Here's a quick resume of what I'd like to hear the opinion of you people so I can decide what to do next to finish repair of the Pioneer SA-710 amp I have here. Attached is an image with the relevant section of the schematic.
- The problem:
it was giving no sound - protection circuit activated; checked components - output power transistors of one channel destroyed (short); detected a burnt 100R resistor (R96 - open - red circle in schematic) and a 330R resistor (R98 - green circle) which had one leg unconnected to the PCB - I figure it was a bad solder joint and over the time the leg became totally loose.
- What I did so far:
Took out the burnt power transistors and checked circuitry, other transistors were giving good values; soldered the 330R resistor and replaced the other burnt one. Then decided to risk switching amp on and yes, the protection circuitry didn't activate and I had the other channel working perfectly, giving good sound on speaker, so the amp itself is working, at least on the good channel.
- What I'd ask for help:
I don't have an oscilloscope so I can't measure if signals are processed correctly until the power stage on the problematic channel or if some other components might also have suffered. Further I don't have enough knowledge to determine if the loose leg of the resistor could have been enough to cause the output transistors to burn. So, I'd ask your opinion about this possibility and also I'd like to hear, if you find it advisable (or even possible) to temporarily install some other complementary pair of power transistors in the output of the problematic channel so to see if I get sound (I have a few lying around here).
I'm asking all this because obviously I'd like to avoid ordering new power transistors, installing them, only to discover that they are destroyed again because I missed some other defect component....
Thanks a lot in advance!
The longer story, for those having time and will to read on: this amp belongs to my mother-in-law, it had always worked fine but one day just made a loud sound and stopped working. She wanted to throw it away, I obviously refused that and said I'd gonna have a look at it. In the worst case scenario I would simply gut it and use the casing and power transformer and nice Fluoro-display to implement another gain-clone or similar. I myself have owned a very similar Pioneer (610) in the past which I also repaired and then sold (as it is not that great in sound, but it's a nice looking vintage amp, I love the display).
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