Hot Rod Deville Buzzing

[dpm309]

I have a Hot Rod Deville  that has a horrible buzzing sound (120Hz). All new tubes were installed before I got to look at it. Cleaned all the sockets and jacks with no luck. Cannot bias it past 14mV. Voltages don't seem out of line. Am getting about -55V on the bias pot with the standby off or on. Checked for cold solder joints and any signs of overheating etc. Filter caps 'appear' ok but they are the Illinois caps which Fender has had issues with. I am suspecting something with the bias circuit. Any suggestions? I have attached the schematic.

[dpm309]

Still getting the buzzing after pulling the preamp tubes, one at a time, and pulling both power tubes.

[acheld]

If the buzzing continues after pulling all of your tubes, you should consider vibrations coming from the power transformer.   Sometimes the plates loosen up, and they can indeed vibrate . . .

I'm not sayin' this is the problem, but if all of your tubes are out, then the buzz is not coming from your circuit . . .

[Latole]

I .

  but if all of your tubes are out, then the buzz is not coming from your circuit . . .

I agreeé

[Lauri]

Do you have an ESR meter you could check the filter caps with? I've serviced lots of hot rod deluxes and devilles and about half of the time one of the filter caps has failed. Usually it's the first one and the amp buzzes a lot. Also check the 1N5062 diodes. 

[Latole]

To hear buzz from filter caps don't you need tubes in the amp ?

Amp buzz with no tubes !

[dpm309]

I just happen to have an ESR meter but it only goes up to 250V and the caps are rated to 350V or 500V. Could I just jumper the filter caps with a good cap?

[Latole]

Look it is not the issue IMO .

[punkykatt]

With all the tubes out, disconnect the speaker. Does the amp still buzz?

[dpm309]

No buzzing with the speakers disconnected. I am not getting any reading on the 1st filter cap (100uf/ 350V) even though my ESR only goes up to 250V. Am getting readings on the rest of them. Looks like a bad filter cap.

[dpm309]

Just to make sure before I start tearing apart the amp, can I jumper a good 100uf/450V cap across the 1st filter cap?

[sluckey]

Yes. Back in the '60s every radio/tv shop had a couple electrolytics with gator clip test leads permanently attached just so you could quickly parallel a filter cap when troubleshooting hum issues.

[dpm309]

Thanks Sluckey. I remember hearing that awhile ago but just wanted to make sure.

[dpm309]

That did it. It is time for a re-cap on this Deville.Thanks,Dan

[Latole]

Do you see any anomalies on the ends of the capacitors (Photo)
No anomalies probably means that the capacitors are good. the problem could come from soldering on the printed circuit

Big job to disassembly a HRD

https://www.bustedgear.com/repair_Fender_deville_jack_2.html

[Lauri]

Just because there's no goo coming out of the capacitor doesn't mean it's not bad. The first filter capacitor has failed to an open circuit and there 120hz pulsating dc on the B+ node. The amp will buzz even without power tubes because now there's 120hz on the output transformer primary and some of that buzz will transfer to the secondary. Those IC capacitors are notoriously bad. Almost every time a hot rod deluxe or deville comes in for repairs it either has bad filter caps or R78 and R79 have desoldered themselves from heat.

[Latole]

Good or bad filter caps ; easy to check with AC ripple.

[acheld]

easy to check with AC ripple.

+1.  Also, easy enough to check ESR (at a minimum) in circuit, and not that hard to lift a lead and check capacitance. 

I personally don't think the Illinois Caps are particularly bad.    However, the HRD is like a souped up hot rod with a stock radiator.

[dpm309]

The first filter cap was definitely bad. Did not get a reading on my ESR meter but the other 5 tested fine. As soon as I jumpered the 100uF/500V cap across the 1st cap, the buzzing was gone, I could bias it to 60mV and it sounded good playing through it. I have run into problems with the Illinois caps in the past. Fender had gotten a hold of a bad batch of Illinois caps when these amps were built. I have not had any problems with the newer Illinois caps. Will replace all 6 of the filter caps as long as I have the amp opened up. Just a matter of time before the rest go bad.

[AlNewman]

I went with Panasonic radials where I could find them.  I am cheap, but also 5000 hrs @ 105 degrees.  The new IC caps were the only brand of electrolytics my supplier had at that voltage. 

[PRR]

I used "IC" caps for decades, even past spec, and never had trouble.

Yes it is possible Fender pushed them on price and got what they (didn't) pay for.