Working on a '79 RP 65 112, GP-2 board, EL34 tubes. Was functional but noisy and weak, after a good cleaning (pots, jacks, tube and opamp sockets, etc.) the tone cleaned up, Reverb and Phasor became strong, but still not as much volume as it should have. Replaced all electroyltics (18 caps in all) and the volume came roaring back. However, two issues:
1 - the Reverb seems much less pronounced now. I pulled the tank to clean the jacks and plugs, and tried connecting it both ways (only works plugged in one way). I'll test with another tank to see if maybe the original tank has a bad/disconnected spring. If it's not the tank, where to check next?
2 - the Phasor has a good range of Depth, however now only starts when the Rate is turned up to 7 or so. It will run at an appropriate speed for that setting and on up to 10. And if I turn the Rate knob down slowly, the Phasor Rate will slow, but stop completely at about 3 or 4. Ideas what to check? I'm thinking a resistor must be out of spec or a cap connection isn't solid.
Thanks.
Reverb tank
If the tank is original, I’d replace it especially if it is the short OC tank. The MOD 4FB3A1B is an excellent tank. I put this in my 100RP and my 112RD65.
Ewan
Checked the tank...
...and it's a full size Gibbs tank with no model # anywhere I can see. Suppose it could be original. Actually very good sounding, just weak. Have to turn the Reverb control up to 10 for an equivalent setting of 3 or 4 on my RD 50.
Thanks for the tank recommendation, I will track one down to try.
Gibbs reverb tank
I’d never heard of a Gibbs tank being used in a Music Man amp. Is this one from a Fender Reverb unit? I came across this saying the vintage Gibbs unit needs higher settings in the Reverb unit reissues. https://surfguitar101.com/forums/topic/2793/
Some folks here report Gibbs tanks but...
...doesn't mean they are factory original. I think all the originals I've seen were Accutronics.
The amp itself looks like it had never been worked on, however the reverb tank bag and cables are definitely not original - so I'll go with the tank not being original as well.
More on Gibbs tanks
It seems that Gibbs tanks were used on blackface and silverface Fenders with reverb. The impedance on these would be wrong (8 ohm in 2,250 ohm out). If the transducers were changed to the 4FB3A1B specs then it would work.
Phasor Issue Resolved! Reverb Awaiting New Tank
Turns out the Phasor issue was gunk in the Rate pot preventing proper contact with the carbon track at lower settings. I knew something was off when I couldn't get a stable resistance reading on my multimeter with the pot set to 0.
Unsoldered the connecting wires, opened the pot and cleaned, lubricated and retensioned. Put it all back together and it's as good as new.
As for the weak Reverb, the pot and all related circuit components check out, so I ordered a 4FB3A1B MOD tank. Hopefully that solves the problem.
Tank Arrived, Reverb Update
The new MOD 4FB3A1B tank arrived, installed in the RP 65 last night.
Definitely a good sounding reverb, longer decay, more pronounced sound BUT still not as strong as it ought to be.
I know the tank (new), cables, and footswitch (removed for testing) are good. The resistors in the reverb circuit tested well within spec, and I've tried swapping ICs around. Is there anything else to check that I'm missing?
Think I'll order some new 1458 ICs, suppose it's possible the originals are all tired (the amps is 40 years old!). If new ICs don't fix the problem I'll next replace the few resistors the reverb circuit. After that...I'm out of ideas.