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So I've got this free MG8080RCD I've been bugging Enzo about 
(Thanks again for the schematic)
It's got no speaker and the input jack was munched, and 3 of the 4 shoulder washers (it seems that there may originally have been 5) have had their sheaths.. like flattened and partially melted...
Amazingly, it seems that the collectors never shorted to the heat sink - the fuse and power supply are intact, and I think the output stage works fine (I hooked it up toa dummy load when I first got it and wiggled an instrument cable around in the destroyed input jack until I got 60hz buzz, which came out of the line out jack (which feeds off the output of the power amp) just fine... altered the tone of the 60hz buzz with the treble knob of the clean channel, and shut the thing off - I didn't want to leave it on for more than a couple of seconds, which was wise, because I discovered upon taking it apart that someone had already done so and (for some unknown reason) had unscrewed 3 or 4 of the xstrs from the heat sink, screwed all but one back in(the middle xstr was hanging in the breeze), munching all but one nylon shoulder washer's sleeve, and moved the mica insulator from the middle (non-conductive case - why was there a mica insulator in the first place?) to another transistor(which now had 2, decreasing thermal coupling). Clearly, whoever did it knew not what they were doing.
Anyway, I think the damn thing works - it just needs an input jack, a few new nylon shoulder washers (#4screws, fits through to-220 and to-3p mounting hole) some fresh thermal compound, and it should be good to go.
2 Questions:
1) Does anybody know a part # for a suitable replacement jack? It's a dual switching type - 1 switch shorted tip to ground with no cable and the other switch shorted to the sleeve (ground when nothing was plugged in (I think the other end of the switch went to a mute circuit later in the amp to prevent speaker pops when a cable was plugged/unplugged. I'll post a picture of where it mounted to the board, but I can't show you the jack - it had EPOXY of all things on it and was a mess - I carefully removed it by crushing and plying it away from the board... now I have 4 leads and I've followed the traces so I know which goes where...
2)Is it even worth it? Should I packrat the board and build something better from scratch inside the chassis, which is in decent condition? I've never heard one of these. Supposedly the preamp is highly versatile and pretty amazing sounding for an LED clipping circuit... it's got an efx loop with a -20db switch, a reverb tank (which may or may not work) seems like a killer little practice amp. I was thinking about lowering the rails and biasing it well into class AB because I don't need 80watts (overkill - I play for myself at home - bedroom levels) and my big complaint about SS amps is that when they're running in class B (or barely in AB), which they almost always are, at low levels like I play, they sound.. thin and breathy... like the kinky, wispy tones that you can get out of a of a moog pulse width modulation synth. I can only assume that this is crossover distortion from cheap ass power amps, but who knows.
Anyway, here's the photos - if anyone knows what I can stick in there, let me know
thanks,
Nevermind, I can't find the effing cable for my digital camera. I'll post pics if no one can help on this one
Worst case scenario, I have jacks I can use... and I can rig up a sort of standby spst switch to hook the mute to ground
Michael Miller
(Thanks again for the schematic)
It's got no speaker and the input jack was munched, and 3 of the 4 shoulder washers (it seems that there may originally have been 5) have had their sheaths.. like flattened and partially melted...
Amazingly, it seems that the collectors never shorted to the heat sink - the fuse and power supply are intact, and I think the output stage works fine (I hooked it up toa dummy load when I first got it and wiggled an instrument cable around in the destroyed input jack until I got 60hz buzz, which came out of the line out jack (which feeds off the output of the power amp) just fine... altered the tone of the 60hz buzz with the treble knob of the clean channel, and shut the thing off - I didn't want to leave it on for more than a couple of seconds, which was wise, because I discovered upon taking it apart that someone had already done so and (for some unknown reason) had unscrewed 3 or 4 of the xstrs from the heat sink, screwed all but one back in(the middle xstr was hanging in the breeze), munching all but one nylon shoulder washer's sleeve, and moved the mica insulator from the middle (non-conductive case - why was there a mica insulator in the first place?) to another transistor(which now had 2, decreasing thermal coupling). Clearly, whoever did it knew not what they were doing.
Anyway, I think the damn thing works - it just needs an input jack, a few new nylon shoulder washers (#4screws, fits through to-220 and to-3p mounting hole) some fresh thermal compound, and it should be good to go.
2 Questions:
1) Does anybody know a part # for a suitable replacement jack? It's a dual switching type - 1 switch shorted tip to ground with no cable and the other switch shorted to the sleeve (ground when nothing was plugged in (I think the other end of the switch went to a mute circuit later in the amp to prevent speaker pops when a cable was plugged/unplugged. I'll post a picture of where it mounted to the board, but I can't show you the jack - it had EPOXY of all things on it and was a mess - I carefully removed it by crushing and plying it away from the board... now I have 4 leads and I've followed the traces so I know which goes where...
2)Is it even worth it? Should I packrat the board and build something better from scratch inside the chassis, which is in decent condition? I've never heard one of these. Supposedly the preamp is highly versatile and pretty amazing sounding for an LED clipping circuit... it's got an efx loop with a -20db switch, a reverb tank (which may or may not work) seems like a killer little practice amp. I was thinking about lowering the rails and biasing it well into class AB because I don't need 80watts (overkill - I play for myself at home - bedroom levels) and my big complaint about SS amps is that when they're running in class B (or barely in AB), which they almost always are, at low levels like I play, they sound.. thin and breathy... like the kinky, wispy tones that you can get out of a of a moog pulse width modulation synth. I can only assume that this is crossover distortion from cheap ass power amps, but who knows.
Anyway, here's the photos - if anyone knows what I can stick in there, let me know
thanks,
Nevermind, I can't find the effing cable for my digital camera. I'll post pics if no one can help on this one
Worst case scenario, I have jacks I can use... and I can rig up a sort of standby spst switch to hook the mute to ground
Michael Miller
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