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First of all I'd like to introduce myself as a new member of the forum. Although a 15+ years El. Eng. (and a 25+ years guitar player) I'm working with tube amps only for the last couple of years. So I would like to share one issue that I came accross....
I bought an all-original 135W 1978 Fender DSR that I found in a really nice price (that was actually a steal) and it came with the original 15'' JBLs in an almost MINT condition so I felt I could'nt resist. The previous owner used it barely 15 years ago...
After I picked the amp, for the first 2 times that i turned it on, it worked (though sounding very weak) however the next time I tried to turn it on it started bringing down the mains automatic fuse, sometimes with a flash between the lugs of the power-on switch!!!! I decided that I should go through a thorough inspection and that's what i did, replacing also the filter caps. Then I started charging the new caps (everything seemed to work fine) with a Variac at 80 volts (I live in Europe) and left. After 15 minutes i turned back just to realize that the room was full of smoke and the chassis close to the PT was very hot.
Turned everything off, did a new isnpection just to realize that the cable going from the electrolytics to the two 100K resistors (after the rectification bridge) was barely holding - actually the minute i touched the wire it fell off the terminal strip - that was actually an open circuit!! The big cement 20W 30K resistor was also way too off (around 2,5MOhm!!!) and when I tried to remove the terminal strip, it broke in half.
I changed the resistors (both 100K and the 20W) and installed new terminal strip. Then I checked the fuse just to find that it was an old slo blo with the wire forced to stay in place (someone did obviously a horrible job - that also misleaded me because a correct fuse would blow and indicate directly a short circuit). I changed the fuse and I tried to power on but....nothing. The new fuse was blown. I tried with another PT that I had spare (for 135W fender) and same thing happened. I went all the way through the power line and rectification and found that the two of the four diodes at the rectification bridge where shorted ...so that was the main problem.
Changed the diodes (all four) with new ones and with the new PT, checked the voltages and everything seems to work fine now. Next thing that i'm going to try is the old PT - if i'm lucky enough it will be working....and I will replace the power-on switch which still shows some malfunction (sometimes it does not power-up). However, since i'm fresh to tube amp maintenance and I may be missing something important here, any ideas or comments would be more than welcome.....
I bought an all-original 135W 1978 Fender DSR that I found in a really nice price (that was actually a steal) and it came with the original 15'' JBLs in an almost MINT condition so I felt I could'nt resist. The previous owner used it barely 15 years ago...
After I picked the amp, for the first 2 times that i turned it on, it worked (though sounding very weak) however the next time I tried to turn it on it started bringing down the mains automatic fuse, sometimes with a flash between the lugs of the power-on switch!!!! I decided that I should go through a thorough inspection and that's what i did, replacing also the filter caps. Then I started charging the new caps (everything seemed to work fine) with a Variac at 80 volts (I live in Europe) and left. After 15 minutes i turned back just to realize that the room was full of smoke and the chassis close to the PT was very hot.
Turned everything off, did a new isnpection just to realize that the cable going from the electrolytics to the two 100K resistors (after the rectification bridge) was barely holding - actually the minute i touched the wire it fell off the terminal strip - that was actually an open circuit!! The big cement 20W 30K resistor was also way too off (around 2,5MOhm!!!) and when I tried to remove the terminal strip, it broke in half.
I changed the resistors (both 100K and the 20W) and installed new terminal strip. Then I checked the fuse just to find that it was an old slo blo with the wire forced to stay in place (someone did obviously a horrible job - that also misleaded me because a correct fuse would blow and indicate directly a short circuit). I changed the fuse and I tried to power on but....nothing. The new fuse was blown. I tried with another PT that I had spare (for 135W fender) and same thing happened. I went all the way through the power line and rectification and found that the two of the four diodes at the rectification bridge where shorted ...so that was the main problem.
Changed the diodes (all four) with new ones and with the new PT, checked the voltages and everything seems to work fine now. Next thing that i'm going to try is the old PT - if i'm lucky enough it will be working....and I will replace the power-on switch which still shows some malfunction (sometimes it does not power-up). However, since i'm fresh to tube amp maintenance and I may be missing something important here, any ideas or comments would be more than welcome.....
. Anything I get for repair (or pick up used) I try to remember to check for proper fuse before turning on. It's amazing how many are the wrong type or rating or not really fuses at all.
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