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DOA Music Man 112-RP-65

Received a DOA 1978 RP65 recently, all the PO knew was that it sat in a garage and would not power on.

Seems to be all original except for two (incorrect) chassis nuts and (sloppily) replaced flyback diode and snubber cap on one power tube socket. Also one opamp (in IC-6 socket) is not the normal LM1458, it's some kind of TI that doesn't cross reference to anything familiar. Came with (probably replacement) clean and clear Telefunken EL-34s.

Cleaned all sockets and jacks, checked for anything loose, broken or burnt. Installed a correct 3A Slo-Blo fuse, turned amp Power on (with Hi-Standby-Low switch in Standby), the pilot light came on and the tubes began to glow.

Could I have gotten lucky that the amp only needed a new fuse to work again?

No. As soon as I switched from Standby to Low power, the fuse blew and the pilot light went out.

Removed all tubes and repeated the power on sequence with the same result.

Replaced the main filter caps with fresh ones, same result.

The rectifier diodes (MR510) in the doghouse measured a little flakey, so I lifted one end of each. Now the amp would stay powered on in Low or Hi power mode. So bad rectifier diodes, right? I replaced them with modern equivalents (1N5408-E3/54 3.0 Amp 1000 Volt from Mouser) and powered on again. And again, blown fuse.

Seemed like I was finding a lot of grounding points on the board under the doghouse, so I studied the schematic and traced the circuit back. Found that the red cloth covered HT lead from the OT that connects to a R/W wire feeding the doghouse board was grounded from the OT! Disconnecting it from the board, power stayed on in Hi or Low mode.

Does this definitively indicate a bad OT? I am getting good resistance readings at the OT wires at the power tube plates, but should the red HT lead measure grounded? Is it possible to open the OT and repair? Or do I just need to track down a replacement 5-75 OT?

Thanks for any advice.


Shorted OT?

The Red HT Lead on the OT should not be grounded. Use your DMM to check if there is a short between the windings of the OT, ie. the Red lead of the OT (tube side) and the BLK lead of the OT (speaker side). It should be infinite resistance. If it measures short circuit, then the OT has failed. If the OT measures open circuit between the windings, look for nicked wires and sloppy solder globs in all the wrong places. Good luck. -mgriffin


Appreciate the quick and

Appreciate the quick and detailed advice.

Both red and black OT wires show continuity to each other and ground...only 8ohms of resistance. So looks like OT is NG. First time I've run into that in working on many old MMs.

The good news is, this must be an early production RP65, it has a 3-65 OT, not the 5-75 OT shown in the schematic. I happen to have a new Mojotone 3-65 replacement OT on hand. Gonna swap it in and see if that brings the amp back to life.


I've heard

It's rare but I've heard they sound really great just before the OT gives out. :) -mgriffin


New OT installed, getting

New OT installed, amp producing good clean sound when guitar plugged into Line In/Out jacks only. Getting volume dependent hum (no signal) when plugged into normal input jacks, but I haven't recapped the rest of the amp yet, so hopeful that will fix that issue. Dead quiet at idle.


Good Job

Good job T'shooting the OT. I'm sure you'll figure out the input problem. -mgriffin


Thanks, your help was key to

Thanks, your help was key to my understanding of the problem's source.


And the answer is...

... IC-1 was bad. That was blocking the signal. Replaced that with a good 1458 and the amp lives again. Fully recapped and biased, sounds great.

Had to also replace the Phasor Rate pot, it only worked at the very end of its range. Disassembling the pot revealed only the very start and end of the track was still intact. Old pot was a 100K RA, I tried all the pots I had and settled on an old 25K audio pot that seemed to work the best in terms of speed range and sweep.

Just need to solder in the new snubber caps and flyback diodes, do some cosmetic cleanup, and declare this one done.

Only thing not great on the amp is its Reverb. It has a Cascade branded tank (made in Wisconsin) which appears to be original. Even on 10, Reverb is subtle. Tried swapping out IC-2, no change. Maybe a new tank, if I can determine which model is correct. Anything else to check?



Try this. 8AB2A1B. That's what's in my 1981 RD65. Late Rev. Schematics for reverb look similar. -mgriffin


Thanks for the suggestion.

Thanks for the suggestion.

The online sites that mentioned MM RP amps all said this was the correct replacement tank:

Mod® 4FB3A1B, Long Decay, 2-Spring

Ordered one, will report back when it arrives.


New Reverb Tank & Gain Mod

Received and tested the MOD 4FB3A1B tank, supposed to be the "correct" replacement for the RP 65.

The good: it has a smoother, more pleasant reverb tone
The bad: it is only barely stronger than the original Cascade tank the amp came with

Just to be sure, tried a different opamp in IC-2, no difference.

May go ahead and order an 8AB2A1B recommended above to see if it makes a difference.

Also did the gain resistor mod, changed the gain factor from a puny 1.8 to a huge (and original) 6.6! Night and day difference in clean headroom and volume. Before, at band rehearsal, I had the RP Volume on 8 with Gain on 0 to keep it clean. Normally my RD50 would be on about 2 in the same situation. Now the RP is as loud, if not louder.

Original RP GF: 6.6 (R48 = 3.3K, R49 = 22K)
This RP GF: 1.8 (R48 = 10K, R49 = 18K)
Modded RP GF: 6.6 (R48= 2.7K, R49=18)

I soldered a 1.5K and 2.2K resistor together in series (total 3.7K), then soldered that in parallel across the 10K R48 to get 2.7K.


Which Version RP

The GP1 and GP2 versions of the RP have a lower gain config for the reverb tank driver. The GP3 and 3A have a higher gain configuration. What vintage is yours? Don't have answers but these are clues. Also, 8AB2A1B has 10 ohm input and 4FB3A1B has1.5kohm input. -mgriffin



My amp is a GP-1, all components date to 1978. Has the earlier 3-65 OT, and lower gain resistors (R48 & R49), which I mentioned previously.

Are you saying I may be able to modify the reverb circuit to increase Reverb strength? That would be amazing!



Yes. I see no reason not to. For GP1&2, R15 (100K) and R17(100K) on IC2 control the gain of the reverb driver and Gain is 1. For GP3, R25(100K) and R27(220K) control gain so driver Gain is 2.2. For GP3A, R26(10K) and R27(270K) control gain so driver Gain is 27. The RD amps also follow the same gain progression through the revisions. Anyway, if you choose to swap out resistors, pay attention to GP1-R16(2.2K) and GP3-R26(4.7K). They are tone modifiers in the feedback circuit to attenuate bass. GP3A doesn't use them. For what it's worth, my RD65 uses the GP3A circuit driving a 10 ohm input tank 8AB2A1B and it's no slouch. Anyway, study the schematics and diagram it out. -mgriffin



Hi there,

I have a GP-1 and the reverb tank appears to be original (OC made in Wisconsin, if memory serves). The reverb tone is not as nice as in my 2 channel MM amps, with the large tank, but there is plenty of effect available. All of this is to say: Inertian - could it be that you have, say, a faulty cap C11 or C14 or a connection problem to the tank? (Failing RCA jacks or wires).



Good Suggestion

I tried different cables and tanks, including the "correct" 4FB3A1B but not the site recommended 8AB2A1B. FWIW, I seem to recall testing the C11 and C14 caps with a meter, measured ok. I can double check or clip in new caps to see if it makes a difference.

Have not yet tried changing the reverb gain resistors, but that is on the list as well.

Have the amp all together for sale, so I have been reluctant to open it up again. I'm not a big reverb user, but I know some people are. And I would like the amp to perform as it was designed.


Good Suggestion

Ooops, duplicate post.

Moderators, please delete.


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