Hello everybody! I am attempting my first repair on the plate amplifier from a QSC HPR 153F 3-way pa speaker.
I recently bought it used and had it freighted to me. It definitely sat in the cold for a little while. When I first powered it on, I noticed very little bass compared to the other HPR 153F I already had. A few seconds later it made a loud honking sound and flipped it's internal breaker.
Taking a look inside revealed a few shorted transistors (2SC5200/2SA1943)
I also noticed all of the 0.1R 2W resistors were measuring around 0.4R.
The pcb has a "hot" side with all through hole parts. I think I'm going to just replace all of the parts on this side. I've also noticed a couple open circuit resistors on the smt side I will need to replace.
Should I be considering replacing the electrolytic capacitors as well?
Cosmetically they look fine.
Also, should I check the resistors in the other speaker I have? Could this have been the cause of the failure of the amplifier?
Thank you!
EDIT: I learned from a friend to check the internal resistance of the meter before measuring something so small. Most of the resistors were actually fine.
I recently bought it used and had it freighted to me. It definitely sat in the cold for a little while. When I first powered it on, I noticed very little bass compared to the other HPR 153F I already had. A few seconds later it made a loud honking sound and flipped it's internal breaker.
Taking a look inside revealed a few shorted transistors (2SC5200/2SA1943)
I also noticed all of the 0.1R 2W resistors were measuring around 0.4R.
The pcb has a "hot" side with all through hole parts. I think I'm going to just replace all of the parts on this side. I've also noticed a couple open circuit resistors on the smt side I will need to replace.
Should I be considering replacing the electrolytic capacitors as well?
Cosmetically they look fine.
Also, should I check the resistors in the other speaker I have? Could this have been the cause of the failure of the amplifier?
Thank you!
EDIT: I learned from a friend to check the internal resistance of the meter before measuring something so small. Most of the resistors were actually fine.
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I was checking parts in the LF amp's bias circuit yesterday and I noticed the trimpot was reading strange. I removed it from the circuit and it's definitely damaged. Reading much higher than normal and goes intermittent at the ends.
I'm thinking this may the root cause of the fried output transistors. I'm wondering if maybe the trimmer was damaged from vibration during shipping. I'm guessing this could explain why it turned on at first, long enough to cook the output transistors, but had no LF audio.
I've ordered all of the parts I need except for the trim pot. When I receive it all, my plan is to put everything except the new output transistors in. Then, power up the amp without the drivers, and measure the bias, and various power supplies. I will probably try to borrow a variac from a friend for this.
I'm thinking this may the root cause of the fried output transistors. I'm wondering if maybe the trimmer was damaged from vibration during shipping. I'm guessing this could explain why it turned on at first, long enough to cook the output transistors, but had no LF audio.
I've ordered all of the parts I need except for the trim pot. When I receive it all, my plan is to put everything except the new output transistors in. Then, power up the amp without the drivers, and measure the bias, and various power supplies. I will probably try to borrow a variac from a friend for this.
Damaged bias pot (and diodes or vbe multiplier) is often collateral damage any time you blow a set of outputs. If I replace a set out outputs, I also replace the drivers, predrivers if there are any, the entire bias stack, and base-to-emitter resistors on both outputs and drivers. That will catch anything that can cause it to blow up again and the extra couple bucks savings in parts isn’t worth the time and hassle of jacking with it for hours.
If emitter resistors open it’s non fatal, so they stay unless there is a reason to change em.
If emitter resistors open it’s non fatal, so they stay unless there is a reason to change em.
I finally received all the parts and replaced them. Everything turns on and there are no shorts, but unfortunately there is what sounds like 60hz hum coming through all three channels. I set the bias voltages to the values in the schematic, but the noise remains.
Any suggestions on where/what to check?
Any suggestions on where/what to check?