Electronics > Repair

HP 8903a "mV" problem, newbie question

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Agosto:
I just got a 8903a. Initial inspection showed that voltage measurement in the "mV" section is far off: e.g. 100mV input shows 17mV on display. If I increase voltage level, and as soon as the unit switches range from "mV" to "V", display shows accurate values. I can hear a faint "click" when the unit switches between "V" and "mV" range, so perhaps a relay issue?
I have the manual but I thought before I dig into the service details I'd ask if someone experienced this issue before.

Any feedback is appreciated! Thanks

DeepLink:
Never worked on a 8903A, but the 8903B is almost the same, on which I've done some work

Look for the problem in the input circuit

Gain is divided into 3 sections
Input attenuator
Differential to single ended amplifier
Programmable Gain Amplifier
(Relays are used in the first 2 sections)

If you don't have the Service manual it can be downloaded here
https://www.waynekirkwood.com/images/pdf/HP_8903A_Audio_Analyzer-Service.pdf
You can control the settings of these via the Special functions (1.1 to 1.19) (page 3.35)
If you go through these settings, you will probably find the faulty section
Select the range with Special function and feed a suitable input for each range
Look at Service Sheet BD2

Agosto:
Thank you! Great info! I was going through the service manual and BD2 and also thought that the issue must be in the input section. Never worked on an HP so this is new to me. I will do as suggested and use special function to trigger the suitable input. So you think the issue is on the input board but not related to the "logic control"?
I did see some Reed-Relais on the board. Mostly for attenuation, though. I am thinking I should look at the gain section.
Do you know a source for these relays? I googled them to no avail...

Thanks again for your input. Seems that I am on the right track. I will only be able to work on this unit after my return from a trip in two weeks. I might need your help again...!

DeepLink:
Never seen a bad relay in the units I worked on...

It is easy to check the logic circuit
The input board are controlled by the Latch board

Agosto:
Great - Thanks again. Will check both boards.

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