Fender pro junior bad transformer and red plating v3 tube

[bluestom50]

I am sure that there is no output from the output transformer-- proper readings into transformer but no signal out-- could this cause red plating v3 power tube? As always any insight will be appreciated thanks, tom

[The Dude]

Do some preliminary testing of the transformer. Open primary? Open secondary? Shorted primary or secondary? Both halves of primary measure approximately the same?

I would first suspect bad tube(s). Swap the tubes around does the red plating follow the tube or the socket? Or just try known good tubes.

[g1]

You never responded to the first suggestion in the other thread ( http://music-electronics-forum.com/t40083/#post392567 )
Did you ever check the grid voltage at pin2 of the power tubes?
It should be about -10VDC at each (that is negative voltage). If it is not proper for V3, that will cause red-plating.

[bluestom50]

I have -9.3 vdc on v4 and - .7 vdc on v3 2.5 vdc at r20 cp1 275 vdc cp2 290 vdc cp3 278 vdc r23 9.5 vdc r21 9.77 vdc r22 9.77 vdc any idea what to check for using this

[52 Bill]

I have -9.3 vdc on v4 and - .7 vdc on v3

Remove the tube (V3) and measure the voltage at pin 2. Does it stay low or does it get back to -10 volts dc?

If it stays low, then you need to find out why. If it comes back up, then you need to try a different tube in V3.

[bluestom50]

With tube removed I had 127 vdc at pin 2 also r20 has 127vdc at both ends i read -6 vdc at r29 any idea where to go next? Thanks, Tom

[The Dude]

Well, I've no idea what those circuit board designation numbers mean without a schematic. Could you please post one so we know what you're talking about?

From what little I can glean with the provided information, we don't have the correct bias on one of the output tubes. I would unsolder the coupling cap from the PI to that grid and see if it goes normal. Maybe you have a leaky cap.

[52 Bill]

If there is positive voltage at both ends of R20, then C8 is leaking voltage or there is a solder bridge on the board that is allowing the positive voltage to get there.

In the other post, you never told us what was changed in the mod that you did. Was C8 part of the mod? Even if it wasn't, you need to recheck the things that you did and try and find the reason for the positive voltage at R20.

[Jazz P Bass]

Well, I've no idea what those circuit board designation numbers mean without a schematic. Could you please post one so we know what you're talking about?

From what little I can glean with the provided information, we don't have the correct bias on one of the output tubes. I would unsolder the coupling cap from the PI to that grid and see if it goes normal. Maybe you have a leaky cap.

Schematic:
http://music-electronics-forum.com/a...atic_rev-f.pdf

$80.00 Fromel Kit:
Fender Pro Junior Modifications ? Fromel Electronics

[bluestom50]

I found a solder mistake where i crossed one trace to another no more red plate and equal -12 vdc at v3 and v4 #2 pins. Still no sound except when testing voltage at c10 r19 and r18 a static sound from speaker? Any ideas? I am not going to give up and will eventually fix this amp. As usual thanks for your help. Tom

[g1]

Well, the good news is you don't have to worry about your output transformer anymore.
Have you checked over the rest of the work you did installing the kit?
If you are sure there are no more mistakes, a good place to start would be to check the DC voltages on the preamp tubes. They are shown in the square boxes on the schematic.

[The Dude]

The upside to that particular schematic is that it also shows signal levels. Inject a signal and trace it through. See where it gets lost. If you don't have a signal generator, you can find MP3 test tones all over the web. Use your PC or cell phone or whatever as the source to play the file. Use your DVM to trace the signal if you don't have a scope. A bit crude, but it works.

[52 Bill]

Thanks for the links JPB, but even after looking at the link, we still do not know what mods were done to the amp. Did he install the $20 kit or the $80 one?

For all we know he has wired the new Switchcraft input jack wrong and there is no signal getting to the amp at all.

[bluestom50]

I made a few changes to the pcb and now my test points are reading correctly except tp 2 at r3 which reads 232 volts on both sides of the resistor. right now I am burning out and hate these cheap pcb's and ribbon cables. I really need some encouragement and any thoughts arereally appreciated. It was the supreme everything fromel kit and I am sure the jack is wired correctly. Any help is appreciated. Thanks, Tom

[52 Bill]

I made a few changes to the pcb and now my test points are reading correctly except tp 2 at r3 which reads 232 volts on both sides of the resistor.

If there is no voltage drop across R3, then the tube is not conducting. Could be a bad tube, no heater voltage, an open ground in the cathode circuit.

I really need some encouragement and any thoughts arereally appreciated.

Hey, you'll get it fixed. Don't worry, when you finally get it sorted out there will be a great feeling of accomplishment. Maybe you can get yourself a little treat once it's fixed as a reward.