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Pioneer A400 power on relay problem

R

redart64

New Member
Hi All,

I'm new to the board and yes I'm starting with a problem - sorry!.
A few weeks ago my much loved Pioneer A400 amp started misbehaving. Occasional loud noises were happening after powering up, then the power up relay click started taking longer than usual to kick in. Then it wouldn't kick in at all. I opened it up and removed the relay (Omron G5R-2282C-PA) for inspection - looked fine. After giving it a squirt of electrical contact cleaner for good measure I re-soldered it back onto the board and powered it up. Much to my dismay resistor R308 - next to the relay - started to smoke !. I replaced it with a new 100R .25W and the same thing happened, in fact the new resistor burst into flames!.

I think this is part of the DC detection or Overload detection circuit. Hoping there's an electronics guru out there that knows a bit about the A400 and can help me trouble shoot. I have schematics if anyone would care to take a look. Thanks.
p.s. does an early 90's amp even count as 'vintage' ?
 
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It sounds as though the protection circuit was doing its job, and the relay wasn't the culprit here! A simple test with a volt/ohm meter would have told you if the relay coil was open. It sounds like a transistor is going, or has gone, and that's why you heard a noise, crackle or pop and so the circuit shut down to prevent any more damage. Now you need to have it repaired. I would check the output transistors and the driver transistors.
 
Hi and thanks for the reply. I'm hoping to do the repairs myself, having successfully fixed a couple of amps in the past. I can follow a schematic and use a multimeter and a soldering iron, just need some help with the theory :)

The right channel output, measured on L402, is steady at -41 V. The left channel (at L401) starts off at about -35 V but then drops slowly to around -20.

Q237 has +41 V on it's emitter, collector and base. Q238 is the same but with -41 V.

L402 and 401 should be close to zero, right?. Any thoughts on what these readings suggest ?.
 
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