I have an old AA-1200 receiver that works intermittently. When it cuts out I can detect a relay on the circuit board changing states. I assume some protection circuit has detected an unsafe condition and shut it down. My problem is I have no information on the unit. Can anyone provide advice or steer me to a source of documentation?
AA-1200, eh? Any evidence of a manufacturer's label?
The relay on the circuit board thinks that there is a steady DC at the speaker outputs and is disconnecting said speaker outputs from the amplifier to keep the voice coils from burning out. This can happen for any number of reasons, including a defective detection circuit. In my experience, this is not so easy to troubleshoot, especially if you have to play the thing for a long period to get it to kick out.
Fool with a heat gun/soldering iron. Note that the relay is not the problem, but the output transistors quite possibly could be. Put a voltmeter, preferably analog, on the power amplifier output (they run into that relay) so you can watch what happens to the voltage as the amplifier is running; possibly you may see a DC voltage developing as a transistor heats up and unbalances the power amplifier.
M Kinsler
The first question is does it do it with no music playing - ie with it just sitting there idling, do you hear that relay drop out ? If yes, and it drops back in on its own, problem could be as simple as a bad joint, likely located in the power amps, possibly in the power supply, remote possibility of elsewhere.
If no, can you get it to do it on a particular piece of music at a particular place ie on a heavy bass line ? If yes, problem may be equally simple - a bad speaker. Disconnect them one at a time, and replay the music that initiated the cut out.
If problem seems totally random, then most likely cause is still bad joints, although it could be an intermittent component failure in the output stages ( not too likely if it works ok when it is working ) or a problem with the actual fault detect / delay circuit. I've had the delay timing cap cause this sort of thing, for instance. If it does turn out to be a genuine component fault, electrolytics on an old unit, are prime suspects. If you do get to the point of deciding that it is just a random intermittent failure, the next move is to remove the covers, and see if you can provoke the problem by firm tapping around inside with the butt end of a screwdriver. I would recommend doing this either without your speakers connected, or with an old pair on it. If you can make it go off, switch to something lighter like a Biro pen for the tapping, to try to localise the bad joint area, before actually looking for it. Most likely places would be around the output heatsink area, and the power supply section. Could even be a slack fuseholder. I have also had bad joints on the relay itself, particularly on the coil pins, on many manufacturers' units. A bad joint there will give the sort of symptoms you are experiencing.
Arfa
The AA-1200 is made by Akai. I put that in the title but neglected to repeat it in the post. Sorry for the confusion. I appreciate the response and will follow up on your suggestions. I have a scope I get on the output to check for DC levels. Any Idea how much the balance needs to be off to kick off the protection circuit?
Most protection circuits trigger with 1 to 2 volts DC at the output.
If the unit uses an STK voltage driver IC, there is a good chance this is your problem, and Akai did use those on some models.
I have also seen flat-pack transistors develop open or intermittent base-emitter junctions which also trigger protection circuits.
This may have already been mentioned, but don't ignore the possibility of bad solder connections, in the regulated power supply areas, at driver transistors and, well, anywhere in there.
Watch out for any residual DC voltage on the caps before you start soldering...
Mark Z.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.