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wrote:
wrote:
This would make the repair too expensive, since these (and the bias
transistor) blew while bench testing after fixing the original
fault.

This would worry me. Even if you could get the parts for a dime a
dozen, don't waste silicon. I bet you did not fix the original fault.

(quoted text cut)
JURB

Thanks, guys,

I'm going to take JURB's advice and do some more testing before I
order those transistors. The reason I was concerned about the
counterfeits was that I only wanted to make one more parts order
for this unit. Having to meet a minimum order and pay $8 shipping
each time starts to add up, so some scope testing with the outputs
removed makes sense.

This Pioneer VSX-454 is a Pro-Logic receiver with a 4 channel amp.
My father-in-law gave it to me to repair, but it looks like he got
it from a flea market or yard sale. ("$7.00 as-is" written in black
marker on the top cover.) So I had no history of what was wrong with
it. The only symptom was that it smoked when it was turned on.

The power amp in this unit is a "module" of sorts: It's like a box,
made of two circuit boards (L/R and C/S amps) and two heatsinks
(all NPN outputs from both boards on one sink, all PNPs on the other
sink). The heatsink fins point inwards and there is a fan at one end
that blows thru the box. The heatsinks are "live" - they are at the
+/- rail voltages, and no insulators are used on the transistors.
This makes testing the amp outside the receiver very risky, as it
can't touch anything, not even the chassis, and the multiple ribbon
cables connecting it to the main board aren't long enough to get the
amp completely out of the receiver.

3 of the 4 channels had already been repaired befo The Right
channel has an off-brand 2SA1302, the Center channel outputs were
subbed with Matsu****a transistors, and the Surround outputs were
subbed with physically smaller Toshiba transistors. A few of the
flameproof resistors on the L/R amp board had been replaced by cutting
the old ones off and soldering new ones to the "stubs". (In order to
get to the bottoms of the boards to do any soldering, all the outputs
& drivers & bias transistors have to be removed from the heatsinks to
free the board up.)

Anyway, what I found was a few 1/8 W resistors on the L/R amp board
burnt to a crisp, and a browned area on the board around them, which
covered a few unburned resistors. This is what smoked when powered
up. The amp circuits looked pretty simple, and I got the burnt
resistor's values from the other amp channels, so I decided not to
order a schematic, and just test parts. Since I didn't know the
extent of the damage, or if more than one channel was faulty, I
checked everything on both boards. I found the damage was confined
to the Left channel, and the Outputs & Drivers were about the only
parts NOT blown. 3 of the 4 "tall TO92" transistors as well as the
little bias transistor
on the heat sink were shorted, 4 47 & 4.7 ohm flameproof resistors
connected to them were open, and 4 zener diodes were shorted.

I replaced all the bad parts, all the resistors in the "burned area"
even the ones that looked & measured OK, and all 4 of the "tall TO-92"
transistors. Powered it up with load resistors on L&R outputs and
speakers on center & surround outs, at 90V on a Variac. It seemed
to work OK, no DC on the outputs, and I was able to get sound form the
Center & surround from its FM tuner, so the rest of the receiver was
working, so I brought the line voltage to 120. I checked the voltages
across the .33 ohm emitter resistors, and it was only 1 mV on all 4
channels. The bias is not adjustable on this model, and all channels
read the same, so I assumed it was OK.

When I turned up the Volume to about 10:00, a flameproof resistor in
the left channel immediately lit up like a sparkler, then the line
fuse blew a few seconds later. I never got around to putting the
scope

on the outputs, but the air blowing thru the heatsinks was coming out
cool, so there probably wasn't any high-level oscillation taking place
(at least until I turned up the volume.)

So I tore the amp module down again, retested everything in the Left
channel, and the only bad parts were the resistor that lit up, and
both outputs and the little Bias transistor on the heatsink were
shorted. Since I replaced nearly everything else, and those parts were
still good, I figured replacing the outputs (and drivers for
insurance)

should take care of it. But there is a possibility that it took a
certain signal level to kick off an oscillation...

Mike
WB2MEP

I tried JURB's technique yesterday and 8 resistors burned up. Use caution.

Mark Z.