SA-6700 blows power fuse

rickl

I'm just a bass player
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I just got a SA-6700 from someone that tried to fix the meter lights and blew the main 3A fuse. Gave up and gave it to me. I got a service manual and have studied it for a few days. I looked at the board and didn't see any burned resistors or bad caps.

I've read a few threads on how to fix these amps (and similar ones):

http://audiokarma.org/forums/index.php?threads/pioneer-sa-6700-project.594128/
http://audiokarma.org/forums/index.php?threads/sa-6700-amplifier-woes.196884/

Per MTF suggestion, I pulled the meter driver Q15 and it was open. Pulled D26 (BCR3AM-4) to isolate the power supply.
I have +40v/-40v at the bridge. DBT didn't glow.

Added D26 and my DBT lit up. The bulb didn't dim so I turned off power.

Not really sure what D26 thyristor does but would like some help on next steps.

Should I check Q19, Q20?

How to test D26? Can I use my DMM? Just got it calibrated so it should be accurate.

R110/R111 (0.5R) are not burned looking but I didn't measure them.

Thanks for any help or advice.
 
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I got the amp working. D26 pulled and all voltages look good. Output bias measured 20mV and 16mV at test points. Left channel worked but right channel didn't. Function selector switch needed cleaning.

Ordering MJE15030G for the 2SB507 Q15 and looking for a BCR3AM D26.

Thanks to Mark the Fixer for past threads.
 
Bam!
With the right search, MTF will live in infamy.
He was on hiatus for some time and we had to use the search function. There's hardly a question that MTF hasn't answered. Just got'a find them.
 
Parts came, thyristor worked great. Replaced the 27v zener and the 250ma meter lamps to leds and a 140R dropping resistor. I think the PO used 250mA (500mA total) bulbs and the meter power supply couldn't keep up. Then he blew the fuse trying to find the problem.

Amp seems to work now. Thanks for the help!
 
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