Pioneer A-449 amplifier repair, no sound (fixed)

[Chungalin]

This good classic stereo amplifier with 2SA1302 / 2SC3281 pair had no sound at all. Power ±42V OK. Speaker relays don't activate. It's some kind of protection. I check power transistors output and I find -42V at Left channel. Right channel normal (0V). So it's a DC protection going on.

No transistors shorted, neither finals or preamps.

I check for cracked solders (long term thermal stress). I find some suspicious ones, but believe me that I know when a solder is really broken, and in this case none seemed completely broken. Also, no changes when knocking the board.

Then I start to measure things randomly at L channel, diode test mode, specially diodes and low value resistors. I reach R247 (100Ω 1/4W) that reads higher. I double check and it's about 650Ω, but it's not burnt or bad looking at all. Same for surrounding components. I replace it without great expectations but surprisingly it's fixed!

Very, very random failure.

[rhomanski]

Re: Pioneer A-449 amplifier repair, no sound (fixed)

Late '70 Sansui had that problem with fusistors. I bought an amp off ebay said to have both channels out. It has a fusister in each channel of the pre-amp and both read infinity. The others on the power boards read about 600 to 1000 ohms instead of the 81 they were supposed to be.

[budm]

Re: Pioneer A-449 amplifier repair, no sound (fixed)

Good fix.
Service manual:

[momaka]

Re: Pioneer A-449 amplifier repair, no sound (fixed)

I reach R247 (100Ω 1/4W) that reads higher. I double check and it’s about 650Ω, but it’s not burnt or bad looking at all. Same for surrounding components. I replace it without great expectations but surprisingly it’s fixed!

Very, very random failure.

I've had that happen on an 17" CRT monitor. In that case, the resistor was a 560 KOhm 1/4 or 1/2 Watt. Wasn't overloaded or burned out. Just went open-circuit all by itself. I didn't have that specific value, but I did have 2x 300 KOhm resistors. The 560 KOhm resistor was part of a circuit that seemed to be flexible with values, so I soldered those 300 KOhm ones in series for total of 600 KOhms. Pressed the power button and hey, it works!

So yeah, good save! Old, simple, stereo amps are always worthwhile fixing.