Pioneer SX-434 - Troubleshooting, fuses blow

[santashooter]

Hi there
I hope this is the right section for this...
So tonight i was out with a friend who has an old SX-434 that her dad gave her.
It was in bad shape electronically, but all symptoms seemed like very bad contacts - so out with the deoxit for contact cleaning and all pots and switches were given the big overhaul.
Then a quick polish of the front and knobs and we assembled it again.

I didn't bring bulbs, and all of these were gone, so we just turned it on in blind - it played, sounded very good and there was nothing in any switches - hooray!

but then, after 5 min there was a slight hum for about a second and then no sound.
I took it off the speakers instantly and measured 3.7 DCV on the output posts - something was clearly wrong.
I have now brought the device back home with me, and it seems fuse #5 and #6 has blown - question is why and how i should go about troubleshooting components.

I have a multimeter and some basic tools at hand, but i would really appreciate some guidance - i really wish i could deliver this back to her, fully playing and solid.

 

[dshoaf]

A good place to start is to follow the AK Troubleshooting Flowchart stickey found here. It is all about eliminating what is known to be working right. By default, whatever is left is the root cause.

Cheers,

David

 

[CT Jim]

Also check into the Pioneer forum for more specific info.

 

[woodj]

Service manual.

 

[santashooter]

Okay - i disengaged the power amp from the power-supply, (removed the +24v and -24v leads) and the fuses did not blow, so i figured the problem resided in the power amp:
I tested the output transistors and the SVT3H diodes, the diodes were fine, but the transistors were all shorted.
SO i replaced the output transistors and gave them new thermal compound. I then recapped the power supply and the amp-board and engaged the power amp again - now the fuses are not blowing and music is playing.
Testing DC and Bias and all is good in reference to the service manual - have had it playing for 5 hours now at good volumes and it is sounding as it should without getting too hot either.
I conclude the culprit was the output transistors and that the device is now healty to go back to my friend.

 

[Edward Musci]

Question , I have one with similar issues, did you replace every cap on the amp board and ps?
Thanks any help would be appreciated.

 

[Shedelectric]

I've got a similar issue here, also after disassembly to clean the pots. I suspect that the issue here is trying to reseat the transistors without the inclusion of new thermal compound. I have the same question as Edward as to how many total caps you replaced as it looks like this is probably the route I have to take...
Thanks!

 

[Edward Musci]

Good update. The operation was a success. I replaced all transistors and updated the caps. Big smiles after I turned the unit on, no blown fuses, really nice warm sound and no extra screws. Whoo hoo.
Can't thank you guys enough. Really appreciate the help and information. Helped a lot with telling me to pay special attention when replacing transistors. It was slow and sometimes painful but the outcome was fantastic.

I replaced just the electrolytic ones on the amp board and all transistors on the same board.