p4spooky
Active Member
Hi All: Bought this on the bay for cheap since it had the protect light on. Here the issues I found (right channel)
1. Final outputs C-E short (2SA1215/2SC2921)
2. BIAS transistor 2SC3423 C-E short
3. Several fusistors open (R356,R357,R358)
Left channel appeared OK and DC offset was 2.3mV measured at R315
Repair Strategy:
1. Replace final outputs with working pull (bought from bay)
2. Replace 2SC3423 with same ordered from bdent
3. Drivers 2SA985 and 2SC2275 were OK - removed from board and tested OK
4. Replace 2SA1049 (Protect) with same part ordered from bdent
5. Replace bias pots with bourns single turn for both channels
6. Replace input cap on both channels
7. Replace diodes and both resistors on Speaker board (10R resistor on right channel was burnt)
8. Replace both 5A relay with 8A (Part# G2RG-2A4 DC24)
9. Replace all caps on PSU section and preamp section (note this was done AFTER the amp was repaired but still has the DC offset issue)
Completely rebuilt right channel with all new MF resistors/diodes/one input cap and new transistors from above.
Power up:
Powered up using DBT and came out of protect on first shot and no smoke
Bias adjusted to 7.14mV (spec is 7mV - pic shows bias while connected to DBT)
Took it out of DBT and readjusted bias to 7.14mV on both channels
Played music and sounds great.
Problem
Right channel has high DC offset (180mV)
Measure all voltages on right channel (see attached) and sure enough they are off. Emitter voltage on final output is 180mV instead of close to zero. Detailed measurements attached. Verified all resistor values and found them to be accurate. Went though and touched up all solder joints. Dc offset does not budge.
Need help to figure this out experts! Amp is running OK, and sounds good although I am yet to connect to scope. Want to fix the offset issue first. Attaching some relevant pics
1. Final outputs C-E short (2SA1215/2SC2921)
2. BIAS transistor 2SC3423 C-E short
3. Several fusistors open (R356,R357,R358)
Left channel appeared OK and DC offset was 2.3mV measured at R315
Repair Strategy:
1. Replace final outputs with working pull (bought from bay)
2. Replace 2SC3423 with same ordered from bdent
3. Drivers 2SA985 and 2SC2275 were OK - removed from board and tested OK
4. Replace 2SA1049 (Protect) with same part ordered from bdent
5. Replace bias pots with bourns single turn for both channels
6. Replace input cap on both channels
7. Replace diodes and both resistors on Speaker board (10R resistor on right channel was burnt)
8. Replace both 5A relay with 8A (Part# G2RG-2A4 DC24)
9. Replace all caps on PSU section and preamp section (note this was done AFTER the amp was repaired but still has the DC offset issue)
Completely rebuilt right channel with all new MF resistors/diodes/one input cap and new transistors from above.
Power up:
Powered up using DBT and came out of protect on first shot and no smoke
Bias adjusted to 7.14mV (spec is 7mV - pic shows bias while connected to DBT)
Took it out of DBT and readjusted bias to 7.14mV on both channels
Played music and sounds great.
Problem
Right channel has high DC offset (180mV)
Measure all voltages on right channel (see attached) and sure enough they are off. Emitter voltage on final output is 180mV instead of close to zero. Detailed measurements attached. Verified all resistor values and found them to be accurate. Went though and touched up all solder joints. Dc offset does not budge.
Need help to figure this out experts! Amp is running OK, and sounds good although I am yet to connect to scope. Want to fix the offset issue first. Attaching some relevant pics
Attachments
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