Coustic AMP190 "Ripping" Sound

[lionfire17]

Hey all,

I have a Coustic AMP190 that I got from my dad a number of years ago after he got some good use out of it. I used it for several more years with no problems until a few months ago when it started acting up.

The Remote connection of the amp was connected to the remote line of my stereo, so when I turn on the stereo the amp would turn on as well. As long as I've had it, whenever the amp would power up, the sub would make a little "bump" sound. I never thought anything of it because lots of audio systems make little pops or clicks when they power up and it seemed totally normal.

A few months ago, that bump sound started happening more frequently, like the amp was turning itself off and on every so often. After time, that got more and more frequent until now when it just sounds like a continuous ripping sound anytime it's powered up. In the process the amp draws a lot of current and gets very warm.

My intuition told me it was a capacitor issue at first since it is a bit aged and electrolytics like to lose their potency over time, so I replaced most of the larger ones. The problem remained so before I replace anything else I wanted to consult the forum.

I've posted some pictures on flickr here:
Coustic AMP 190 - a set on Flickr

I have very little amp repair experience but I do have good general electronics knowledge, soldering skills, and access to power supplies, signal generators and o-scopes.

Any advice is most appreciated!

[Perry Babin]

I'm assuming that you've checked the output transistors and none are shorted.

In these amps, the bias pots failed fairly often. You could try cleaning and readjusting the bias pots to see if that helps. If it does, replace them.

One more note... I think the channel layouts in this amp are mirror images of each other. This means that you will increase the bias on one channel by turning the bias pot clockwise. You will increase the bias on the other channel by turning the bias counter-clockwise. You can't simply set both fully counter-clockwise to reduce the biasing while working on it like you can for other amps.

[lionfire17]

I haven't actually checked the output transistors but that would make sense that if something were shorted and drawing too much current, the voltage supply for the whole amp would drop causing it to imitate the signs of "motorboating" like it says in the manual. From reading around I've taken those to be some of the parts clamped to the heatsinks on the sides. On each side there is a set of two IRF530's, a TIP142, TIP147, and a small SA13. Are all of those the output transistors or just one of them?

EDIT: BTW I just posted a couple more pictures on the same Flickr set showing those parts more closely

[JLH]

The outputs are TIP142 and TIP147

[lionfire17]

Great, thanks to you both for the tips. I will try out the transistor testing and come back with the results.

[lionfire17]

Output transistors appear to have passed the test with no continuity between any pins. Moving on to playing with the bias pots.

[JLH]

Something looks like it got hot near one of the voltage rails. I've circled it in orange. I also circled the bias pots in red for you.

[lionfire17]

Hot dang! I fiddled with the bias pots and low and behold, the current draw has dramatically reduced. I anticipate when I hook it back up to the sub I should be in good shape. I will go shopping for some new pots but in the meantime, and for when I get the new pots, is there a recommended guide for how to correctly set them?

[Perry Babin]

Digi-Key - 3309P-102-ND (Manufacturer - 3309P-1-102)

If I'm not mistaken, these are the ones I used the last time I needed replacements. Confirm dimensions and value before ordering.

The following demo shows one simple way to set the bias. Turn it until you just barely see an increase in the idle current on the 12v power supply amp meter.
http://www.bcae1.com/temp/ausettingbias.swf

[lionfire17]

OK I'll double check the dimensions of these pots to the hole pattern on the board and get them ordered, as well as play around with the adjustment. Thanks again for all your help.