Help troubleshooting sub amp

[dB]

Long time no post....

I have a PE 120W sub amp. Probably 5 years old. I think I drove it too hard yesterday...

I measure about 12V dc at the output. That probable means the output transistors are bad, right?

The numbers on the devices are 2SC5200 and 2SA1943.

Do you think I'm on the right track with my troubleshooting? Is it worth it to try to replace them?

Any help and advice appreciated.

dB

[AJ]

Re: Help troubleshooting sub amp

An offset of 12v could mean that anything is wrong. Driver transistors, pre drivers, dead caps, etc...

Make sure the power supply is working correctly first. I'm not totally familiar with that amp, but make sure you have a + and a - supply voltage in reference to ground at the main filter caps.

Also, if you do order new output transistors, order from a reputable source. Toshiba isn't making those transistors anymore and there are a lot of fakes floating around.

[Æ]

Re: Help troubleshooting sub amp

Long time no post....

I have a PE 120W sub amp. Probably 5 years old. I think I drove it too hard yesterday...

I measure about 12V dc at the output. That probable means the output transistors are bad, right?

The numbers on the devices are 2SC5200 and 2SA1943.

Do you think I'm on the right track with my troubleshooting? Is it worth it to try to replace them?

Any help and advice appreciated.

dB

Hey, how is it going?
It's easy to replace a couple of output transistors, but. . . .
How do you know that is the problem? You have to check out everything first. You really need the schematics. DC at the output is bad, don't just assume it's the transistors.

[Fred T]

Re: Help troubleshooting sub amp

Might be a shorted output trans.To check set your DMM to diode test position and see if there is a dead short between the pins.

[AJ]

Re: Help troubleshooting sub amp

I doubt any of the outputs are dead shorted... If they were he would have the full rail voltage at the output or blown fuses. They are either leaking, or not the problem at all.

[Fred T]

Re: Help troubleshooting sub amp

I doubt any of the outputs are dead shorted... If they were he would have the full rail voltage at the output or blown fuses. They are either leaking, or not the problem at all.

Correct the fuse should blow with a shorted output unless perhaps each rail has a separate fuse so only the one for that polarity opened?This would leave the other polarity operational causing the offset.Also assuming the outputs are fine and if it did have a fuse for each rail, if one opened this could cause hi offset.As you suggested previously check for the presence of both the pos and neg voltages.

[dB]

Re: Help troubleshooting sub amp

OK, thanks for the help and Hi Æ.

I first removed the output transistors and checked them. Sort of, I don't have a clue really, except that there aren't any shorts between the emitter, base and collector -- if those are the right terms?

Next, I measured the voltage at the power supply caps. (Is this the a+ a-?) One was 48V and the other was 32V. Hmmmm...Close to my 12V offset.

So I removed the power supply caps. The caps values are supposed to be 6800 uF. I measured them using my multimeter and one is shorted and the other reads 70 nF.

So, I'll replace the caps.

Hopefully we will succeed thanks to your superb recommendations.

dave

[Æ]

Re: Help troubleshooting sub amp

OK, thanks for the help and Hi Æ.

I first removed the output transistors and checked them. Sort of, I don't have a clue really, except that there aren't any shorts between the emitter, base and collector -- if those are the right terms?

Next, I measured the voltage at the power supply caps. (Is this the a+ a-?) One was 48V and the other was 32V. Hmmmm...Close to my 12V offset.

So I removed the power supply caps. The caps values are supposed to be 6800 uF. I measured them using my multimeter and one is shorted and the other reads 70 nF.

So, I'll replace the caps.

Hopefully we will succeed thanks to your superb recommendations.

dave

Dave, do you really know what you are doing?
Typically you would check the AC voltages of the power transformer secondaries, then you would check DC voltage at the rectifier (bridge) and then the filtered DC after the capacitors. Capacitors don't usually give out part of the way. When they blow, they really blow or leak all over the place. Bridge or single piece rectifiers give out more often.

How did you check your capacitors? Could you be more specific? Hopefully your multimeter has capacitor testing function (cap meter) otherwise your readings might be misleading.
Did you discharge the capacitors before checking them? They will hold a voltage until you discharge them. Any voltage present would really screw up your measurement and could possibly even damage your meter. Best to check capacitors/capacitance out of circuit.

[dB]

Re: Help troubleshooting sub amp

Yes, I did pretty much what you said. Once I removed the caps I measured them with my DMM which has a cap function. I did discharge them. One read 70 nF and the other wouldn't give a reading. The one which didn't give a reading I measured for resistance and it was zero -- both ways of polarity. So, I'm pretty sure the caps are bad. Whether or not that's the entire problem with the amp I don't know.

DAVE

[Æ]

Re: Help troubleshooting sub amp

Yes, I did pretty much what you said. Once I removed the caps I measured them with my DMM which has a cap function. I did discharge them. One read 70 nF and the other wouldn't give a reading. The one which didn't give a reading I measured for resistance and it was zero -- both ways of polarity. So, I'm pretty sure the caps are bad. Whether or not that's the entire problem with the amp I don't know.

DAVE

Good going so far.

Get replacement caps with a higher voltage (if possible).
I've got several brand new spare caps, snap in size that I could give you. But you would have to tell me the exact hole spacing etc. Can you measure the physical dimensions of your caps and/or take pictures of them too?
I haven't forgotten about those rotary switches you once sent me.

There still may be other things wrong with your amp!

[dB]

Re: Help troubleshooting sub amp

Good going so far.

Get replacement caps with a higher voltage (if possible).
I've got several brand new spare caps, snap in size that I could give you. But you would have to tell me the exact hole spacing etc. Can you measure the physical dimensions of your caps and/or take pictures of them too?
I haven't forgotten about those rotary switches you once sent me.

There still may be other things wrong with your amp!

Thanks for the offer Æ but I ordered a pair of caps on Thursday from Mouser. Too bad PE didn't have them because I needed to order some HDMI cables too.

dave