HP 3325 Fail 24 solved

[RogerThat]

Hi all,

I bought a 3325B with a known fail 24 thinking it would be an easy "adjust the PSU, change a cap or two, reseat the flat cables", but no.

The unit gave a "Fail 24" on power up and when changing function (sine, square, ramp etc). Self test listed fail 28,24,41. There was no output wave form, it was only possible to change the output by doing DC offset large enough for the relays to actually move the output.

While following the service manual I checked all the relevant boards. The service manual on Keysight's page is badly scanned on the schematics pages but it's possible to figure out what goes where by cross reference with parts list and looking at the board.

I found that the DAC output set all the sample and hold circuits to -10v. This is found on figure 8-I-3 in the service manual, on the left side of the schematics there is plenty of test points to check the sample and hold outputs(inside the blue circle in the picture). Following the signal upstream I found the DAC integrator and this where the issue was(yellow circle in the picture). While measuring resistans over the 0.01uF cap(C43) there was 0.7 ohms. After desoldering it it was still 0.7ohm and zero capacitance. This is a film capacitors so I find it a bit odd for it to fail.

I replaced it with what I had: a 0.01uF 63v(original is 100v) Wima MKS4. And, it solved all the issues and the unit now works flawless. In the picture the new capacitor (red square) is already in place

Oh, on a side note, the Schaffner input filter made a nice smoke show half way thru the repair....

[wn1fju]

Good diagnosis and fix.  I've had a couple of those integrator capacitors fail in other HP pieces.

As you might be aware, the ribbon cables connecting boards are a point of failure on the 3325B.  I had a terrible time with my unit (Errors 23 and 26) and finally got so fed up reseating one of the cables, that I finally removed it and installed 21 individual jumper wires.  So keep an eye on your unit.

I also have the earlier 3325A and never had a problem with those cables.

[Martin Miranda]

when i got my hp 3325B it throws all kinds of error. but before i troubleshoot mine. i replaced ALL tantalum caps first. it will FAIL, and WILL FAIL. at any time.

[RogerThat]

A follow up here.

I replaced my temporary fix with the proper capacitor. I replaced all other polystyren caps at the same time. Of these only one started to have high ESR, otherwise they where fine.

I've also had a few fail 26 when the unit have been running for some time. Turn it off, wait and then on again sorts it. I think I've traced it to fuse F4 (just above the output amplifier). Its resistance is quite high at close to 4ohm when warm and 2,5ohm when cold. Next time I order parts I will have a new one to put there.

[SilverSolder]

Mine had high ESR in the big power supply capacitor, worth checking the caps on the PSU board as it runs a little warm.