I had started a post yesterday regarding a 1981 GP-3A board 112 RP SIxty-Five I recently acquired that had a constant, volume independent buzz, no guitar signal. I cleaned and recapped it, but no change.
In the process of asking for help and posting the measurements I'd taken, an idea struck me. When I tried it, it led me to the subsequent successful repair of the amp. A zener diode (D19) had shorted, grounding out itself and some other connected components, allowing an AC buzz and blocking signal flow. Replaced the offending zener, and the amp is working well.
Two minor issues left:
1 - The Phasor is a bit weak even at its full setting (10), with the trimpot adjusted for maximum "swirl". All components in the Phasor circuit chain check out OK, cleaned and swapped around ICs, no change. Maybe an iffy solder joint somewhere?
2 - Even with the Gain set at 0, (with master Volume at 10) a guitar signal passes through at what I'd call "living room" volume.
Ideas welcome.
112 RP SIxty-Five Back to Life Update
The 112 RP SIxty-Five is fully up and running!
The weak Phasor was due to a bad solder joint between the .01 cap and 100k resistor in that part of the circuit, I pulled the board and saw a number of wire "patches" done previously that I cleaned up. Got plenty of swirl at all settings now...
The remaining issues are minor, but would like to address them if anyone has suggestions:
1 - As mentioned above, even with the Gain set at 0, (with master Volume at 10) a guitar signal passes through at what I'd call "living room" volume. Shouldn't there be no volume at a Gain setting of 0? Maybe just a bad pot?
2 - When measuring bias across the 3.9 ohm emitter resistors, setting the lower of the two 6L6s to 25mV gives me a higher tube reading of 33mV (8mV difference). The measurements don't follow the tubes, i,e, when I swap them one for the other. Bad JE1692 driver, or one or more bad 470 resistors in the circuit? The MM Service Bulletin says the higher of the two should not exceed 55mv DC, so technically I'm ok but...
It's the 470 ohm resistors
Hi Inertian, The bias imbalance you see is caused by the 6-470 ohm and 2-3.9 ohm resistors not being perfectly matched. For the Gain thing, check the Gain Pot(s) and/or look for wiring shorts. -mgriffin
Both 3.9 ohm resistors have
Both 3.9 ohm resistors have drifted to about 4.6 ohms. Five of the six 470 ohm resistors measured a bit low, one somewhat higher. However, the measurements at Pins 5 and 8 on both 6L6 tubes are identical, so I'm thinking the error is just in reading the bias, and not the actual bias of the tubes themselves. Amp sounds great, tubes are not running hot.
Supposed the Gain pot could be bad, or some short somewhere, this amp was definitely worked on before I got it.
Thanks.