RP112 100 WATT COMBO NO OUTPUT

Regortteggel61

Good Morning, the amp in question when purchased had really low volume with the master on full and channel volume on 10. All the electrolytics have been replaced including the dog house and all the ics and the 2 transistors. There was significant evidence of heat round the valve clips and base. When the amp was turned on after the work being completed the bias reading across both 3.9 resisters was 81 and 86. On resetting a comfortable 25 mv and  26 mv was achieved. When an input was tried there is no volume at all running into an 8 ohm speaker. Measuring voltages on the valve bases resulted in,

pin 3 low position  460    high position  692.

pin 4   -  227

 pin 5  - 16

 pin 8 - 36.

 other pins 0.

Could anybody point me in the right direction on where to check next please, this is the amp from my previous post about the missing component , which Mike helped to discover was a thermal switch. Could this be an output transformer problem although there was output before the cap replacement was carried out,. Many thanks for reading this and fingers crossed.

Full Name
Mike Griffin
Music Man Equipment
MM 112-RD-65

Member for

11 years 4 months

mgriffin155 Tue, 07/15/2025 - 11:16

Pin 5 should be closer to +22V.  Per GP3A schematic, check the Zener D18 1N478.  The zeners voltage regulators are a common failure point in these amps.  BTW, the tube voltages look acceptable except pin 5.  It should be closer to +22VDC.  -mgriffin

Full Name
ROGER LEGGETT
Music Man Equipment
Music Man 2 x 10 65.
Music Man 4 x 10 65
Music Man RD50 1 X 10
Music Man 1 X 12 Extension cabinet.
Music Man HD150 Reverb head
Music Man 412 GS cabinet
2 x Fender Silverface 100 watt Twin Reverbs.
2 x Fender Silverface 135 watt Twin Reverbs.
1 x Sound city Concorde.
2 x Wem Dominator 50 watt.
1 x Wen Dominator Bass
1 x Wem Mk1 Audiomaster.
1 x Wem Mk2 Audiomaster
1 x Marshall JCM 800 100 watt head.
1 x Marshall JCM 800 50 watt head
1 x Marshall JTM 45 head.
1 x Marshall 4 x 12 A.
1 x Marshall 4 x 12 b
1 x Marshall 2 x 12 Vintage.

Member for

3 years 1 month

Regortteggel61 Tue, 07/15/2025 - 21:58

Good Morning Mike, thanks for your reply, I have tracked the problem down to a short on the 16v positive rail. After disconnecting the resisters and capacitors soldered to the rail I still had the short, rail to chassis. After checking many times all around the exterior of the track relative to other tracks I could not find any short. As a drastic last resort I made small cuts in the track prior to the 3 positions feeding the op amps, got a result, there was no short then, on the op amp side of the track. I then continued to make cuts in the track to eliminate other areas on the track, finally nailing it down to a short length of track feeding the long run of opamps. This seems to go against all logic as nothing is connected to the offending remaining piece of track, but the meter does not lie, there are no more shorts anywhere else now, only on the isolated remaining length. I intend  today to bypass the offending piece of track repair the cuts and see if I now have 16v positve at the opamps and hopefully a working amp. All the diodes were changed so fingers crossed.

Full Name
Mike Griffin
Music Man Equipment
MM 112-RD-65

Member for

11 years 4 months

mgriffin155 Wed, 07/16/2025 - 15:33

If it's a GP3A, was D19 16V Zener Diode voltage regulator verified to good?   Does it look like it has gotten hot or scorched?  Sometimes they short out when they fail.  You mentioned the diodes were changed.  All of them?  Which ones?  Double check the work for solder blobs.   Also, make sure all zener diode polarities are correct.  It's reverse of normal diodes.  Make sure to follow the polarities on the schematic.   Good luck.  -mgriffin

Full Name
ROGER LEGGETT
Music Man Equipment
Music Man 2 x 10 65.
Music Man 4 x 10 65
Music Man RD50 1 X 10
Music Man 1 X 12 Extension cabinet.
Music Man HD150 Reverb head
Music Man 412 GS cabinet
2 x Fender Silverface 100 watt Twin Reverbs.
2 x Fender Silverface 135 watt Twin Reverbs.
1 x Sound city Concorde.
2 x Wem Dominator 50 watt.
1 x Wen Dominator Bass
1 x Wem Mk1 Audiomaster.
1 x Wem Mk2 Audiomaster
1 x Marshall JCM 800 100 watt head.
1 x Marshall JCM 800 50 watt head
1 x Marshall JTM 45 head.
1 x Marshall 4 x 12 A.
1 x Marshall 4 x 12 b
1 x Marshall 2 x 12 Vintage.

Member for

3 years 1 month

Regortteggel61 Wed, 07/16/2025 - 21:44

Good Morning Mike, amplifier is now fixed, after cutting the track in various places to try and eliminate different areas it came down to a small piece of track about 1 inch long which was shorting, nothing was connected to that piece and it seems to go against all logic, but bypassed it and short was gone. All voltages were restored, back to one nice sounding amp. On my other Music Man restorations I replaced all the pots with Cosmos Tocos pots which are great sounding and mechanically smooth, so just that to do now and to replace the thermal switch. Many Thanks for your help. I have another RP112 100 coming this morning with a non functioning power transformer, so no dought more fun and games, but we have a good company who will rebuild to the original spec and original look, once again many thanks for your help, ps. the Newark website is down at the moment so could not check your link but its saying its only temporary. Regards, Rog.