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Emerson Research Amp repair

I recently was given a "poor man's home theatre" system... it's made by a company called Emerson Research (yeah, Emerson, not the best name in audio). It had had the plug cut off, I assumed it to be non-functional.

this unit appears to contain a ported woofer or subwoofer, a crossover network, and a switch for multiple stereo inputs and outputs as follows:

2 rear speakers 2 "micro" speakers - assume satellite tweeters with high crossover freq. 1 (7 or 9-pin) DIN connector labelled "center channel". ???

I opened it up and, yup, there are two 5A fuses, both blown, one on each outside lead of a CT transformer prior to the rectifying diodes. Diodes checked out fine, output transistors (there are six of them, TIP41C/TIP42C pairs) are all blown.

Each transistor appears to be fed from either the + or - rail to the collector. Each base connects to an output jacks via a 0.22ohm resistor, none of the resistors are open.

These transistors are cheap, and I'd like to get the unit going to use with my computer at work. I have a pair of unpowered satellite speakers and I never listen to them because there is no bass whatsoever and because the amp on my soundcard sounds like crap at the best of times. I know I'm never going to get high fidelity but hey, the amp was free.

Before I replace the transistors, anything else I should check? Can anybody tell me (without seeing a schematic) if crossing any of the outputs could cause all of the transistors to go? As there's no SMPS, I can set it up with my trusty ol' series light bulb I guess and see if there's any shorts. Do I need to load the outputs with a dummy resistor? If so how what size and rating?

And what's up with that "center channel" DIN jack?

Any replies greatly appreciated.

Dave

Reply to
Dave
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If one pair is shorted, all may appear shorted in your test. Did you open them up from the circuit for your test?

Typically, if the drivers and bias transistor check OK, you are just down to replacing the outputs and any resistors which may have failed from the over-current condition. These often include a resistor in the 220 to 470 ohm range located between the emitters of the driver transistors, or resistors in the 2.2 to 4.7 ohm range going to the base of each output transistor, and sometimes a resistor in the range of 1 to 10 ohms feeding the B+ or - to one or both of the driver transistors' collectors.

Mark Z.

Mark Z.

Reply to
Mark D. Zacharias

I removed them all from the circuit, they all have either a) dead short (zero voltage drop) between B-C-E or b) open between B-C-E, no drop either direction between any leads.

Each power transistor has a small transistor (TO-92) next to it, resting against but not in any way affixed to the giant aluminum common heatsink, those would be the bias transistors? I guess any driver transistors would be located in the general vicinity as well. I'll keep looking.

I find it really weird that ALL of the transistors were baked, not just one channel or one single output... even the pair that drives the internal woofer which is not accessible from outside the case.

Dave

Reply to
Dave

I presume thats a typing mistake.

I dont see how it would. Does seem odd all 6 are gone.

Is this a capacitor coupled amp or dc coupled? IF the former, you only need check the output stage is ok. If its a dc coupled amp, bin it now, theyre nightmares to work on.

No.

NT

Reply to
meow2222

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