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

[jason123]

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

Any specific varnish type? I took it out of circuit - seems fairly rigid. Maybe the audible noise was coming from somewhere near it also, so I'm not really sure

[jason123]

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

Hi R_J, the mic picked up the noise from that area - so I don't know if it's the transformer or regulator or FETs, ... I took T1301 off the board, and I was hoping to see a crack or something physically bad with it, but I don't - C1324 (560u) is one of the electrolytics and I just replaced with a brand new capacitor. there is a 10u in parallel, but I've never heard of 10u Ceramic with audible noise?? I see some posts online about people complaining about a noisy "Linear Regulator" but referring to +/-15 V circuits - not sue if it's audible noise or noisy signal. That's for designs with tantalum capacitors - this version of the board (MY44 serial) seems to have replaced all Tantalum caps with ceramics)
Not sure where to go next

[R_J]

You can try scoping the output of the transformer, before the diodes and see what the signal looks like, is it a smooth sinewave or does it look chopped up etc.

[CapLeaker]

Any specific varnish type? I took it out of circuit - seems fairly rigid. Maybe the audible noise was coming from somewhere near it also, so I'm not really sure

What kind of transformer is it? Maybe take the rectifier diodes out and just feed straight DC from the bench power supply?

[stj]

replacing tantalum with ceramic was a downgrade

[jason123]

You can try scoping the output of the transformer, before the diodes and see what the signal looks like, is it a smooth sinewave or does it look chopped up etc.

Will share shortly - I had already taken the transformer off the board, in hopes of finding a replacement, but that's dead-ended -It's a specially made transformer for Agilent, and manufacturer only sells it to them.

[jason123]

What kind of transformer is it? Maybe take the rectifier diodes out and just feed straight DC from the bench power supply?

Fairly elaborate function, many rails are created from this transformer - Page 177 of manual attached

[jason123]

replacing tantalum with ceramic was a downgrade

I guess many people were reporting charred tantalums on the main board

[CapLeaker]

Your saying you swapped your friends power supply into you rig and then it is still making that noise?

[R_J]

There are two (2) power supplies, for this unit, one to take line voltage down to 12v and the one on board that takes the 12v and converts it to the other +/- voltages.

[jason123]

Your saying you swapped your friends power supply into you rig and then it is still making that noise?

Yes, the initial suggestion was that the noise maybe coming from the main supply (AC to +12 Vdc)... i swapped that with my friends main supply and the audible noise was still present on my unit (noise followed the main board - not the AC/DC supply.) Seems noise is from / near the transformer on the main board. Trying to determine the "noisy" component

[CapLeaker]

I just wanna clarify which PSU you switched. Yes there are 2 PSU’s here an AC to DC 12V and then the DC to DC converter in the schematic. I looked at the datasheet of the LT1683 and it sez it switches from 20khz to 250khz. This is outside the normal human hearing range, unless you’re like 20 years old to actually hear 20khz.
I think it’s time to break out an oscilloscope and see what’s happening.

The feedback is based on + and - 15vdc rails that controls the PWM. The problem may not be there, it could be on the positive 5V rail and then there are 2 more smaller rails from it or the -5v rail.
I think you guys already covered input capacitance, slew caps and resistor etc.

looks like CR1311 has a hole in it?