Ampeg ReverbRocket Reissue - weird buzz problem

Kyle B

Silver Supporting Member
Messages
5,099
Soooo...... I took on a charity job. I'm doing it/did it for free. The guy who contacted me said he bought it from his friend, 'cuz his friend needed $$$ for medications (he has cancer). He wants it fixed up, and he's gonna give it back to him at a benefit they're throwing next month, tell him not to sell it again! There's gonna be a couple bands - he's gonna put the amp on-stage and present it at some point. Good man :) He wasn't looking for a freebie, but after that story I just had to do it. He didn't even have the scratch to do whole cap job, even at my ridiculously low prices. Enough said - You get a free work. I get good karma :)

It's an Ampeg ReverbRocket reissue (Yes PCB).

It was brought in completely non-functional (bad tube). The pots were all scratchy & intermittent beyond usefulness, so I opened 'em all, cleaned them, bent the wipers a hair so they hit virgin material, and they all work like new now :)

The line-in jack was wonky too --- The internal switch barely making contact, causes a periodic wildly loud hum. Easy fix

It really sounds nice now

EXCEPT

There's a fairly loud 120Hz buzz on the clean channel only.

It's there with all the knobs @ zero and guitar plugged in. Plugging in a guitar doesn't change it at all. You can't hear it when playing, but in a quiet room it's pretty annoying.

When you flick to the dirt channel, all knobs still on zero, there is NO buzz.
Very strange.

The buzz does not get louder as the volume is turned up (clean channel). When the dirt channel is turned up loud enough I can hear the same buzz - but it doesn't seem "abnormal' in that case. On the clean channel it's omnipresent and doesn't respond to any controls EXCEPT
1) Changing the channel switch to dirt makes it go away completely
2) Turning up the reverb to full makes it go away A LITTLE.

The amp doesn't really have two channels -- It just plays around a little with shorting out resistors and pots. That's why it's so perplexing that I hear this on clean channel only, and not the dirt channel. For the most part, all the same components are still in-circuit!


I tried rolling tubes, resoldering sockets, replacing a few (but not all ) electrolytics, etc.... To no avail so far....

Heck - I'm not even sure if this is 'abnormal' operation. I gotta think it IS, that they wouldn't market the amp with a buzzy clean channel, but I've heard crazier things.

This is the schematic - It seems accurate:
http://www.el34world.com/charts/Schematics/files/ampeg/ampeg_r-12r_212r_50h_reverbrocket_ri.pdf


I put a shorting-plug into the LINE IN, and the buzz went away completely. Suggests the problem is in the preamp.

The caps I replaced were C33, C34, C35, C36, C41, C42 and C38. I can see the buzz on certain components come & go as I flick the switch -- It's a 120Hz rising sawtooth waveform. I saw something of it on all those caps, so I figured it was a good shot to try 'em.

I don't see any noticable ripple on the B+ rail

The noise seems to be a rising sawtooth @ 120Hz. I can find it on quite a few components (but oddly I can't seem to find it in the signal path anywhere). There's a volt or so of this sawtooth on D6. That means the relay coils are somewhat buzzing -- would this somehow translate into the signal path perhaps?????

I saw a little ripple on the quad-opamp (U2) power pins, so I thought maybe the op-amp power supply was noisy, main reason that I changed all those smaller caps. There's noise on pin 7 only --- none to be see on any other pins.

Any thoughts?? :)

It's just a little frustrating since I'm not sure what "normal" operation is.

If I can't solve it relatively soon, I'm just giving it back as is. It's in dramatically better shape than it was 4 days ago! But still, I just hate to do that if I know it can be made 'perfect'.
 

Nige

Silver Supporting Member
Messages
41
I know this is an old thread but I have the same amp with the same problem! Any resolution to this problem?
 

Kyle B

Silver Supporting Member
Messages
5,099
No :(

It was a Charity Repair (i.e. free) so I decided I'd spent enough time on it and cut bait. Sorry! I disassembled & reassembled it like 8 times before I'd decided enough was enough.

If you do ever find the solution please contact me though :)
 

UsableThought

Member
Messages
1,649
I am working the same issue. I have had some success, but still not all the way there yet.

www.diyaudio.com/forums/instruments-amps/285959-ampeg-reverberocket-vs-hum.html
I have just posted over there suggesting that you use a signal tracer + spectrogram rather than trying to assemble the noise waveform via scope only - that much work is like Hercules cleaning out the stables and you will never get as complete & easy to read "picture" as with an FFT spectrogram. Plus a couple of other suggestions. This thread is pretty ancient so I won't repeat it all here.
 
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