SX-780 troubleshooting guide

dave_n_s

Active Member
I'm a competent systems troubleshooter, but have to work at it at component level. I have a good DVM, scope & signal generator and a dead SX-780

It came in this state; I knew it wouldn't power on, but as a classic for few $$, it seemed an interesting challenge.

It's blowing the 4A fuse, so I've done my homework already and know I've got one or more Darlingtons dead, potentially along with associated components.

However my thinking is that I ought to establish everything that needs work, apart from pots and switches, before investing time and money in the power stage.

Does that make sense for bringing up a unit that I've no prior history of?

It doesn't make sense if it was my long term unit that quit one day - then I'd go straight for the modules.

This receiver doesn't have a pre-out, power-in to enable me to check out the tuner, phono in, switches and controls. A bad tuner or phono stage would have me thinking that this was something to put to the back of the queue.

Is there a tried and tested process for going through one of these that won't power up, that ensures that any seriously bad news is known before starting the refurb process?
 
If the 4amp fuse is blowing then either one or both STK's are fried, also check the emitter resistors they usually go when a module blows up. Also check the 3 transistors attached to silver heatsinks, they get so hot the solder sometimes fails.
 
You can troubleshoot the pre and driver stages of the amp without the modules installed.
You can add a 1K resistor between 0 and 8, and between 1 and 3. And run it. This will simulate the modules installed.
Check the emitter resistors for value then power up and check the amps transistor voltages.
 
both, thank you, good information. I'll document my findings

And zebulon1, talking of having to wait, I have a Yamaha CR 1020 (nasty hum, filter board I guess) and a woefully cared-for Marantz 2325 (one side no audio and a bit of a mess) waiting in line. Going to be fun!
 
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