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RP 112 65 red plating on one tube

I recently aquired an RP 112 65.
Sounded GREAT!!

However after playing it for a few hours it started to hum. Now I new it most likely needed a cap job, so no surprise.

I have replaced all electrolytics in the amp. Some values seems non existant though so I took what I could get.
Some differences in values now.
The 150uF caps next to the diodes are now 100uF (would it be better to go up in value?)
The 400uF next to them are now 470uF

and all the 20uF caps are now 22uF.

I measured the old caps and they had drifted enormously! The 400uF caps were at 512uF!

I started the amp up and it hummed loud and red plated on the first of the two 6L6 GCs.
Turned it of, switched positions on the tubes. Same thing. Red plating on the first one.
So no bad tube.

Looking at the pcb there are a burn mark under the two large zeners so I replaced those.
No difference.

I'm trying to take measurements on pin 5 and get very different results on the two tubes. Also when I measure across the bias resistors.
I have tried to alter the bias with the bias pot and got rid of the red plating but the amp now humms in what sounds almost like tremolo oscillations.. Like a wobbling humming.
I'm not that knowlegeable on MM amps and believe that the tubes are somehow driven by the transistors below the bias pot?

I am lost right now however and need help with suggestions..
Only thing I can think of now is that the new cap vaules made it go bananas.. But the burn mark tells me that the old caps perhaps has been drifting for a while..


It is the RP 2165 with GP-3 board.


Shorted Transistor?

Double check voltages at the zeners and ensure they are all correct. For the tube that is red plating, check the power transistor that feeds it to see if it has shorted out or is shorted to ground. Also, it would be a good idea to put in the correct value caps in the hi voltage power supply. BTW: the work you've done looks very clean. Good luck. -mgriffin


Do you know what voltages I

Do you know what voltages I should see there? Sadly the schematic does not tell anything about voltages at different spots..
I am suspecting the transistor as well! Is this a common thing to have issues with on these amps? I think I've read somewhere before about driver transistors gone bad.

A thing that puzzles me is the tape underneath the transistors. There were tape under one of them, so I did a continuity check on the right one. It has continuity between the heatsink touching the chassis and a couple of the leads. My guess is that it had tape but it fell of when I replaced caps. That means it must have had a short to ground. The black tape is my temporary solution, but it seems the damage is done.

Yes! I ordered caps with the correct values. Hopefully they arrive soon. I agree that it would be better.

Thank you for that! I appreciate it very much! I take pride in tidy work ☺️


DC Specs

To see the voltage specs, take a look at the lower right section of the schematic. Look for D11, D12 and D7. http://www.pacair.com/mmamps3/sites/default/files/docs/2165-rp_and_2100-...


Just so that we are on the

Just so that we are on the same page. I believe I have the GP-3A version. Since it has 6L6s.
The diodes you mention would be from the GP-3 model? With 6CA7 tubes?

Sorry, I know I said GP-3 at first but I believe it must be GP-3A.


So now it is V2 that is red

So now it is V2 that is red-plating...
V1 doesn't do it anymore.

No tubes in I get 22,5V at pin 5.

Tubes in and in standby I get 11,2V at V1 pin 5 and 8,7V at V2 pin 5.

On the transistors, the middle leg has 10v on the first one connected to V1.
But on the second transistor connected to V2 I get about 0,11V on the middle leg.
I believe this is connected to the cathode of the tubes?

Thing is I can't figure out if the bias pot is balance or actual bias.

This is in truth strange..

Oh and I put in the correct cap values!


Usually when red-plating

Usually when red-plating occurres I look for negative voltage or the lack of it causing the tubes to go wild.
In this amp it is more complicated or different at least..
I can't find where to measure negative voltage in the bias circuit. Or is it as simple as measuring at the bias resistors by the transistors and skip looking for negative voltage?


Bias Reset

Measure bias at positive side of C35. It should be between .6V and 1.4V. Mid point is about 1V. If I may be so bold, may I suggest we back up a little to get a solid footing. 1) Turn the amp off and remove the tubes. 2) Center the bias pot to its mid point. 3) Ensure both transistor insulators are properly installed and use a DMM to verify that the collectors are not shorted to ground. 4) With power on and tubes out, use a DMM to verify that there is +16V at D19, -16V at D20 and +22V at D18. Those 3 voltages must be there before proceeding. 5) If that goes well, power the amp off, install the tubes and power it on. If either tube red plates, shutdown immediately. Most likely one or both power transistors are bad and will need replacement. Replace both transistors with 2N6292. See parts list on page 7 of schematic. 6) If the tubes don't red plate, then follow the bias adjust procedure at the top of this document from Downloads and Resources. http://www.pacair.com/mmamps3/sites/default/files/docs/MM%20Service%20Bu... Good Luck. mgriffin


Thank you!!

Thank you!!
This is precisely what I need! Great step by step info!
IF the transistors are bad I have the replacement recommended by MM at a later point in time. I believe it is the 2N6488 or similar number. The ones later used by MM. Should work as replacement.
I can't wait to get this beautiful beast up and running again! It really really sings.


So i get these readings:

So i get these readings:

D18: 22.4vdc
D19: 26.8vdc
D20: -31.2vdc

The caps are brand new spragues installed by myself and I feel confident that they are good.
The 390ohm 3w resistors though.... They measure fine, but that does not mean they do their job..
Also the diodes: D14 - 17 might need replacing.
That is what comes to my mind at least. Any ideas?


D19,20 Zeners

The voltage at D18 22V Zener Diode is good. The voltages (26.8 & -31.2) at D19 and D20 16V Zener Diodes are not good. Those voltages indicate those 2 Zener Diode regulators are not working. Were they replaced with 1N5353 16V Zener Diodes? BTW: The 390 Ohm resistors are OK. Also, since you are measuring 26.8 vdc and -31.2vdc, the D14-17 full wave rectifier circuit is working perfect. Double check and make sure 1N5353 Zeners are installed for D19 and D20. If you have to replace them, I would recommend standing them off the PCB with a 3mm air gap between the zener diodes and the PCB. As your picture shows, they do get muy caliente (hot) under normal operation. -mgriffin


Hmm.. The ones installed now

Hmm.. The ones installed now is 1N5383B zeners.
When I ordered them I was under the impression that they were equal the 5353 version. But I might have gotten it wrong. You think that could be it?
At least as far as these voltages.

Great tip about standing the diodes! The heat is no joke!



That explains a lot as those will not work for this application. 1N5383B is 150V 5W so they will never even turn on. Remove those and replace with 1N5353 or equivalent 16V 5W zeners. -mgriffin


Ah... *Slaps forehead*

Ah... *Slaps forehead*
You know I was thinking that they would handle 150V without breaking. Diodes is apparently not my field of expertise.
I ordered 5353s now though and hope they will turn up soon.
This makes me think that the zeners were the issue all along and perhaps why I got red-plating.
However there seems to be something fishy about the transistors, so first things first. Stabilize the voltages.


One step at a time

Live and learn. The 3A schematic is a bit misleading as it incorrectly uses diode symbols for zener diode voltage regulators. Check the GP3 schematic and it shows the correct symbol for zeners. As for the 2 power transistors, they must have insulators between the mounting tab and the sheet metal chassis. The mounting tab is the collector and should never touch ground (earth). Also, make sure to use nylon screws and nuts when securing the transistors to the chassis. It's also a good idea to uses some insulating heat conductive paste between the insulators and the collector tabs of the power transistors. -mgriffin


Yep, every mistake is a

Yep, every mistake is a lesson learned.
In a way I'm glad I made this mistake because I feel I got a better understanding of the circuit.

I thought about that and put thermal paste under them. I have measured and they are isolated from ground.
The screws are metal but there are plastic bushings from both sides that locks into the holes so that the screw can't move and touch.
One of the isolators were missing though so I have temporarily put a piece of electrical tape in its place.
Seems to be working fine for now, but with heat I'm not so sure.
I'll see if I can find a better solution.


I've installed the correct

I've installed the correct zeners now and the amp works like a charm again!
Thank you so much for your invaluable input!

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