Could be just a noisy transformer along with another problem which is killing the output.
Know how to drain caps, handy with an ohmmeter? Got a copy of the schematic?
Looked inside the chassis? Anything burned, smelling funny?
It might be fun replacing the cap cans, but spending $100 on 'em just to find the O/T is shorted would diminish the euphoria. And if you are not handy enough to do some basic fault finding, you might find replacing the caps a moisture releasing experience.
Ever worked on electronics?
Check the simple stuff. All tubes light? Patch cable to the speaker cab is good (the Mig is a head, isn't it?) Just this weekend I opened up the belly of a guitar amp, only to find my guitar cable was shot. Usually, I am smarter than the average beer, but sometimes not...
Know how to drain caps, handy with an ohmmeter? Got a copy of the schematic?
Looked inside the chassis? Anything burned, smelling funny?
It might be fun replacing the cap cans, but spending $100 on 'em just to find the O/T is shorted would diminish the euphoria. And if you are not handy enough to do some basic fault finding, you might find replacing the caps a moisture releasing experience.
Ever worked on electronics?
Check the simple stuff. All tubes light? Patch cable to the speaker cab is good (the Mig is a head, isn't it?) Just this weekend I opened up the belly of a guitar amp, only to find my guitar cable was shot. Usually, I am smarter than the average beer, but sometimes not...
Yeah. i know enough not to be too terribly dangerous. The noise from the tranny is definitely not normal. i'm going to test the caps tonight, and if they check out i'll test the OT. Either way though the caps are reaching the age when they should be replaced and i'm really just trying to find out what the effect of raising the uF value would be or if i need to find an exact replacement.
The tolerance of those caps is probably horrible. If you stay within 20% you are probably within factory specs. C19 and C20 are parallel - anywhere from 100u to 150u (total - no need for dual caps) you will not know the difference.
C18 is split. 100u goes to the output screens, 30u goes to the driver tubes. These need to be separate caps - either two discrete or another dual can.
The schematic suggest bias test points, which should read about 80mv. If you get about that, you would clear the B+ and your output tubes, suggesting either the O/T or something in the driver stage.
C18 is split. 100u goes to the output screens, 30u goes to the driver tubes. These need to be separate caps - either two discrete or another dual can.
The schematic suggest bias test points, which should read about 80mv. If you get about that, you would clear the B+ and your output tubes, suggesting either the O/T or something in the driver stage.
Burntheflags,
Perhaps repeating, but it is never a good idea to just start replacing stuff until the cure is found (like auto-repair 'experts' .... oops, sorry!).
If the filter capacitors are at fault, first thing is you will get hum (sound) and the transformer will get hot! As suggested, have a little fault-finding and measuring first.
Perhaps repeating, but it is never a good idea to just start replacing stuff until the cure is found (like auto-repair 'experts' .... oops, sorry!).
If the filter capacitors are at fault, first thing is you will get hum (sound) and the transformer will get hot! As suggested, have a little fault-finding and measuring first.
What Johan said:
Schematic for reference:
http://www.schematicheaven.com/newamps/sovtek_mig60.pdf
I would be carefully checking fuses 1 and 2 and in particular that they have the correct rating fuses in them.
You would not believe (or perhaps you might) the number of blown up, severely damaged guitar amps I have seen where after a minor problem had caused a fuse to blow someone decided to fit a fuse 10 times larger in value and thus increased the damage and repair cost by around the same (times 10) figure.
Also carefully check the diode bridges D1-D4 and D5-D8.
I would also check the 200mA 3kV protection diodes on the anodes of the output tubes. Shorted protection diodes often cause problems.
Cheers,
Ian
Schematic for reference:
http://www.schematicheaven.com/newamps/sovtek_mig60.pdf
I would be carefully checking fuses 1 and 2 and in particular that they have the correct rating fuses in them.
You would not believe (or perhaps you might) the number of blown up, severely damaged guitar amps I have seen where after a minor problem had caused a fuse to blow someone decided to fit a fuse 10 times larger in value and thus increased the damage and repair cost by around the same (times 10) figure.
Also carefully check the diode bridges D1-D4 and D5-D8.
I would also check the 200mA 3kV protection diodes on the anodes of the output tubes. Shorted protection diodes often cause problems.
Cheers,
Ian
Yes, i do agree with you Johan that the problem should be found before you go and start replacing stuff. i pulled the caps last night, tested them and sure enough two of the three were bad. So a recap should be in order. Just as a reliability issue i replace all of the crappy russian diodes a while back. Hopefully it will be up and running again this weekend.
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