HP 8903a "mV" problem, newbie question

[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.

[Agosto]

@DeepLink: It has been while since I had time to work on the HP. Today, I worked through the service manual and used SPCL function as per your suggestion.
Checked the values of Table 8-12 (Amplitude Limits A2TP5): All correct, within spec. However, values of Table 8-13 for 1.17 and 1.19 were much too low.
According to the manual, this hints in the direction of an error in the "Differential-to-single ended amplifier".

I found out that depending on the selected voltage range (via SPCL function) the problem occurs. "1.17SPCL" (equals voltage range 0.189V) and up (= lower voltage ranges) causes the display to show erratic values while 1.16 and below (= higher voltage ranges)  show consistent correct values.
While in "1.16 SPCL" I can reduce input voltage below 0.189V while still getting correct readings. However, as soon as I hit "Automatic Operation" at an input voltage below 0.189V, the display returns to show erratic values.

In mode 1.16 SPCL, connecting a scope to A2TP6 and A2TP7 shows stable voltage amplitudes change proportional to the input voltage. If below 0.189V and switching to "Automatic Operation" the voltage amplitude at the TPs starts to jitter which I believe is the reason for the false display reading. Perhaps a bad capacitor? Is there a commonly known culprit?
I have yet to find out what difference the signal flow between "SPCL" operation and "Automatic Operation" is.

Any recommendation for next steps is highly appreciated.

[Wallace Gasiewicz]

Just glanced at the schematic of the input amp and attenuator.  P 314
I would check the relay K6 perhaps they do not fail often but I would check it anyway.It is also possible it is not getting a signal to actuate, The signal would be a near ground at the anode of CR2, Check the volts at CR 2 and compare to CR 3 and CR 4 as these relays are used in higher volt ranges.

[Agosto]

Thank you for your input! Relay K6 turned out to be ok, switching voltage is around 3V AC at input. The issue I have/had appeared below 190mV at input. However, I found Relay K1 to be permanently open. Bridging K1 provides correct values. Looking for a suitable relay right now (Magnecraft W107 DIP-1). Any sourcing suggestion is appreciated!

Thank you both for your help!