Another Fender Tube AMP on the bench, this time a perfect condition Fender Twin Reverb. This amp seems to blow fuses when power is supplied, therefore ending up in my hands.
Wow, quite amazing, I may be the first one in this amp, as it would seem. This however lends itself toward the problem as the original Electrolytic caps cant possible be good. The explosion under the cover confirms my findings. This is a complete recap job (so far as electrolytic caps go).
I can never bring myself to remove the old caps without testing them, so naturally they go on the Genrad and the IT-11 for expected awful results. But hey, why not. This also included all of the Electrolytic caps needed to the Cathode bias, each was removed and replaced. Finally the last electrolytic cap in the bias was swapped AND upgraded to 100uf.
I did also check all of the coupling caps; listed and tested in the IT-11, and they did turn out to be in perfect order so they were not replaced.
Not everything is fun. Like having to measure every resistor, but yeah...... funny thing though, there were more out of tolerance resistors than I expected. So swap-outs definitely needed to be made before continuing. Some resistors off as much as 80%
Funny how one POT was found to have whiskers all over it and none of the others. a quick observation.
Naturally tube testing is done, this is one of the last videos to exclusively use the Century tester. Not that i'm getting rid of it, but the Hickok 6000 will be talking over soon. All tubes tested good.
With that the VARIAC dial up of the unit begins...... aaaand its shut down. It seems the plate voltage appears regardless of the standby switch position. Looks like its time to troubleshoot a short. Not only that but the voltage on the plate is the rectifier voltage. I wont spoil it, the troubleshooting and issue is found in the video.
I did break out the Pevono PS 305-H to search for the short instead of using mains voltage and the lethal voltage in the AMP.
With the short repaired from the last video, we see the working standby functioning, all of the tubes are loaded in for a complete observation. Everything now working as expected.
The is the magic moment where I hook up my garbage test speaker, for initial evaluation. Wow, all of the potentiometers sound like, and feel like they haven't been moved in a while, and will need some Deoxit, but the Amp itself it totally quiet.
As expected, Deoxit does the job. This brought the project to the complete voltage annotation phase. Corrections are then made to bring everything within spec.
During this time, I am monitoring the current of the output tubes so I can see what the deviation is across all four.
This is followed up by a test of the amplifier functions. Some requiring a shorting plug for lack of a foot pedal. With that its time for a chassis re-installation and a test run by the wife for correct operation.
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