Hybrid Music Man 212-65 1980 Build - Learning As I Go, Some Questions

[keithb7]

I am reporting back on my amp repair. Hoping to to document my findings for future Music Man 212 Sixty-Five amp owners.

I did source 7 new NOS Motorola LM307H op-amps from E-bay. They are stamped "Made In Korea". I put them in the amp and turned it on. Seemed like I had loose connections somewhere.
I removed all the Motorola op-amps again and cleaned them with de-ox-it. Reinstalled and voila! We gad all amp functions working properly and sounding good. I still had a little
touch of unwanted distortion at all times. I adjusted the output tube bias, cooled it down a bit and the amp sounds great. Exactly what I expected it to sound like. The amp is very loud and clean. As I turn up each
channel's volume the pre-amp section starts to distort a little. At about 4.5-5 on the channel volume knob.
Seems this is expected in the original design and it sounds nice. I used the master volume to control overall volume while cranking up pre-amp volume.

A couple of things in summary:
The current made in China LM307H op-amps did not work well in this MM amp. They emitted some nasty distortion. By swapping the old and new Chinese op-amps in, I was able to confirm the new Chinese quality was certainly inconsistent. All the NOS Motorola op-amps seem to work great and the amp now sounds great.

A list of parts I did replace while servicing my amp:
All e-caps on main board, the bias board, on phase inverter board, B+ caps under dog house on bottom of chassis.
All new Diodes through out the various circuits
7 New NOS LM307H op-amps
A matched pair of EL34 power tubes
2 PnP JE1692 Transistors on PI board heat-sinks
1 qty 10K Linear Master volume Pot
1 qty Channel 2, Double gang volume pot (stock spec is one 10K linear & one 10K audio pot. Could not locate. Installed a dual gang 10K linear. Sounds great)
Cleaned any connections, jacks, pots, with de-ox-it
Set Bias

Thanks for your help folks. You steered me in the right direction with the NOS op-amps.

[loudthud]

With the quasi-square pattern on the PCB, you can remove the socket and just solder the new DIP opamps to the PCB as follows. Spread pins 2, 3, 6 and 7 to hit the corresponding pads on the PCB. Mount the chip as high as the pins will allow, while still making good solder connections to the PCB. Bend the other pins out so they don't touch anything. The only other pin you have to connect is pin 4. Run a short wire from the PCB to the IC. You can cut pins 1, 5 and 8 off the IC.