Rockford 250, circa 2001, PC-3080-E. Had a blown IRF540 on right channel, blown trace & diode on the RCA shield, etc. Replaced the 540, diode, trace, and checked everything I thought necessary.
Amp works and produces clean output, but I have dc offset 10mV on the left channel and 100mV on the right. I checked and even swapped MMBT5088L Q202 and Q102 side to side with no change, and also compared them out of circuit and they seemed fine. I was looking at these because when I was checking components I get different readings from left & right when I have positive probe on the base and negative on the collector. On the left channel I get around 22k ohm and on the right I have around 6k ohm, but I haven't found anything else out of spec in close proximity to these.
How do I track down DC offset issues when it's relatively minor and still have clean output?
Amp works and produces clean output, but I have dc offset 10mV on the left channel and 100mV on the right. I checked and even swapped MMBT5088L Q202 and Q102 side to side with no change, and also compared them out of circuit and they seemed fine. I was looking at these because when I was checking components I get different readings from left & right when I have positive probe on the base and negative on the collector. On the left channel I get around 22k ohm and on the right I have around 6k ohm, but I haven't found anything else out of spec in close proximity to these.
How do I track down DC offset issues when it's relatively minor and still have clean output?
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Do you mean mV instead of mA offset?
Are the 2 degeneration resistors in on the emitters of the differential amplifier transistors within tolerance?
Is the resistor Rx22 within tolerance in each differential amplifier?
What is the DC voltage across that resistor after the end of the mute delay?
Are you measuring DC offset directly across the 2 speaker terminals for each channel?
Are the 2 degeneration resistors in on the emitters of the differential amplifier transistors within tolerance?
Is the resistor Rx22 within tolerance in each differential amplifier?
What is the DC voltage across that resistor after the end of the mute delay?
Are you measuring DC offset directly across the 2 speaker terminals for each channel?
Yes, sorry, mV not mA.
I'm not sure I know which you are referring to in the next two questions. Are you talking about the same sets of resistors with those questions? The 33ohm resistors (R220, R222, R120, R121) all read right at 33ohm for all four (both channels). For some reason Rockford didn't seem to mirror the numbering scheme on this like they have in the past.
I'm not sure I know which you are referring to in the next two questions. Are you talking about the same sets of resistors with those questions? The 33ohm resistors (R220, R222, R120, R121) all read right at 33ohm for all four (both channels). For some reason Rockford didn't seem to mirror the numbering scheme on this like they have in the past.
Does the voltage match across all of the 33 ohm resistors?
Voltage across R276 and R122?
Are you measuring DC offset directly across the 2 speaker terminals for each channel?
Voltage across R276 and R122?
Are you measuring DC offset directly across the 2 speaker terminals for each channel?
Voltage across the 33ohm resistors is:
R220 0.038
R222 0.036
R120 0.039
R121 0.036
R276 12.16
R122 12.16
Yes, measuring DC across speaker terminal outputs
R220 0.038
R222 0.036
R120 0.039
R121 0.036
R276 12.16
R122 12.16
Yes, measuring DC across speaker terminal outputs
Didn't you say that you earlier swapped one transistor from each of the two differential amplifiers and there was absolutely no difference in the offset you read across the speaker terminals?
What's the DC voltage across the two bases of the differential amplifier transistors for each channel?
What's the DC voltage across the two bases of the differential amplifier transistors for each channel?
Yes, I swapped Q202 and Q102. There was maybe a slight change. I now have 94mV instead of 100mV.
Across right channel Q202/Q203 bases: 0.006v
Across left channel Q102/Q103 bases: 0.005v
Across right channel Q202/Q203 bases: 0.006v
Across left channel Q102/Q103 bases: 0.005v
If you drive the amp to clipping into a speaker/dummy load, does it drive cleanly to both rails?
If you heat the differential amplifier transistors, one at a time, does heating one cause the offset to go towards 0v DC?
If you heat the differential amplifier transistors, one at a time, does heating one cause the offset to go towards 0v DC?
It appears to be clean up to clipping. It's clipping a bit below rated output, but that could be my test setup power supply wiring is not very heavy gauge at the moment.
Heating Q202 causes DC offset to rise. Heating Q203 causes DC offset to to drop and swing negative, but I can get it to hold near 0v if I back heat off to just the right amount.
Heating Q202 causes DC offset to rise. Heating Q203 causes DC offset to to drop and swing negative, but I can get it to hold near 0v if I back heat off to just the right amount.
It seems that everything beyond the differential amplifier (responsible for DC offset and error correction) is working properly.
If there is no excessive DC leaking through the coupling capacitors, the problem may simply be a mismatch of the differential amplifier transistors. The 10mv channel doesn't need to be resolved but the other does. The transistors should be from the same production batch. You may have to do a bit of trial and error to get the offset to an acceptable level.
If there is no excessive DC leaking through the coupling capacitors, the problem may simply be a mismatch of the differential amplifier transistors. The 10mv channel doesn't need to be resolved but the other does. The transistors should be from the same production batch. You may have to do a bit of trial and error to get the offset to an acceptable level.
That sounds like I should try swapping Q203 & Q103, to see if the problem follows (since I already swapped Q202 & Q102). I will need to order these if they are the problem.
Which caps are you referring to?
Which caps are you referring to?
C111 and C223.
You can swap the others in the amp or could try whatever transistors you have on hand (for testing) to see if you can get acceptable offset.
You can swap the others in the amp or could try whatever transistors you have on hand (for testing) to see if you can get acceptable offset.
Ok, the DC offset followed Q203 when I swapped it with Q103.
Thanks Perry, always on the right track
Thanks Perry, always on the right track