Coustic AMP-360 repair

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I have a Coustic AMP-360 in need of repair. The amp powers up and idles at 800mA with 13.8VDC. Both voltage rails are good at -32.5VDC and +32.5VDC. The right channel plays fine and sounds good. The left channel is a different story. The strange thing is the left channel is fine until I apply an input signal via the RCAs. Once I apply a signal, I get the full voltage rail on the speaker output and the amp begins to draw current in an erratic manner bouncing around 3 to 5 amps. It continues to do this until I power it down. When I power it up again, there is no DC on the speaker output until I apply a signal again. Very strange

The first thing I did was pull both the TIP142 and TIP147 output transistors. The TIP142 only measures about 1 ohm between collector and emitter, so I believe it is shorted. To see if this was the only problem I swapped the good transistors from the right channel in. However, it behaved the same way with the good transistors. I pulled the transistors out believing I had probably damaged them, but after reinstalling them back into the right channel, they still work fine. At this point I'm guessing I have a small signal transistor that is causing the problem in the left channel. Most of the small signal transistors are MPSA06 and MPSA56. The thermal Vbe transistor is a MPSA13, and then there are some LM9014. Does anyone here have the schematic for this amp? The going will be a little slow because I only have time to work on this amp during the weekend. Thanks!
 
Do you only get DC on one channel?

You may want to replace the bias pots. The Digi-key 3309P-102 should be the correct part.

The bias pots in this amp are mirror image adjust (clockwise = increase for one channel, decrease for the other channel).

The drivers' collectors connect to the outputs via a 1.5 ohm resistor.
 
If its a small transistor then I would pull them one at a time and check them for hfe on a meter.
Had a similar problem myself and by sods law it was the last transistor of 7 pulled !
It had an hfe of 1.

If you get them all out and none are faulty then while they are out check all resistors for correct value and all capacitors for shorts. If the amp is of any age I would replace the electrolytics anyway.
 
Do you only get DC on one channel?

You may want to replace the bias pots. The Digi-key 3309P-102 should be the correct part.

The bias pots in this amp are mirror image adjust (clockwise = increase for one channel, decrease for the other channel).

The drivers' collectors connect to the outputs via a 1.5 ohm resistor.

Correct. Only the left channel outputs DC on the speaker terminal. I swapped in a good set of TIP142/147 and DC was on the speaker terminals again. However, DC only showed up once I applied a signal to the RCAs. If I just power up the amp, then there is no DC on the speaker terminals.

I pulled that 1.5 ohm resistor because it looked like the body had shorted through to the 0.1 ohm emitter resistor's lead wire. The black mark ended up being the lovely flux that is all over these amps.

I'll inspect the bias pot and see if it is the cause of the problem. Thanks for the help. Will give an update as I go along.

Rgs, JLH
 
If its a small transistor then I would pull them one at a time and check them for hfe on a meter.
Had a similar problem myself and by sods law it was the last transistor of 7 pulled !
It had an hfe of 1.

If you get them all out and none are faulty then while they are out check all resistors for correct value and all capacitors for shorts. If the amp is of any age I would replace the electrolytics anyway.

It's always the last one! That has happened to me too. Once I check the bias pot, I look into checking the small signal trasistors.
 
An update on this repair. I believe the trouble maker was the Vbe multiplier transistor - MPSA13. I replaced it and the TIP142/TIP147 and all is good now. This is the third AMP-360 I've repaired and most of them have only had about 20mV of DC at the speaker terminals. This one is between 60mV and 63mV. This isn't the end of the world, but I'd like to lower this a little bit. As the schematic for these amps doesn't appear to be available, does anyone here on the forum know what needs to be adjusted to lower the DC on the speaker terminals? Thanks guys.
 
Dunno if I should post in a new thread, but since it almost covers the same issue I'll post here ;)

I found an old Coustic AMP360 and one channel is out. Reading 27V+ on left channel speaker output and the red "peak" LED lights up as soon as I power it up, even without load. At idle it draws ~1A so there's no abnormal current draw.

I'm not very good at troubleshooting electronics but I assumed it was shorted output transistors and replaced them (TIP142/147) but it did not help, turns out the old transistors was ok. I measured the voltage over the output transistors with an o-scope and it shows a steady DC 27V over both Base, Collector and Emitter. Removed the transistors again and there was still 27V DC on the base, I suppose it should measure close to 0 when there's so signal? I measured the resistance of the small transistors around the output stage but nothing seems shorted.

No visible sign of damage anywhere on the board, checked all the big caps, the bias pot and the small transistor in the middle of the output transistors but they seem all to be ok.

Anyone got a schematic over the board? Any ideas?
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.