Coustic AMP-360 repair - diyAudio
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Old 11th September 2011, 05:52 PM   #1
JLH is offline JLH  United States
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Default Coustic AMP-360 repair

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!
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Old 11th September 2011, 06:11 PM   #2
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Just checked the TIP147 and it tests as it should. Only the TIP142 in the left channel was shorted. I'm guessing the driver transistor for the TIP142 is defective, but don't which one that is on the board.
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Old 11th September 2011, 09:20 PM   #3
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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.
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Old 11th September 2011, 09:50 PM   #4
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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.
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Old 11th September 2011, 11:08 PM   #5
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Quote:
Originally Posted by Perry Babin View Post
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
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Old 11th September 2011, 11:12 PM   #6
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Quote:
Originally Posted by nigelwright7557 View Post
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.
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Old 12th September 2011, 03:10 AM   #7
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Also,if this is the same amp I'm thinking of,there's a pair of 15V? zener diodes (for op-amps,etc.) that I've seen fail in a few of them..worth checking out and making sure they're okay. (they probably are,since one channel is working fine.)
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Old 25th September 2011, 08:03 PM   #8
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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.
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Old 25th September 2011, 09:13 PM   #9
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There are two 2SC9014s with their emitters directly connected. Those are the transistors in the differential amplifier. I'd suggest replacing those first.
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Old 26th September 2011, 11:58 PM   #10
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Thanks Perry.
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