Hello,
I an new to this forum, so bear with me. I have a 1993 Rivera R30-112 amp that has a problem in the reverb:
- no noise in normal operation, with reverb at 0
- hum (looks like 50Hz) when turning on reverb, increasing with reverb setting
- hum persists with the reverb tank disconnected, so its not a lead problem.
I opened up the amp and did the following repairs:
- replace the V4 12AX7 lamp, no change. Old one is back in place
- replaced some of the big filtercaps near the rectifier diodes that looked dodgy (although they were still functional otherwise the amp would have more problems than just the reverb I suspect). (2 x 47 uF @ 450 V). No change.
Although a very neat PCB, desoldering is tricky with components sometimes soldered on both sides and/or glued to the PCB. With the problem still persisting I am now looking at the caps near the V4 lamp, especially C127 (coupling cap) and maybe C126 which looks to me to be a filter cap for the reverb return. C124 looks unlikely as it would also impair non-reverb operation, and the same goes for C123.
I have a basic knowledge of electronics (components), but little experience and knowledge about typical circuit design.
My question is: am I on the right path (C127)? Suggestions about other places to look at?
Many thanks in advance
I an new to this forum, so bear with me. I have a 1993 Rivera R30-112 amp that has a problem in the reverb:
- no noise in normal operation, with reverb at 0
- hum (looks like 50Hz) when turning on reverb, increasing with reverb setting
- hum persists with the reverb tank disconnected, so its not a lead problem.
I opened up the amp and did the following repairs:
- replace the V4 12AX7 lamp, no change. Old one is back in place
- replaced some of the big filtercaps near the rectifier diodes that looked dodgy (although they were still functional otherwise the amp would have more problems than just the reverb I suspect). (2 x 47 uF @ 450 V). No change.
Although a very neat PCB, desoldering is tricky with components sometimes soldered on both sides and/or glued to the PCB. With the problem still persisting I am now looking at the caps near the V4 lamp, especially C127 (coupling cap) and maybe C126 which looks to me to be a filter cap for the reverb return. C124 looks unlikely as it would also impair non-reverb operation, and the same goes for C123.
I have a basic knowledge of electronics (components), but little experience and knowledge about typical circuit design.
My question is: am I on the right path (C127)? Suggestions about other places to look at?
Many thanks in advance
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If you short circuit the reverb return lead/socket and the hum stops. the circuitry is probably OK and the return coil from the tank may be open circuit.
Hi Jon,
wow, that was quick. I will try what you suggest and let you know.
FYI, the reverb still works though very noisy, can this be the case with the tank output transducer broken? Also, why do I still have noise when the return lead of the reverb tank is disconnected?
thnx
wow, that was quick. I will try what you suggest and let you know.
FYI, the reverb still works though very noisy, can this be the case with the tank output transducer broken? Also, why do I still have noise when the return lead of the reverb tank is disconnected?
thnx
Because an open transducer in the pan is not really much different from and open circuit with the pan disconnected. So if the thing hums with nothing plugged into it, then it likely will hum with an open transducer plugged into it.
SO please do try what Jon suggested and ground that return jack or the grid of the tube directly. to see if it kills the noise.
And I suggest not focusing laser-like on it being a bad part. A broken ground connection can just as easily cause hum. As can lead dress and other factors.
SO please do try what Jon suggested and ground that return jack or the grid of the tube directly. to see if it kills the noise.
And I suggest not focusing laser-like on it being a bad part. A broken ground connection can just as easily cause hum. As can lead dress and other factors.
Thanks Enzo,
you are right. I tried shorting the RCA reverb return jack which should ground it, but the hum remains the same. The hum level only depends on the reverb setting, and is independent of the channel gain or volume settings. I analyzed it with an RTA and it is 50 Hz!
Could C123 (10 uF @ 450 Volt) close by be worth replacing?
Again thanks for any advice, learning!
you are right. I tried shorting the RCA reverb return jack which should ground it, but the hum remains the same. The hum level only depends on the reverb setting, and is independent of the channel gain or volume settings. I analyzed it with an RTA and it is 50 Hz!
Could C123 (10 uF @ 450 Volt) close by be worth replacing?
Again thanks for any advice, learning!
Hi again,
checked the grounding of the reverb return jack, and its OK. RCA signal to ground is 20 mV AC when reverb tank not connected, so looks Ok (or is it?).
checked the grounding of the reverb return jack, and its OK. RCA signal to ground is 20 mV AC when reverb tank not connected, so looks Ok (or is it?).
Power supply ripple would be 100Hz, so your 50Hz hum would be more likely grounding somehow or issues of radiated hum.
New info that maybe relevant: reverb pot is now clearly scratchy, so it may get subjected to DC. I'll try to measure it (without electrocuting myself). If so then it is likely it the cathode coupling cap C127 after all.
I will also check the grounding of the tube metallic protectors.
Maybe several things are at play at the same time.
The search goes on ...
I will also check the grounding of the tube metallic protectors.
Maybe several things are at play at the same time.
The search goes on ...
Hi guys,
I did electrocute myself, luckily without anything worse than a big fright and a very rapid heartbeat for a few minutes ...
Here's what happened.
I replaced the C127 cap, taking all the normal precautions (unloading caps, one rubber gloved hand, etc. ..). Then remounted the PCB and reconnected everything. Plugged the guitar in and took one glove of to fingerpick some strings. No sound, but I had turned down all the volume controls. Touched the bare volume control POT and BANG ... 300 Volts is not funny.
Think I will give this one to a professional ...
Thanks for all the help.
I did electrocute myself, luckily without anything worse than a big fright and a very rapid heartbeat for a few minutes ...
Here's what happened.
I replaced the C127 cap, taking all the normal precautions (unloading caps, one rubber gloved hand, etc. ..). Then remounted the PCB and reconnected everything. Plugged the guitar in and took one glove of to fingerpick some strings. No sound, but I had turned down all the volume controls. Touched the bare volume control POT and BANG ... 300 Volts is not funny.
Think I will give this one to a professional ...
Thanks for all the help.
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