HP 33120 Function Generator Problem

[Henry Finley]

I did something to my function generator and don't know where to start hunting the problem. I suspect power supply. Here's what happened: I had just finished aligning the 60kc IF on my Hammarlund (with clock). The clock is fed by 120V and the contacts are sticking right out in the open. After I finished my alignment, I unhooked the signal generator probes from the radio and dropped them on the table, But by a stroke of bad luck, the negative clip on the generator probe brushed one of the hot contacts of the radio clock. POP!. A big spark, a loud pop, and the AC electrical panel breaker kicked out. I couldn't duplicate this fateful accident in a million years.
     Now the HP 33120A function generator comes on, and in about 30 seconds it shuts down. In that 20-30 seconds I believe Im getting proper signals out of it, then it shuts down. It's been a while, but I believe I remember I can't reboot it till it has sat a good while. Then does it again. Any ideas where to look?
     Thank you.

[Electro Fan]

Just a SWAG, but the situation sounds somewhat similar to here:
https://www.eevblog.com/forum/testgear/siglent-sdg1032x-help-needed/

[bd139]

The 33120A has a floating output so the entire generator portion of it is on a separate transformer tap to the CPU etc and there's an optoisolator between each half of it. Need to isolate which side is fecked.

Service manual is here: https://ese.wustl.edu/undergraduate/Documents/lab_support/HP_33120A_Function_Generator_Service.pdf

As it seems to survive for a bit, see page 21 and run a full self test on it. (hold down shift at power up for at least 5-10 seconds) and see if that throws any error codes. codes are listed in the back. Failing that I'd look at the power supply monitoring IC in the power supply. There's a PFAIL line there which looks suspiciously guilty on shutdown conditions!

That should give you an idea where to look. Inspect the board around the attenuators as well. They frequently go up in smoke.

[Chris56000]

Hi!

Full component–level parts lists & circuit diagrams are provided in the Service Manual linked to in the previous post, so if  there IS internal damage you'll not be guessing in the dark!

I would personally begin by dismantling the instrument and inspecting the main pcb for damaged components, burnt–out lengths of earth–print, etc., etc.

As with bench DVMs where there's "in–guard" (floating) and "out–guard" (earth–referenced) sections or blocks of circuitry, damaged or floating components can cause erroneous signal–levels to be translated back to the micro, causing self–test failures, unexpected shutdowns, etc., etc.

Read thro' the S.M. & examine your main board carefully, working back from the waveform output–sockets your probe was plugged into, and if you find no obvious physical damage or self–test failures, myself or other Members'll be pleased to offer help with this!

Chris Williams

[Henry Finley]

This is me, 3/4 of a year later, back with the same machine and a revision to the original post, This is a friend's HP33120A, not mine. And I haven't seen it since I gave it back. He had another man take a look at it, with no changes. But here's what it's doing: Boots up with RS 232 error, but otherwise still puts out signals and seems to work normally from the front panel. Today I loaded up a bunch of Keysight software on my Windows XP SP3 computer, and hooked up the function generator through my 232 to usb adapter (that I know works) but the Keysight software doesn't recognize the generator. I'm not sure the original accident in the first post had anything to do with it. This generator seems to work fine otherwise. I wonder what the problem could be.