Problem with Musicman RP-65 (2165-RP) volume.

[PfeifferElectronics]

I've been restoring a 2165 and I'm now stumped as to what is wrong.

The problem is the volume is way too low until I turn the gain knob all the way up, then it jumps to full volume.

I have replaced the output xfmr, replaced all the electrolytics accept a few that check out ok with my esr meter. (2uf) I've replaced all the op-amps with TL072s. I've replaced the JE1692s with 2N6488Gs. Replaced the Gain pot. Checked the tubes with tube checker. Replaced IC3 and IC7 sockets.

Readings (low/high)

6L6 pin 3: 500/669
6L6 pin 4: 250/369
6L6 pin 5: 16/16
TR1: 1.1V
IC7: pins 1,2,6,7 all nothing.
Voltage across opamp pins 8 and 4 is 26V

There is also a "knee" in the volume knob with a scope attached to C24 or C25 coming from IC7 pins 1 and 7. What I'm seeing is at about 3 or 4 the sine wave turns to a complete blur.

thanks for any help you can provide!

[Enzo]

You replaced all that stuff chasing this??

COmplete blur? Like a wide band of light instead of a clean line on your scope? Sped the sweep way up and see if that complete blur resolves into a waveform. It sounds like you are describing the amp going into oscillation at RF.

If so, those TL072s could be at fault. The amp used 1458s, and the 072s I think have a higher open loop gain and may tend toward the unstable. I know ther are some Marshall SS models where a 4558 in place of a 1458 is enough to make the amp unstable. Note the gain pot is not a voltage divider like the volume control after the next stage.

On my schematic there are HI and LOW level loop jacks. The HI jack right before that master volume. Connecting into that ought to disconnect the preamp. Can you drive a test signal into that and bring full output to the power stage?


IC7 pins... "nothing" um, does that mean zero volts or something else? I don't like to see "nothing" as a reading.

IC power rails, we generally would prefer to see separate readings for the two rails to let us know the rails are balanced. Proabably OK here, but can;t hurt to scope the rails to see if any signal is riding them.

I doubt it is involved, but I am unsure about that transistor cross at only 80v. Not sure how high signal peaks go there, but I usually use something higher voltage like a common MJE15032.

Your tubes are probably OK, but be careful using a tester. Better testing is putting the tubes in another amp that works. Tube testers usually do not put real world voltages or loads on tubes. If a tester says a tube is bad, it usually is, but when a tester says good, you can;t be sure without also doing a functional test in real world conditions.

A lot of this hinges upon what you were actually seeing on your scope. If it is indeed RF oscillation, we wold want to go back to IC1-1 and then IC1-7 while we ran a strong sine into the amp. And ultimately IC3-1. We want to see how far the clean signal will progress through the amp before turning into that blur.

And look at the schematic, whatever appears are IC7-1 ought to appear at IC7-7, after all the second half of IC7 gets its signal from pin 1.

[PfeifferElectronics]

Thanks for the info Enzo. I replaced the opamps chasing this, the other stuff needed to be replaced anyway. This amp is for a friend of mine and he said that it was in a flood. Had to clean everything. The amp worked for a few minutes, then this issue started. Then I changed the opamps but that didn't help.

I get a clean sine wave up until I get to IC7, then the wave voltage drops off. I'll do some more scoping and get you some better info.

Injecting a signal into the hi jack exhibits the same issue.

16V at 6L6 pin 5 seems too low. Thinking it might be the zener diode.

[Enzo]

COnsidering that the zener is a 1N4743, which is a 13v, I'd say that 16v is darn close to right on. AFter all, what is on the cathodes, +90 or something?

[PfeifferElectronics]

My schematic has D9 = 1N4748. I just did some more scoping and the signal on pin 7 of IC7 is so low my scope can't measure it.

[Enzo]

AH, my drawing looks like 4743, but I guess that 3 could be an 8, the print is not that good. That would be a 22v zener then. yes, low, not thinking it is the problem though, but should be fixed.

[PfeifferElectronics]

I replaced IC7 and IC3 with the old LM1458 and the oscillation problem is fixed! Now I still have the problem with the gain knob. Low volume until the gain is turned all the way up, then very scratchy/distorted output. With my scope I get a good (4V) signal out of all the op amps until I get to IC3, then it drops. On the the transformer side of R50 (which is the shielded 150K lead) I get a .5V signal. On the IC7 side, 0V. Same with R49 18K resistor, 0V on the pin 2 side. This is with the gain knob at 5. When I turn the gain up to 7.5, then I start getting a nice sine wave on IC3.

[Amp Kat]

Are you sure the pot is ok ? Would also put the 1692's back in as a flood really can't harm them and they very seldon go bad. Monitor the signal on pin 3 of the pot and see if it's dropping at the pot or at the wiper. If it's good up to the pot then the pot is bad. Grounded grid amp so the signal comes in at the power tube Cathodes which are controlled by the 1692 Xistors so they may not be turning on correctly until full signal is applied.

[Enzo]

It is unusual when you cannot "upgrade" your op amps to a more modern type, But here is the perfect example. I have a tube full of 1458s in my 4558 drawer for just such occasions.

[PfeifferElectronics]

Update, the GP-1 schematic is wrong? I was told to turn C24 and C25 around and the amp started working! I'm one step closer but there are more problems.

I still am not happy with the sound I'm getting from this amp. There is a weird 'farting' sound when the gain is at 8. It also seems that there isn't enough volume until the gain is at 9, then the amp 'turns on' and the volume jumps quite a bit.

Since the schematic is wrong as far as the orientation of C24 and C25, I'm wondering if other errors exist. What about C16 and C18 on the gain and volume pots? Is the schematic for GP-1 correct?

Here is what I'm observing at this time. I bias the amp with the lowest side at 25mV and the higher side is at 31mV or so. When I apply a signal and crank it up, the low side hovers around 100mV and drops as soon as I stop playing. However, the high side climbs to over 300mV and does not seem to drop when I stop playing. I've also observed that the high side tube is beginning to red plate. I've measured resistance on both sides across the 3.9 bias resistors and they are at 4.5 ohms. The 470 ohm resistors measure the same on both sides. I've swapped the two power tubes and the same the problem remains on the same side, so it isn't a bad tube.

[g1]

a) Did you sort out the zener diode issue?
b) Did you put the original 1692's back in as Ampkat suggested? Some of these amps were very picky about subs. for those transistors causing oscillations.

[PfeifferElectronics]

Yes, I installed 1N4748. Unfortunately, I threw away the original JE1692s. I installed 2N6488s. I hooked up my oscilliscope and observed a 'blur' on the positive going sine wave when the volume gets to about 7 or 8. Not sure what that means.

[g1]

That means you have an oscillation issue. You can ignore what I said about the 6488's, I think we were using 6292's as subs. and having problems with them. Musicman later
recommended the 6488's as the best sub so you should be good with those.
Is it for sure a GP-1? GP1,2,&3 used EL34's, GP3A used 6L6. Attached are schems. for GP-3 & GP-3A.
As far as the oscillation (and bias), please see service bulletins 1,6, & 7 here:
Music Man Amps Discussion Forum: Music Man Service Bulletins
They do note in bulletin 1 that IC7 must be a 1458.

[PfeifferElectronics]

I've tried swapping IC7 with other 1458s with no change. I have a GP-2. On the schematic for the GP-2, the 20uf electrolytics are not shown as polarized like the GP-1 so I'm not sure what direction things are supposed to go. Also, R48 is 3.3k for the GP-2 schematic, but a 10k for the GP-1. I have a 10k here. I think I'll try a few more 1458s for IC7.

[g1]

The caps should be oriented the same as the other models. I'm a bit puzzled as to why they were in backwards or why they wouldn't work backwards.
Does the setting of the bias trim affect the oscillation? From what I gather from the bulletin, bias is min. 25mV, max. 55mV.
You have 6L6's in an amp that is set up for EL34's. Have changes been made to accommodate them? Have you tried it with EL34's?