Amplifier loud hum.

[fossilshark]

Hi all, I'm really struggling to figure out this Bugera V22 combo. There is a nasty hum that is really loud when the master volume is up enough for signal to go through. There is definitely some guitar single coming through under the hum. No other controls affect it except for a little squeal when I dime or back off on the presence.

I replaced all the filter caps, all the tubes, and resoldered all bad connections. I also went through this http://www.geofex.com/ampdbug/ampdebug.htm and could not diagnose the hum.

Could it be the output transformer? how would I check the output transformer? I am a little confused at this point, any help is appreciated.

[Latole]

"Could it be the output transformer? "
- Not in my opinion.

1- How is the hum with no guitar plugged ?
2- How is the hum if you remove V2 ?

Schematic;

[pdf64]

No controls will work but is the sound ok if you plug into the fx loop return?

[fossilshark]

Latole, the hum is very much still present with no guitar plugged in and taking out V2 does not help.

[fossilshark]

No controls will work but is the sound ok if you plug into the fx loop return?

wow amazingly the amp sounds fine when i plug into the FX loop return. That means my hum is in the preamp somewhere right?

[Latole]

No controls will work but is the sound ok if you plug into the fx loop return?

wow amazingly the amp sounds fine when i plug into the FX loop return. That means my hum is in the preamp somewhere right?

If you are right, with removing V2 you must not hear hum !

[Keppy]

My money's on the +-15v power section that feeds the transistor that drives the effects send. Have you checked the caps and diodes in that section, or metered the voltages there?

[fossilshark]

My money's on the +-15v power section that feeds the transistor that drives the effects send. Have you checked the caps and diodes in that section, or metered the voltages there?

i replaced the 2200uf filter caps, but I have not checked the output. Could it be caused by a bad regulator? what am I looking for?

[Keppy]

i replaced the 2200uf filter caps, but I have not checked the output. Could it be caused by a bad regulator? what am I looking for?

The problem could be the regulator, or the transistor, or the 10uF caps on the regulator output, or maybe the Zener diode on the ground pin of the regulator. Check the regulator ground pin for continuity to ground, and check the voltages on T8, plus the voltage across the emitter resistor. Should be close to this:

• Collector: 15v (because it's connected to the regulator output)
• Base: 0v (because it's tied to ground through R70)
• Emitter: -0.6v (because the emitter is always a diode drop below the base)
• Emitter resistor: -15v (because it's connected to the -15v source created by the zener diode on the regulator's ground pin)

[Latole]

No controls will work but is the sound ok if you plug into the fx loop return?

wow amazingly the amp sounds fine when i plug into the FX loop return. That means my hum is in the preamp somewhere right?

There a not much amplification using FX loop return. You may not hear the hum wich is there

[fossilshark]

i replaced the 2200uf filter caps, but I have not checked the output. Could it be caused by a bad regulator? what am I looking for?

The problem could be the regulator, or the transistor, or the 10uF caps on the regulator output, or maybe the Zener diode on the ground pin of the regulator. Check the regulator ground pin for continuity to ground, and check the voltages on T8, plus the voltage across the emitter resistor. Should be close to this:

• Collector: 15v (because it's connected to the regulator output)
• Base: 0v (because it's tied to ground through R70)
• Emitter: -0.6v (because the emitter is always a diode drop below the base)
• Emitter resistor: -15v (because it's connected to the -15v source created by the zener diode on the regulator's ground pin)

Awesome so I measured ~6v from ground to the emitter as well as the base of T8 and 15v on the collector.

Replace the transistor? This definitely seems like a tiny tranny for the job

[Keppy]

Yes, as long as you've ruled out the base and emitter resistors. If the base resistor is grounded, and the emitter resistor connects to -15v, then those voltages say the transistor is bad.

The size of the transistor doesn't really matter, since it's not set up to dissipate a lot of power. It's just buffering the effects loop output. You will want to make sure you get one with a high enough rating for max C-E voltage though, since a lot of common types are only rated for 25v and the power supply suggests 30v+ for that spec.

[fossilshark]

Plopping in a 2n3053 because its all I have on deck that will work. I will update if this doesn't solve the problem.

[PRR]

While you have the transistor out, put 1K in series with the collector. Leave the C lead up in the air, use a 1k to connect to the C hole on the PCB.

There's just enough energy there to blow that transistor on "spike" waveforms. With 1K as shown it is not likely to pass over 15mA, well within any Si transistor's ability.

If you are out of 1K, 470 to 5k.