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Stuck on Stereo Repair

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Dr. Shark

Sun Sept 24 2006, 02:34PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

Guys, I need your help once again... A friend of mine asked me to repair his compact stereo, and in order not to look like an idiot I guess I better succeed. The problem is this: Everything works normally, disks spin up, the display starts counting, even the digial VU-meter mockup starts pulsing, but no sound comes out. Neither the phones output nor the speaker output work, and that's with all input sources (casette, tuner or CD). Thus I blame it on the preamp. Still I think this should not be technically possible, since the pre-amp is stereo, and both channels giving up at the same time is unlikely.
Are there any clever things I could try out? Sams Repair FAQ does not say much on audio equipment, but maybe someone knows a better source of information?

Any help appreciated!

Bjørn

Sun Sept 24 2006, 03:02PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

Follow the signal from the input and when you find out where it disappears you will know a lot more.

Bennem

Sun Sept 24 2006, 03:13PM

Registered Member #154 Joined: Sun Feb 12 2006, 04:28PM
Location: Westmidlands, UK
Posts: 260

Its possibly a power amp problem,
If the power amp is in form of a power output IC,
check its supply voltage,
it might be missing due to a blown fuse.

Marko

Sun Sept 24 2006, 04:03PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

If you havve a simple signal generator you can test everything in order from output amplifier.

Amp should even give out hum if you touch the input with finger, but be careful with that.

From what you say it seems that CD head, DAC's and stuff are OK and problem shouldn't be too irrepairable.

If you post clear pics and maybe names of critical components I guess sound-masters like steve conner could help you a lot there

Checked fuses, voltages and 'regular' stuff?

Dr. Shark

Sun Sept 24 2006, 04:03PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

The power amp is in fact an IC, so I will check that. Since the headphone output is also not working, I might have disregarded the possibiliy of a blown power amp too early.
BjÃ¸rn, your approach would be my last resort, since the whole thing is so damn complex, I woldn't even know where to start (i.e. where the DA chip sits and spits out the first audio signal).
The whole control section of the stereo is digital, which I am not an expert in, so I hope the problem is somewhere in the analogue circuitry.

Marko

Sun Sept 24 2006, 04:18PM

Registered Member #89 Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145

If vu meter works then digi stuff is probably OK.

Scope the inputs and outputs of amplifier chips as bjorn suggested, if you are 'lost' simply enter part names in datasheetcatalog, and with little luck you'l have your pinout.

Some pics may also help if you bother.

FastMHz

Sun Sept 24 2006, 06:30PM

Registered Member #179 Joined: Thu Feb 16 2006, 02:08AM
Location: Hagerstown, Maryland - Close to Prime Outlets
Posts: 287

Instead of trying to start at the audio source and tracing to the end, start tracing from the speaker output and work back. It may be easier especially if the fault lies at the end somewhere.

Steve Conner

Sun Sept 24 2006, 08:05PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

Are there any fuses blown inside? If so the odds are that the power amp IC died and shorted the supply rails. Or they could be speaker fuses.

Does the system have a speaker protection relay? Do you hear it click in a second or two after powerup? If not, either a power amp died, or the relay control circuit is faulty.

Sometimes that relay's contacts go dirty, or a dry joint on the circuit board kills the sound. Try wiggling and tapping the board and any large heavy components with power applied, to see if it restores sound. (warning: I once destroyed a speaker like this because a loose component made the power amp suddenly apply 55 volts DC to the speaker)

Do you hear anything from the speakers at all? A working power amp, even with no signal going in, always produces a quiet hiss and sometimes a little hum too. Turn the volume up and put your ear to the tweeters, or listen carefully in the headphones. Do you hear any hiss or hum? If so, the power amps are probably OK, and the fault is more likely to be in the preamp and signal routing. (warning: scopeboy.com accepts no responsibility for deafness if it decides to start working at full volume while your ear is pressed to a tweeter)

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