Pioneer A109 Stereo Amp - blown R channel

[Driver]

Hi Guys.

I recently acquired a Pioneer A109 stereo amplifier that failed to power on.

I'm no electronics expert, but some basic visual inspection revealed a blown 800mA fuse on the pre-transformer/mains PCB, and some very toasted looking PCB board around the Right channel power mosfets on the main board. These mosfets were shorted too, so I sucked them out and replaced the fuse.

Then bingo, the unit powered up and I got audio from the left channel only! So I ordered some replacement mosfets and fitted them, and everything worked fine for the 15-20mins I had it running on the workbench. During this time, I noticed the Left channel heatsinks seemed hotter than the newly replaced Right channel, so I decided I'd spend another $3 on more new Mosfets for the Left channel too.

So after swapping them out, I powered the unit up and the fuse blows again. Seems the right channel Mosfets were shorted again. Once removed & fuse replaced, it powers up again, left channel only again.

Any ideas as to what's going on here before I scrap the entire unit?

[David_AVD]

You may have a bad driver transistor or bias control which is resulting is excessive bias current and thermal runaway.  Sometimes the bias trim pot can go open circuit either by itself or as a result of an output stage failure.

[4thDoctorWhoFan]

You need to check the driver transistors and the 4 .22 ohm resistors.  Once you know for sure the output transistors, driver transistors and four resistors are good, you will probably have to adjust the DC offset and bias if this unit has a bias adjustment.

A quick check for DC offset is simple:  Power the unit up with no speakers attached and the volume at minimum, then measure the DC voltage at the Right & Left speaker terminals.  It should be low.  It varies from unit to unit but the DC offset is typically somewhere between 5mv to 30mv.

Also, make sure your speakers are okay.  If they are no good, it could be blowing the amp.  Another thing, don't use 4 ohm impedance speakers as your amp lowest recommended impedance is 6 ohms.  Using 4 ohm speaker could also damaged the amp.