J_Verschueren
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what is discrete amplifier
Hi,
I'm trying to repair the amplifier of a 12-channel mixer/amplifier for a local theatre company. Circuit diagrams : **broken link removed** (2.2MB pdf).
The problem concerns the right channel power amplifier (page 9). When it came to me it had a DC voltage on the output, triggering the protection circuit (page 10).
The only things I could find wrong in it were Q31 and Q32 (page 9 again), which were very low on DC current gain (12 instead of a minimum of 50). Replacing them "sort of" fixed the problem in that the protection circuit is no longer triggered, however, the bias current through this amplifier is now 850mA (should be 17mA), which is way too much, obiously, and there's still quite a lot of DC on the output compared to the other side (70mV compared to 5mV on the left side).
A quick (in-circuit) check of the passive components doesn't reveal any obvious errors (I tested all of the transistors out of the circuit) and comparing set point voltages between the two sides doesn't throw up big differences except at the collectors of Q33 and Q34. In the left (good) side (page 8 / Q11&Q12) they are +2.00V and -2.00V repectively, whereas in the (faulty) right side they're +1.75V and -0.57V. I'm wondering whether this is actually an indication of a faulty component somewhere or just a result of having way too much bias current through the power section.
Is there any intelligent way of sorting this out (i.e. without desoldering and checking every component on the PCB + checking all the tracks)? The PCB design is poor and the quality of the PCB itself is poor as well. I would like to avoid doing a lot of soldering in this thing. I also don't want to spend a disproportionate amount of time on it, if possible.
Any comments/hints appreciated.
Hi,
I'm trying to repair the amplifier of a 12-channel mixer/amplifier for a local theatre company. Circuit diagrams : **broken link removed** (2.2MB pdf).
The problem concerns the right channel power amplifier (page 9). When it came to me it had a DC voltage on the output, triggering the protection circuit (page 10).
The only things I could find wrong in it were Q31 and Q32 (page 9 again), which were very low on DC current gain (12 instead of a minimum of 50). Replacing them "sort of" fixed the problem in that the protection circuit is no longer triggered, however, the bias current through this amplifier is now 850mA (should be 17mA), which is way too much, obiously, and there's still quite a lot of DC on the output compared to the other side (70mV compared to 5mV on the left side).
A quick (in-circuit) check of the passive components doesn't reveal any obvious errors (I tested all of the transistors out of the circuit) and comparing set point voltages between the two sides doesn't throw up big differences except at the collectors of Q33 and Q34. In the left (good) side (page 8 / Q11&Q12) they are +2.00V and -2.00V repectively, whereas in the (faulty) right side they're +1.75V and -0.57V. I'm wondering whether this is actually an indication of a faulty component somewhere or just a result of having way too much bias current through the power section.
Is there any intelligent way of sorting this out (i.e. without desoldering and checking every component on the PCB + checking all the tracks)? The PCB design is poor and the quality of the PCB itself is poor as well. I would like to avoid doing a lot of soldering in this thing. I also don't want to spend a disproportionate amount of time on it, if possible.
Any comments/hints appreciated.