Sony STR-KM77

[Sparkshower]

Brought this was supposedly working (luckily the seller was the good sort, got some of my money back) Sony stereo yesterday. So when i first powered it up it went into protect mode then shut down so unplugged it, plugged it in them turned it on and it started clicking like the main replay was going on off on off it must've done it like 7 times before it shut itself down so i unplugged it again, then curiosity got the best of me so i plugged it in again so it turned on and stayed on but it froze up you couldn't change the input or volume but the volume dials lcd would flash like they do when there changed so that was just what happened for a few cycles of turning it on and off the i moved it to show the previous owner and while we were messing around it had been left on for like 10 mins by this stage and it started working as normal apart from any sound coming out the speakers, anyway after playing around with different setting it started inconsistently pushing sound out the speakers so this is where i am at now.

I will turn it on sound works fine for 15 seconds gets less and less consistant then nothing untill you turn it off again, then the cycle repeats once you leave it for a bit

Any idea what area to look at specifically?

 

[Diver300]

The main power supply smoothing capacitors would be worth looking at.

 

[Nigel Goodwin]

The main power supply smoothing capacitors would be worth looking at.

Sorry, but I would disagree, Sony fit high quality capacitors and it's almost totally unknown to have capacitor failures - and I've never replaced a main electrolytic in a Sony product in 46 years as a Sony service dealer.

As it's going in to protect mode, I would check the voltages on the output transistors, to see if one amplifier channel is duff that's what the protect circuit is checking for.

Assuming one of them reads duff, then I'd check for dry joints around that channel, I have had instances of dry joints in output stages causing intermittent protection issues.

At least the service manual is freely available for download:

Electronics Search

Schematics and repair manual for STR-KM77

english.electronica-pt.com

 

[Sparkshower]

It only went into protect mode the very first time i turned it on it doesn't do that now unless i turn it right up and sound decides to come out the speakers

At the moment the sound is intermittent and comes on less and less the longer its on until you leave it of for a bit then the cycle restarts

 

[Nigel Goodwin]

As I said, check the voltages on the output transistors.

 

[Sparkshower]

So now its not going at all when i plug it in it clicks and half the lcd display lights up but from left to right the digits get darker and darker until not being lit, this has happened before and i unplugged it and plugged it in again and it worked slightly better but its not doing much at all now