Agilent 33220A 20 MHz Function Generator

[jason123]

I can't find a forum to discuss "instrumentation Troubleshooting", but PS maybe the closest.
So, I'm having an issue with about 13 year old Agilent Function Generator. it works, passes self-test, but it has an annoying "audible" noise, that changes as I change the output frequency setting of the instrument. Prominent around 600 Hz and up (at least that's what my ears are picking up.) I saw on some forms that it could be fan, linear regulator, dried up caps,... I can't seem to find the source of audible noise. Here is what I tried:
1. Turn the output off - still noisy
2. Disconnect fan. Still noisy
3. Replaced all 8 Electrolytic caps - Still noisy (all removed tested ok for Capacitance and ESR)
4. Tried to probe around the circuit for source of noise with a scope. Tough to find, as I'd imagine this is complex frequency that I'm hearing only part of it.

Any thoughts here are appreciated.

[R_J]

The problem is likely in the power supply, grey box on the left, I would start by replacing C1324 (560µf/35v) in the primary, then check the secondary caps

[jason123]

Thank you R_J. Happy Canada Day!
I've attached some close up photos of the power supply as I can't quite see the reference designators on the board. On the secondary (output) I see 680u @35V, and 330u @ 35V. On the primary, I'm only seeing the large 400V cap. There is the small cap near the standby board. I don't know which, on the primary side, you suggest I replace?

[jason123]

So just while I wait for a response, I went ahead and replaced the 680 and 330 with what I had available (560 and 470) caps, and that didn't alleviate the problem. Finally, I added an additional cap next the 680 (unstuffed cap) and that also, didn't make much difference.

[jason123]

A friend next door has an identical function generator - we just completed the experiment of swapping the internal power supply, and unfortunately, that's not it. Quite on his, and a musical instrument on mine.

[jason123]

Going Hi-Tech here , I got a microphone and started sniffing around the board - seems to be coming from the Big transformer in the middle of the board
T1301, Agilent PN: 9100-6089, Description: XFMR-PWR 11.88-12.12V/18V, Manuf Code: 53471, Manuf. PN: 31482R

Can't seem to find it anywhere. Any help? Could it be the transformer, or something else near it? mic can't be more precise I'm afraid.
Thanks

[CapLeaker]

I suppose you could take the transformer out and soak in in varnish.

[R_J]

I was looking at the wrong power supply schematic, If the noise is coming from T1301, then it might be C1324, which is the vcc supply for the ic that drives Q1302 and the transformer. sometimes if that cap is weak the drive to the Q1301 transistor can be off and not smooth causing the transformer to make noise, or it could just be the windings in the transformer vibrating.
Here is the service manual if you don't have it, looks like they do not include the power supply module itself