On Mon, May 23, 2022 at 04:23 PM, Chris Smith wrote:
Hi all,
Just obtained a rather sick HP3465A (the 4.5 digit one, not the big 6.5 digit unit). It is functioning but there is a one sided offset on measurements which is proportional to the divider/amplifier arrangement.?To clarify, if I short the inputs at 2V it floats reading +.0035. If I short at 20V range it sits around +0.648 volts. These measurements are much larger at turn on and drift down slowly. If I stick a 9V battery on the inputs it reads 9.995 in one polarity and -9.206 in opposite polarity as an example.
I am currently devoid of a scope and am working on this with a rather shonky Fluke 25 at my disposal so running through the SM diagnostics is problematic. I have a better DMM in flight (34401A) and am sourcing a scope at the moment.
I have checked the following things:
1. Power supply -> OK
2. input to Q16 -> seems ok (caveated)
3. input to U3 -> seems ok (caveated)?
4. COMPTP point -> seems ok (caveated)
5. AZTP point -> seems ok (caveated)
caveated = limited measurement capability here.
Any help appreciated. If anyone has any tribal knowledge or debugging approaches I would appreciate it.
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Since you already confirmed some of the internal node voltages, is it correct to assume you followed sections 5-31 and 5-32 in the service manual? Could you not zero the offset with the "ZERO ADJ" and what was the offset of the input amplifier ?(DCTP) when Q16 input was shorted?
Ozan
