Coustic amp460 phhht!

[anatech]

I have a Coustic amplifier that blew one channel. TIP147's are shorted and I still have 5V DC offset with the blown transistors out of circuit. It appears to have a discrete power amp stage - which I could trace. I don't want to have to keep pulling the board to chase down any other faults. Does anyone have a schematic for this beast or know where to get one? I have searched a bit on the web.

Thanks in advance for any help - Chris

[Dr. Photon]

I usually try to repair without a schematic.

Are you talking about a DC ofset on the output or on the base lines to the output devices with NO output devices installed? That's gotta confuse the feedback loop. What I do when I *think* the rest of the amplifier is OK, but I don't want to risk a new pair of outputs, is I solder in some resistors, around 100 ohms, from the emiter to base where the outputs are supposed to go. This will let the driver transistors power the output a little. This should let the feedback loop get the output to zero volts, and will letit pass some kind of signal (but not into any sort of real load). The 100 ohms resistors should prevent more explosions, or at least act as fuses if the drivers short too

Also measure between the bases of the output transistors (or between the bases of the drivers, and/or also between the collector and emiter of the bias transistor) to make sure that the bias is OK and that it isn't going to run away and nuke the new transistors. I also use the diode tester on the meter on all the transistors just to make sure.

also check the big emiter resistors (they sometimes open up), and the base resistors (they sometimes catch fire if the device shorts B-C or BCE). especially in a car amp, all solder joints are suspect.

When I finally put the new transistors in, I'll only put a pair in (if it's an amp with 4 6 or 8 outputs per channel), and I use a small fuse in the amp, so that hopefully that blows before the amp.

After putting the thrid replacement set of outputs into a Yamaha home amp, I stuck some 10A MINI car fuses in the collector lines. this amp has the bias, predriver, and drivers on a little plug in card. it's prone to loose connections and crappy soldering, and when certain lines open, the output stage blows. gah.

[anatech]

Dr. Photon,
The rest of the driver & o/ps are okay, I'll change all the outputs and drivers. Resistors are okay. There is a 5VDC offset at the output. Maybe a diff pair problem (it looks discrete, op amp filters).

I'll power it up with an external variable bipolar power supply. I just hate taking things apart many times if I can avoid it. I power up home amps the same way - switchers too. At least Yamaha is silk screening the boards now. We fought for that for years. We died under warranty - with the damn manual. Out of warranty could be fun too. Remember the darn compu tuners?

-Chris