digital board. Added a schottky diode over the +5 V adjustment pots for safety.

Otherwise those units are working - after the recommended service incl. reseating of all modules. I also replaced the 150K input protection resistors and some other carbon resistors that had drifted more than their 5 % tolerance. Day-to-night temperature changes in our lab are 2 or 3 °C and those units shift 0.7 ppm when logging an ovenized 10 V reference. I think they are very quiet and good for 2 uVpp.

Regards, Dieter

dietert1:
About the relays: As far as i understand the 8502A schematic they tried to use the monostable input relays at constant current in order to reduce shifts from thermal EMF. So they are running the coils at a current that doesn't turn on a relay but holds a relay that has been turned on before. Then the action is by short pulses - either reducing current to zero or increasing it for a short time. Seems to work.

Regards, Dieter

joeqsmith:
With the later revision boards using a latching relay, there's no need to provide a hold current.  Just a pulse. 

Found the last problem with the current source and have the meter now working on all ranges.   Time to find some low tempco parts and then it should be able to be reassembled and finish the alignment.   I have some low tempco parts I had characterized with an 8.5 place meter that I will use to align it.     

The meter uses RS232 rather than GPIB.  I think the next step is to see if I can put some simple program together to tend the data.   

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