Got a customer's absolutely gorgeous MM 212HD 130 combo on the bench. Looks like this one has lived a really easy life, but it's having some issues and I've gotten about as far as I can with the diagnostic. Any help from the forum would be greatly appreciated! There are no problems with channel one. The dry signal on channel two works just fine, but I'm not getting any reverb or tremolo effects.
I've been able to trace the signal all the way to the op amp just before the reverb tank. Looks to be a transconductance amplifier(?) configuration. The signal goes in a sine wave, comes out a square wave, but this might just be a property of the way that op amp stage is put together. When a reverb tank is attached the square wave at the output to the reverb tank (pin 6 on the op amp) changes to the wave in the first photo, the hand drawn waveform. Not sure if this is correct or not since the op amp is now under a load.
I've replaced the coupling cap at C21 heading into the makeup gain stage. The value on the cap read .1uF, but it was only reading .047uF out of circuit. The schematic doesn't have a value listed that I can see.
My next steps are to check the summing op amp up where the reverb signal is meeting up with the dry signal. Also, the foot switch does vary the signal on the scope at the volume pot so it's getting at least that far.
Going through the service bulletin and following the calibration procedure, I'm getting no change in the signal on the scope at Point D, the output of the preamp board when I vary the trim pot. All of the resistors around the low frequency oscillator 2N3391A (marked TR-1 on the schematic) are either within tolerance or have been replaced, so the voltages should be correct for the low frequency oscillator to work.
The customer replaced this transistor as well as the JFET at TR-2, but I can't verify that the replacement JFET is working properly as apparently the reading on these can JFETs can be skewed if you don't place the part in ESD safe foam. It's possible that this part is fine and I can't test it properly or that the replacement part was exposed to ESD when the customer tried replacing it themselves. I ordered a new JFET just to test everything out.
Unless I'm reading the schematic wrong, it looks as if the "wet" guitar signal doesn't ever enter the tremolo circuit. I'm planning on checking the cap at C25 tonight as I've read here on the forum that this can cause issues with the tremolo, but most just noise/hum not necessarily cutting the functionality of the tremolo altogether.
I appreciate any insight those who are more familiar with this amplifier and circuit are able to give! Let me know if anything in the above isn't clear or if you have anyquestions!
Link to the schematic:
- The chassis model is 2475 and the board number is GB-1
- All electrolytic caps have been replaced at some point in the past
- Customer replaced the 2N3391A and 2N4091 Transistors
- Reverb tank has been replaced at some point in the past
- Tried the original foot switch, as well as a replacement foot switch
Just wanted to post a quick update! I've been able to get the amp up and running 100%!
I was able to get the tremolo circuit working after replacing C25 along with the previous replacements. Everything is biasing and calibrated correctly now.
The reverb was a little bit trickier until I realized that the make up gain stage after the reverb tank wasn't amplifying the signal at all. Replacing the resistors attached to the op amp as well as the C20 cap to ground got the op amp working. I also found a few dead/skewed capacitors up near the first summing op amp, C15 and C16.
Tremolo and reverb sound amazing now and I hope this post helps some diagnostic times in the future!
Congrats on fixing it, major kudos!