Pioneer A400 power on relay problem

[admin_exported]

Hi,
A few weeks ago my much loved A400 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 power it up. Much to my dismay R308 started to smoke !. I replaced it with a new 100R .25W and the same thing happened, in fact the 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.

 

[Anonymous]

redart66:Hi,
A few weeks ago my much loved A400 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 power it up. Much to my dismay R308 started to smoke !. I replaced it with a new 100R .25W and the same thing happened, in fact the 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.

The relay is there to protect your speakers from DC if a fault occurs. If DC is present on either output then the relay remains off. The most likely cause is an output transistor, so measure the voltage on the speaker output for each channel, then test the transistors in the channel that has a voltage on it.

It isn't a job for a non-expert IMO.

 

[Big Chris]

I would seriously stop fettling with it before you either make things worse or set yourself on fire.

If you're not looking to replace it, try to find a specialist who can give it the once over and a cost for rectification...... Or should that be resuscitation?

 

[PJPro]

Relays are sometimes used in circuits designed to protect the fuse from the in-rush of current when you switch an amp on as the large power caps and transformer take on charge. This is called a soft start circuit.

If you take a look at my useful threads and read the thread on the Dual Mono Power Amps, you'll find an explanation of this type of circuit on page 5.

The soft start circuit uses a small cap to time activation of the relay. Maybe this is the source of your problem.

Needless to say, on the mains side of the transformer you'll be dealing with 240v so don't fiddle with it unless you're confident you know what you're doing.

 

[Anonymous]

Hi Alfie and thanks for the reply. Both channels outputs, measured on L401 and L402, have voltage on them. The left is sitting at -41 V steady. The right starts at about -30 V then drops to -20 V. Not good. I also measured the voltages on transistors Q237 and Q238. These have +41V and -41V respectively on the emitters as per the schematic, but also on the collectors and bases, which again seems odd. I put together a decent schematic of it but I can't figure out how to upload it to this forum, so the only link I have is this one http://www.hifiengine.com/manuals/pioneer/a400.shtml

Any more thoughts on where to go from here (repair shop isn't an option !) would be much appreciated.

 

[Anonymous]

Oh dear. Both channels. You could try disconnecting the output transistors and checking them.

If it was me I would put it on Ebay for spares and buy another example, if you are really that set on one.