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212 Sixty-Five Channel 2 - Problems

Hi folks I recently purchased a used 1980 212 Sixty-Five amp. Solid state phase inverter. Chassis 2475-65.
When I got it, it worked but seemed a bit anemic. Yet everything seemed to work. I ordered new e-caps and diodes. All in the amp were original from 1980. I installed all the new e-caps only first, and flashed up the amp. Channel 1 is working great. Seems fine. Channel 2 is not working correctly. Lots of distortion and lower volume. Turing the dual-gang pot on channel 2, the low volume also cuts in and out. Severely distorted guitar signal. I went bank in and swapped in some of the new diodes I planned to change. I also swapped around some positions of op-amps. I set the bias to 25mV. No improvement to the problems with channel 2. I rechecked my work and polarity of all caps and diodes that I installed. All seems right to me.

I am new to solid state amps. Most of my hobby experience comes from building and repairing vintage Fender tube amps. I do have some diagnostic tools. I read up a bit on op-amps and went back to the circuit to test and learn some more. I plugged in a 4 ohm speaker, chassis out and up on the bench. I measured +16V and -16V power for all the op-amps.That seems fine. I hooked up a signal generator and input a fixed Hz signal into channel 2. I set up my CRT Oscilloscope to monitor the AC wave signal going in to the amp, channel 2 input. I chased the sine wave signal to the #3 op-amp input. The signal was the same as input at line 2 in-put jack. I then checked the output sine wave at op-amp 3 and saw the large increase in gain at pin 6. I believe this tells me op-amp 3 likely is working. (op-amp 3 is next in line right after channel 2 input). I tested the resistors around all the op-amps, all appear to be within spec. I tested the dual 68K channel input resistors, all good. I lifted 1 leg of all the coupling caps around several op-amps. All measured with my DMM. Nano-farads appear accurate as labeled on each cap. I am a bit stumped as my knowledge is limited. I can't quite find an obvious problem here.

I turned off the power and started tracing component connections by continuity checking. To learn the layout of the circuit compared to the schematic. I think my amp may be slightly modified as I am getting reverb out of both channels. I don't think this normal, when studying the schematic. I see a couple of wires and resistors added on top of the board, likely related to reverb to both channels I suspect. I suspect something is amiss with the dual gang volume pot at channel 2. The center lug on one pot, has no wire hooked up to to it. There are no spare loose wires anywhere in the chassis.

The past couple of days I have been troubleshooting. Temporarily inserting some new parts where I suspect there may be a problem. Any replaced parts that don't improve, are removed again and stock put back in. In my mind, I keep going to the channel 2 dual gang pot. It's making squawky noises now. As of this writing, now no signal at all is being amplified through channel 2. Possibly a component was failing, and now it's completely failed.

The dual gang pot. I suspect 1 pot is for volume control. Does the other pot somehow control the wet reverb mix coming back into the signal? I can upload photos and my schematic inquiries if someone here is able to assist with guiding me through this. A led light behind the PCB has really helped with quickly identifying the circuit track.

Thanks very much.

RD112's picture

BackLight - Great Idea!

I can’t help with the diagnostic part…but the use of the backlight on the PCB is genius.
You don’t mention anything about the Tremelo/vibrato circuit. It appears the foot switch activates the Reverb and Tremelo/Vibrato circuits…with Reverb available to both channels, and Tremelo only on Channel 2?
Good luck with the repair.


212-65 Follow Up

Through trial and error I have discovered that I have issues with either the op-amps or their related sockets. I have the amp working now. I’ll try getting some Deoxit to clean the socket connections. I did buy five new LM307H op-amps from Ali-Express. Those helped me narrow down the problem. Does anyone know if new op-amp sockets are available? I spent some time searching Google. No luck. Thx.

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