Fender Twin Red Knob : switching channel fuse issue...

[samouraiYo]

Hello,


I recently bought a Fender The Twin Red knob in *mint condition*, honestly this amp looks like it never been plugged...
Anyway throught it sounds pretty good, I've noticed that channel switching system does not work, neither channel LED.


I checked the schematic online http://forum.musikding.de/cpg/albums/userpics/16766/FENDER_THE_TWIN.pdfand i guess the fuse T305 was blown : i was right, so I changed it and started the amp again.
The Clean channel was shinning but after 10secs, the fuse went out again.


As the switching system PSU in shared with the BIAS system, i guess i could have faulty power tubes...


Does anyone has any clue about this issue and the way to investigate ?


Best Regards,
Yoann

[sluckey]

Anyway throught it sounds pretty good, I've noticed that channel switching system does not work, neither channel LED.

...i guess the fuse T305 was blown : i was right, so I changed it and started the amp again.
The Clean channel was shinning but after 10secs, the fuse went out again.

I assume you mean F305 rather than T305? F305 does feed the negative bias supply and the positive low voltage switching supply. When that fuse blows you lose both supplies. I would not expect the amp to sound pretty good. I would expect the output tubes to red plate and die pretty quickly. Pull the output tubes until this problem is resolved.

It's unlikely that there is a problem in both bias supply and switching supply. Divide and conquer. Remove diode D306 to isolate the switching supply. Replace the fuse. Does it hold? If so, the bias supply is likely OK. Measure the negative voltage at pin 5 of every 6L6 to be sure. If this all looks good, turn your attention to the switching supply. Most likely suspects are C303 and C304 and the diode you removed. Check those components first.

If the fuse still blows with D306 removed, the bias supply is at fault. Most likely suspects are D305, C305, and C202. Check those.

Report your findings please.

[PRR]

I too would expect loss of bias to be a PROBLEM. Tubes melting very quickly.

So I wonder if that fuse really blew. Or if there is some other problem.

52V at 0.25A means the fault resistance must be less than 208 Ohms. There are three paths from the 52VAC winding. One through a 3.9K resistor. Two through diode-cap rectifiers then through 1.2K or 10K resistors. None of the resistors, even all three together, are low enough to blow this fuse.

I believe your bias is working, but the switching supply has failed. I have re-drawn this messy schematic to highlight this system.

Point "D" (junction of R306, C303, D108) should be +25V to +30V DC.

Footswitch jack should have 30V AC. With no plug, there will also be a negative voltage.

[samouraiYo]

Thanks guys,
So much useful advices, i ll definitly give a shot when ... i ll have new fuses to blow