Hi there, new fault hardware over my desk.
I'm talking of a Marshall Guitar Amp 75 Reverb.
Turning on the amp a "uuummmm" high level noise come out from the speaker. If I connect a source at the input nothing change.
Attached you can find the schematic.
I checked all the +/- voltages, 15, 24 and 45 and all are OK. The input fuse is not burned.
With the speaker connected or disconnected the R44 R45 R46 R47 are soon (10 seconds) very hot.
With the speaker disconnected I measure 45 volt DC on the speaker output terminals. So I checked the two darlington MJ3001 and MJ2501 disconnecting them and to me they seem OK. I heard that a positive multimeter check on a darlington doesn't assure it to be good.
Base-Emitter measurements in diode mode are:
0.6 V and 1.3 V PNP
0.6 V and 1.6 V NPN
Base-Emitter in resistence mode:
4.3 Kohm PNP
10 Kohm NPN
All other junctions are easier to check and are OK.
I also checked "in board" all the other BJTs and diodes, other than shorts on all electrolytic caps with no signs of fault. I haven't yet checked the values and ESRs on caps. But I think that 45 VDC on the output is a short on some component rather than a modified value.
How you think I could proceed with other measurements?
I'm a beginner in the world of audio amplifier repairing.
Thank you.
I'm talking of a Marshall Guitar Amp 75 Reverb.
Turning on the amp a "uuummmm" high level noise come out from the speaker. If I connect a source at the input nothing change.
Attached you can find the schematic.
I checked all the +/- voltages, 15, 24 and 45 and all are OK. The input fuse is not burned.
With the speaker connected or disconnected the R44 R45 R46 R47 are soon (10 seconds) very hot.
With the speaker disconnected I measure 45 volt DC on the speaker output terminals. So I checked the two darlington MJ3001 and MJ2501 disconnecting them and to me they seem OK. I heard that a positive multimeter check on a darlington doesn't assure it to be good.
Base-Emitter measurements in diode mode are:
0.6 V and 1.3 V PNP
0.6 V and 1.6 V NPN
Base-Emitter in resistence mode:
4.3 Kohm PNP
10 Kohm NPN
All other junctions are easier to check and are OK.
I also checked "in board" all the other BJTs and diodes, other than shorts on all electrolytic caps with no signs of fault. I haven't yet checked the values and ESRs on caps. But I think that 45 VDC on the output is a short on some component rather than a modified value.
How you think I could proceed with other measurements?
I'm a beginner in the world of audio amplifier repairing.
Thank you.
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