I have an old Technics SA-380S lying around and I want to fix it

[audiosavant]

Hi everyone,
I'm relatively new to fixing electronics and amplifiers but I love listening to the sound produced from vintage amplifiers
My dad had this Technics SA-380S lying around and I took it out trying to fix it.
At first, it was covered in dirt, so I cleaned it with a blower(used for painting), then hooked it up.
It was working in AUX mode but only in the right channel, very feeble audio.
Then I probed here and there, checking for any loose connections, and after pushing the volume seeker cable, the audio volume increased dramatically.
I hooked some speakers to the right channel in mono and was enjoying listening to it.
The next morning, the volume meters in front of it was getting stuck at 0db level, so I smacked it trying to get it unstuck but the 1.6A fuse went off.
I put in a new fuse but one of the resistors or diode in the Amplifier circuit burned up.
After that, I'm just hearing humming from the speakers and within 2-3 seconds, the speaker protection circuit goes off too.

Does anybody have any advice on how to fix it and where the problem can be? And please, if anybody has the circuit diagram, I would be grateful if you could send it to me

 

[Catmanboo]

That's not a very worthwhile fixer-upper, unless there's an intermittent connection issue easily solved. How hard did you swat it? May have caused the smoked part. Also, it has the despised STK audio power modules in lieu of discrete transistors. If one is defective, since replacements are mostly unavailable, it may be a candidate for the e-waste bin. Can't hurt to poke & prod around, looking for obvious physical connection issues, clean controls & switches & such. You may stumble upon something loose & .maybe causing the smoked resistor.

 

[audiosavant]

I smacked it a bit hard, I suppose
I cleaned it up yesterday with some alcohol and some qtips and everything else looks good, only problem seems to be the blown resistor but I don't know what caused it
I'm afraid even if I replace the resistor and turn it back on, it would just get smoked again
Could you please give me some advice on how to check the amplifier unit if any excess voltage or current is passing through it?

 

[audiosavant]

Also, the multimeter I have doesn't have a capacitance mode to check the caps
Are there any other way to check them, all look good physically

 

[Catmanboo]

Also, the multimeter I have doesn't have a capacitance mode to check the caps
Are there any other way to check them, all look good physically

A dedicated capacitor tester is called for here. And to properly clean pots, switches & such you'll need spray contact cleaner & possibly lubricant. Re: checking pin voltages of the STK packs, you'll need a schematic diagram particular to the equipment that shows what you should have where. Be aware that defective parts often can't be detected visually, especially if they fail open.
Go to the DIY forum section & read up on tools needed, troubleshooting tips, soldering advice, etc. Lots of learnables posted there for your convenience.

 

[audiosavant]

Thank you, I'll be checking the DIY forum section and learning how to check for problems
And I think I'll buy a multimeter capable of testing capacitors, then I'll continue with this expedition

 

[audiosavant]

Are there any alternatives to the IC TA75559S? I think one on the Aux lines is toast but I can't find a replacement due to it being so old and not produced anymore...