Coustic AMP380

[frankusb]

Hi,

I've got a Coustic AMP380 car amplifier with a user manual but no schematic. It's got two output transistors shorted that I replaced a few years ago. It gets driven pretty hard by my son, but I'm curious if someone can tell what the two potentiometers might be for. They appear to eventually feed into the small transistor between the output transistors. The two TIP147s are shorted. When replacing do I need to match?

[R_J]

Re: Coustic AMP380

The two pots are likley idle current for each channel's outputs but could also be dc offset

[frankusb]

Re: Coustic AMP380

Well only one TIP147 was shorted. I replaced both and found that the amplifier was still pulling way too much current sitting idle, voltage would drop to around 10. So I removed both TIP147s and then the rest of the amplifier powered up seemingly good.

Any hints what to check next?

[R_J]

Re: Coustic AMP380

How was the transistor shorted? if it was shorted C-B it is likely the driver transistor is also bad, Check all resistors and transistors in that circuit and compare to the working channel if needed. What are you using for a voltage supply, maybe it can't supply enough current?

[frankusb]

Re: Coustic AMP380

The transistor was shorted C to E. I used a jump pack as a power supply when it worked (output stage removed) and when the voltage dropped (output stage installed). I also tried the car battery thinking that maybe the jump pack could not supply enough.

That was my thought as well, was to remove the driver transistor output to the output stage input connection and retry.

[frankusb]

Re: Coustic AMP380

I traced out portions of the circuit and took some measurements of both channels. I lifted on side of R51 feeding the TIP147 pairs base to allow the system to power on.

My thinking is the the base of Q12 is suspect and I need to follow that.

[frankusb]

Re: Coustic AMP380

The base of Q12 is fed by the emitters of TIP142/147 (after the 5W 0.1 ohm resistors and through R43) and on closer measurement of the TIP142 pair, resistance C to E does not match the good channel and does not differ based on polarity. I think one or both of the TIP142s is causing the problem.

[R_J]

Re: Coustic AMP380

Many times when one output (tip147) shorts, the excess current can damage the other one (tip142).

[frankusb]

Re: Coustic AMP380

OK, sure enough, one of the TIP142s was bad. I replaced both and the amp now powers on and engages the speaker relay.

I am curious what the potentiometer adjustment should adjust. It seemed to influence the DC offset of the output. The good channel is +/- 1. The repaired channel is close to zero.

[R_J]

Re: Coustic AMP380

It could be for adjusting the DC offset which would be 0 volts across the output, OR it could be to adjust the idle current which would be the voltage across one of the (0.1Ω)emitter resistors, usually around 20~30mv (with no input signal) Check the voltage across the good channel emitter resistors

[frankusb]

Re: Coustic AMP380

Huh, I'm measuring 1mV across both the good channel and repaired channel emitter resistors. The speaker outputs measure ~12mV on both channels.

It sounds okay under a light load.

[R_J]

Re: Coustic AMP380

It seems to be repaired, operate it for a while and see if anything is getting abnormally hot. 1mv is should be ok,
I was thinking of other amps that usually have a higher value emitter resistor and a higher voltage across them.