A friend recently purchased a Pioneer VSX-452 Receiver at Goodwill for $15 without being able to listen to it fully. After taking it home, and hooking up some headphones, and speakers, he noticed the left channel sounded terrible, where the right sounded fine. He mentioned this to me the next day, and I figured “hey, I can fix that”. Especially after just reading a pretty decent book called How to Diagnose and Fix Everything Electronic. Anyway, I took the unit home, and set about diagnosing it.
First thing I did, since I figured this wouldn’t be the only stereo receiver I ever fix, 
was pick up two 8R 20W resistors from Radioshack to act as dummy speakers. So I wired those up, hooked up my function generator to the CD inputs with a 1Vpp 1Khz sine wave, hooked my scope to the positive terminal (grounded the probes to the chassis of the receiver) and got a nice waveform that didn’t look quite right.
Thankfully, I was able to find a service manual for the receiver, and a schematic/circuit diagram for the amp. Note: I’m only looking at the Left Front Channel, which is the top section of the schematic. I figured the issue was in the power amp board (rather than the input stages), and confirmed that by hooking my scope up to the inputs to the power amp board (base of Q501) and saw the whole waveform was there. So then it’s just a matter of tracing the signal through the power amp board, which the schematic makes dead simple since it’s got arrows on it! So after Q501, the signal heads off to the base of Q507, at which point, I got a waveform that looked pretty much like the one posted above. Hmm. I posted in the trusty eevblog forum, and the main suggestion for a while was to check the output transistors (Q1 and Q3), which tested fine. After some time, someone piped up saying these amps were notorious for having crappy amplifier transistors, and to just replace Q507 and Q505 (the ones that the signal disappeared on). I did note that those transistors, and their mates on the other channels, showed significant “browning” on the PCB under them.
So, jumped on eBay, and purchased 10 pairs of the 2SC2705/SSA1145 NPN/PNP’s, and waited. Getting stuff from China is always so hit and miss… but, the parts arrived, I swapped out all 3 sets of those transistors ((which, the PCB had to be one of the shittiest I’ve ever worked with. I must have lifted at least 4 traces in replacing the transistors, which required some bodge wiring)), and fired the unit up to a good looking waveform! Total cost of repair, about $5 in parts and shipping, and a couple hours of troubleshooting/soldering.
Friend got the unit back, and has been quite happy with it. I don’t know if the transistors will last or how long, but I guess if they fail again, I have spares! =)
