EV AP2300 Solid State Power Amp help

[davohilts]

Hello,
I have an AP2300 power amp that is not working. When powered up both R91 and R92 heat up and begin to smoke. I measured 45 VDC across the resistors that measure the proper 220 ohms. I assume something is very wrong somewhere.
Can anyone offer any trouble shooting advice on how to find out what the problem is??
Thanks for any help.
Dave

AP2300 schematic.pdf

AP2300 power.pdf

[Enzo]

I bet your 15v supplies are low too. Possible short from +15 to -15.

Is this unit in the as-failed condition? Or has someone been in it trying to fix it before you got it?

[davohilts]

I bet your 15v supplies are low too. Possible short from +15 to -15.

Is this unit in the as-failed condition? Or has someone been in it trying to fix it before you got it?

I was using it and one side had low output. I put it on the bench and opened it up and powered it up and after 5 minutes the smoke started rising from R91 so I shut it down and checked all the connections. Powered it back on and took a voltage reading across R91 and it started smoking again so I shut it off.

[g1]

You said both those resistors have 45V across them, or just R91?
Don't concern yourself with heat, that is normal. But smoke and 45V across is not.
You will need to post the input board schematic.

[davohilts]

You said both those resistors have 45V across them, or just R91?
Don't concern yourself with heat, that is normal. But smoke and 45V across is not.
You will need to post the input board schematic.

yes both resistors. R91 has 49VDC and R92 has 42 VDC.

I'll attach a schematic of the Input Board.
Thanks!
AP2300 input.pdf

[bsco]

FWIW...I had a problem with a solid state guitar amp where the low voltage supplies were very low and blowing low voltage fuses in the low voltage supply.....I decided to de-solder the supply pins of the op amp chips...Pins 4 and 8....then I proceeded to resolder one chip at a time and then check to see if it was causing the problem.....I had actually found 2 op amps that were causing the trouble......I am not sure about your issue here but it might be something to check.....My problem was easier to work on as it was all through hole.....I assume your unit is the same??? It would be more difficult to do that with SMD op amps....Just a thought...Let me know how you make out with it...I would be very interested....

[davohilts]

bsco, thanks for your input. I will take a look at them and let you know if I find any change.

[bsco]

bsco, thanks for your input. I will take a look at them and let you know if I find any change.

When I did it, I desoldered all of them and then resoldered them back into ckt one at a time.....not saying that is your problem but there is something wrong with your low voltage supply somewhere....and the problem has to past the resistors.... and if one of them is shorted and they are all on the same rail, them all of them will show a short when measured with a DMM......

[davohilts]

When I did it, I desoldered all of them and then resoldered them back into ckt one at a time.....not saying that is your problem but there is something wrong with your low voltage supply somewhere....and the problem has to past the resistors.... and if one of them is shorted and they are all on the same rail, them all of them will show a short when measured with a DMM......

I have egg on my face, I carefully looked at the ribbon cable connecting the input board to the main board and noticed I had installed it with on pin shifted off to the left after removing it as I took the amp apart. When I installed it correctly the voltages across R91 dropped to 8.06 VDC and R92 measures 6.42.

Now I'm back to the reason I opened up the amp and that is low output on channel 2. I input a signal on channel 1 and I get good output, when I input a signal into channel 2 I barely get a output.

I am lost on how to troubleshoot this issue but I suspect that a output transistor may be the issue. I have two pair of toshiba 2SA1302, and 2SC3281 for each channel.
Any help on how to proceed would be greatly appreciated!

[g1]

You have 1 good channel to compare to, so that makes things easier.
Power supply should be good as it is the same for both sides.
First thing is to find out if the problem is on the input board or the power amp board. Connect a constant test tone into the good ch.1, set it up for a reasonable output volume. Measure the AC volts at R19 on the power amp board, right where the signal comes in from J5.
Now move the test tone to Ch.2 and set the volume up the same as it was for Ch.1. Measure the voltage at R119 (right where signal comes in from J5).
How do those 2 AC voltage readings compare, close, or way different?

[davohilts]

You have 1 good channel to compare to, so that makes things easier.
Power supply should be good as it is the same for both sides.
First thing is to find out if the problem is on the input board or the power amp board. Connect a constant test tone into the good ch.1, set it up for a reasonable output volume. Measure the AC volts at R19 on the power amp board, right where the signal comes in from J5.
Now move the test tone to Ch.2 and set the volume up the same as it was for Ch.1. Measure the voltage at R119 (right where signal comes in from J5).
How do those 2 AC voltage readings compare, close, or way different?

g1, you nailed it. I ran a signal thru channel 2 and used my chopstick on R119 and got a loud pop. Turned everything off and checked the value and it was within 10 ohms so I pulled the board and resoldered the resistor, put things back together and bingo, all is well again.
Thanks for the help on where to look, you are amazing!!

[g1]

Some mixed metaphor here about a stopped clock & getting lucky.
Glad to hear you got it working.