Fender BG Hot Rod Deluxe noise issue

[threenofingers]

Hi everyone

I'm new to this forum... first post.
I recently bought a second hand BG HRD and kind of got duped. No point going into details.
The amp plays great on the clean and drive channels, but when I change to the "more drive" channel either on the amp switch or the footswitch, it makes a horrendously loud noise and won't react to the master volume, gain or any other dials.
I've searched Youtube for possible similar situations but nothing as specific as this issue pops up.
Any advice is appreciated.

Cheers
Andrew

[Jon Snell]

Hi and welcome to fhe forum.
No idea if you are experienced with high voltage equipment or not but if you are, check Q2, a J111 FET and its drive components. Removing C9 should stop the noise if it comes from the control lines.
Strange the master volume has no effect ... it should do as the master volume is just before the effects loop and main amplifier, after the FET.

[galaxiex]

Might want to check this first before diving deep into the amp.....

https://music-electronics-forum.com/...g-jack-problem

[g1]

I recently bought a second hand BG HRD

Do you mean the GB (George Benson) model?

[threenofingers]

Yes, the George Benson model.

[threenofingers]

Might want to check this first before diving deep into the amp.....

https://music-electronics-forum.com/...g-jack-problem

Thanks gents.
I'll check the jacks before I delve in any deeper....

[Mick Bailey]

Does the problem persist with all controls on zero?

I've never seen a schematic for the GB version, so not sure how it differers from a regular HRD III. The master volume not affecting the noise is puzzling. If you plug your guitar into the 'power amp in' socket and engage the 'more drive' do you still get the same problem? Doing this isolates the preamp, so there shouldn't be any noise. It's useful to do this to eliminate the possibility of the power amp being involved.

Also, do you know if the amp has been modified at all?

[g1]

I've never seen a schematic for the GB version, so not sure how it differers from a regular HRD III.

From what I have read, the HRDIII schematic is correct. There is a 12AT7 installed for V1, the speaker is different, and the cab is built from different material. That's it.

[threenofingers]

Apologies re the time difference... I tried it again with all the controls on zero, I've discovered the noise is affected the most by the drive control that is raucous on 1 and only disappears when I increase the drive past 2-4. Also the bass wigs out as it increases around 4...Good news is none of the other controls are affected and the master just increases the volume as expected. Obviously when I first tried it, I turned the master down(which did zilch) but I didn't think to turn the drive up I tried a patch cable from pre amp to power amp jacks but this didn't change much. I'll give Mick's suggestion a go.... "If you plug your guitar into the 'power amp in' socket and engage the 'more drive' do you still get the same problem? Doing this isolates the preamp, so there shouldn't be any noise. It's useful to do this to eliminate the possibility of the power amp being involved."

[threenofingers]

Plugged my guitar into the 'power amp in' socket and the issue did not occur....

As far as I know the amp hasn't been modified, it has apparently been serviced.
I'll pull the back off and have a look now I know the jacks aren't changing anything.

[Mick Bailey]

We can rule out the power amp - the problem is most likely within the preamp. You say that originally the master did zilch, but now its working. This, along with the operation of the drive control (raucous on 1 and disappears past 2-4) may point to a pot issue or the ground reference for the drive and master being lost. With either control, if the pot is correctly grounding then when turned right down no signal can pass to the power amp, so there should be no sound.

Do you know how to safely work on a tube amp, and do you have a DMM?

If so, my next step would be to discharge the amp and measure the resistance from the pot wiper (centre lug) to chassis ground for the drive and master pots respectively to make sure that the wiper grounds at zero rotation and then increases smoothly as the pot is rotated.

[threenofingers]

Sorry Mick, I probably didn't explain that very well...
When I switched from the gain to the "more gain" channel and the noise started, i immediately turned the master volume down but probably not completely, and wasn't aware it was the drive control at a low setting that was the main offender.
I'm not sure if that clarifies it a bit better.

[g1]

You say that originally the master did zilch, but now its working. This, along with the operation of the drive control (raucous on 1 and disappears past 2-4) may point to a pot issue

Sounds like the pots might just need cleaning?

[threenofingers]

Sounds like the pots might just need cleaning?

Sorry I'm a complete newbie to amps, I have played around with guitars quite a bit.
What's the best product to clean the pots with? I assume contact cleaner, I can get some WD-40 contact cleaner locally. I hope this is not a silly question, do the pots need cleaning from inside the amp or from the top...or both?

I've not worked on an amp before, as you can already guess, I do understand the capacitors hold a charge so I'll be very careful.
Since buying the amp I understand there's a lot of issues with factory build in regards to cheap leaking capacitors, diodes and resistors that should have clearance from the circuit board due to heat issues. At least I can inspect these areas and have an understanding of the work required if there's damage.
I've downloaded the schematic for the HRD so I can get a better understanding of the advice.
The amp is in great condition cosmetically, just need to check out the internal condition.

I really appreciate everyone's help.
I did make a video, but unfortunately having issues uploading it from the iphone.

[Mick Bailey]

I've never used WD-40 contact cleaner, but my experience with pots is that a specialist cleaner is the best thing to use - either Deoxit or Kontakt PR.

You clean from inside the amp. The pot has a gap in the shell where the legs exit and this is the entry point for cleaner. The straw needs to be bent at the end to get it to squirt into the opening. Sometimes wrapping a spiral of stiff wire round it helps, but for this series of amps I've not had to do this. It helps if you lay the amp flay on a bench or table face down. Only the briefest squirt of spray is necessary. Less is more. Too much will strip the lubricant from the pot shaft, resulting in a loose feel and depositing unwanted grease on the track. After a squirt, exercise the pot through its full range of movement and test the results. Better to do a couple of attempts rather than overdo it.