HP 34401A Reset Loop

[FeedMyZappetite]

Hey, I recently bought a broken HP 34401A DMM off of ebay, it turns on and displays the GPIB address then resets and continually turns on and off in this same sequence.

After messing around with it a bit I found that if I quickly hit DCV -> Shift -> Hold it will stay on and I can navigate to the settings. I can change most of the settings and they seem to stick when it resets but navigating to the GPIB-address it freezes on the PARAMETERS screen. Leaving the settings screen causes it to go back to its reset loop.

Feeling confident that the issue is somewhere on the Ground-Reference Circuit I began checking voltages against the reference schematics but so far haven't seen anything obvious. The transformer does seems to be a bit low and is reading 4.6V but this seems close enough to 5V to not the cause of the problem.

So has anyone know of what may be causing this? I can tell the reset is caused by the reset control circuit but I'm not sure what conditions will initiate the reset control?

Service Manual with Schematics: https://www.ecb.torontomu.ca/guides/instrument-manuals/Agilent-HP_34401A_Service_Guide.pdf

[Kean]

First, that link is to Edition 6 of the service guide from 2005.  There is later versions, such as Edition 9 from 2014 which I am looking at.
Should be available on Keysight website and others - but I can provide if you would like it.

In both editions on page 106 "Power Supplies", it states:

The reset output of U553 will change to logic LO when the unregulated dc input to the regulator falls below 5.5 volts.

So, when you say:

The transformer does seems to be a bit low and is reading 4.6V but this seems close enough to 5V to not the cause of the problem.

Which "transformer" output are you referring to, and what exactly is measuring 4.6V?

There are two 5V rails in the power supplies schematic on page 9-15:
- the output of U751 called "+5V_0" earth referenced power supply - going to sheet 7 (page 9-14)
- the output of U553 mentioned above called "VCC" for the floating logic power supply going to the other sheets.

U553 also outputs XPONRST which resets the main mcu U500 and ASIC U501 on sheet 6 (page 9-13)

[FeedMyZappetite]

I got that newer manual, thanks.

I found that note about U553 little bit ago and have been going through that and the components near it.

I measured the pins of U553 and it is diffidently the component sending the reset signal so everything I was saying about the ground circuit system is wrong.

The voltages I was measuring was for the ground circuit power from the transformer. The voltages going to the floating circuit are roughly what they should be but the wires feeding U553 are reading about 4.9V but I'm getting the correct voltage out of the diode bridge. The filter capacitor C556 seems to be the cheapest component to replace that controls the voltage to U553 so I'm going to replace that first then the transformer if that doesn't fix the problem.

[Kean]

The transformers should rarely die, unless abused - e.g. 220/240V applied when set to 100/120V.
I agree that C556 would be the first thing to check/change.

Are you able to measure the voltage across C556 with a different DMM?  I'd expect at least 7V DC going into U553, possibly up to 10V.
Maybe check both DC and AC voltages?  If the cap is bad, you'll see high AC ripple.

Do you have a LCR meter or capacitor tester?  If C556 is bad, there is a good chance some of the other electrolytic capacitors have also gone bad with age, especially the other large ones like C551, C553, C752.  It might be worth replacing them all together.

In another thread, there is mention of C558 being bad and causing a reset signal issue - although I think different symptoms, not looping.
https://www.eevblog.com/forum/repair/help-me-repair-agilent-34401a/

[FeedMyZappetite]

Didn't have time to keep troubleshooting this meter so i set it aside for a while, but recently was looking at it again and made some discoveries. The voltage regulator was not the issue neither was the power going into the meter. The rest pin on U553 is showing 5V but I lifted that leg off the board to verify that 5V was being generated by U553, and well turns out it wasn't.

There is still 5V on that circuit but it wasn't coming from U553. It appears to be coming from the microcontroller or the ASIC but I couldn't find anything in the manual about a condition that would cause either one of those to output 5V to the reset circuitry.

So, I think one is outputting the 5V and the other is initiating the reset but I'm not entirely sure which one, or why for that matter. Either there is some reset condition that I have not been able to identify or one/both of the chips are bad.

Not exactly sure where to go from here, maybe a ram issue but I'm not sure if I can test that without going down a swaptronics rabbit hole.

Figured I would just add an update incase anyone ran into a similar issue.

[Kean]

U553 is an LM2925 which should have an internal pullup to 5V on its RESET output (approx 30k per datasheet).

XPONRST signal from U553 connects to XLPORST via R593 (215Ω) which feeds into U500 (N80C196KB uController) and U501 (ASIC = SCX6206AK0 gate array).

The 80C196 should have an internal pullup on its RESET# input pin.  R_RST is specified as somewhere between 6k and 50k in the datasheet 8XC196KB.
I don't know much about the SCX6206AK0 gate array used for U501, and its RST pin isn't mentioned in the limited datasheet I have looked at.
Pretty sure that neither of these ICs will drive their reset pins, so it will only be generated by U553.