Hello all, this is my first post despite lurking here for a few months now, so obligatory capsule introduction: I'm an old hand with electronics though pretty new to solid-state amps. I've got a reasonably solid grasp of transistor theory but getting a blown-up power amp back into the saddle is still hit-or-miss for me.
So I'm begging the forum's collective indulgence for help with my homework: an old SWR SM-400 that's on my bench today.
It came to me in terrible, terrible shape. Both amplifier modules blown, filter caps blown and half the chassis covered in remnants of corrosive fluff from the caps. Repairing that damage was time-consuming but rewarding: this beast at least looks like a bass amp once again, the power supply's alive again, and all preamp functions have been restored.
I've rebuilt one of the power amp modules and now I've got trouble. I'm powering the chassis with a series 100W bulb for current limiting, and the rails are at +/- 43.3V. Output transistors are MJ21194G's purchased recently from Digi-Key (it seemed much less risky than buying 2SD424's on eBay). I replaced all of the other semiconductors as well, plus new electrolytics and several new resistors. I started with very nearly a bare PCB!
Now, the measurements:
The output is pegged just 0.5V below the +ve rail, and yet none of the transistors seem to be blown. Referring to the attached schematic, Q14-C, Q7-B, Q4-B, Q4-E, and Q3-C are all at 43.3 V.
Q9-B/Q10-B are 42.8V, as is Q9-E and Q11-C. Ditto for Q5-B and Q6-B.
So the high side drivers seem to be fully on, bottom side drivers definitely off, and I'm baffled why.
Any suggestions?
So I'm begging the forum's collective indulgence for help with my homework: an old SWR SM-400 that's on my bench today.
It came to me in terrible, terrible shape. Both amplifier modules blown, filter caps blown and half the chassis covered in remnants of corrosive fluff from the caps. Repairing that damage was time-consuming but rewarding: this beast at least looks like a bass amp once again, the power supply's alive again, and all preamp functions have been restored.
I've rebuilt one of the power amp modules and now I've got trouble. I'm powering the chassis with a series 100W bulb for current limiting, and the rails are at +/- 43.3V. Output transistors are MJ21194G's purchased recently from Digi-Key (it seemed much less risky than buying 2SD424's on eBay). I replaced all of the other semiconductors as well, plus new electrolytics and several new resistors. I started with very nearly a bare PCB!
Now, the measurements:
The output is pegged just 0.5V below the +ve rail, and yet none of the transistors seem to be blown. Referring to the attached schematic, Q14-C, Q7-B, Q4-B, Q4-E, and Q3-C are all at 43.3 V.
Q9-B/Q10-B are 42.8V, as is Q9-E and Q11-C. Ditto for Q5-B and Q6-B.
So the high side drivers seem to be fully on, bottom side drivers definitely off, and I'm baffled why.
Any suggestions?
Comment