Advice on Technics SU-V4X not coming out of protection mode [solved]
A while back I decided to upgrade the speaker terminals on my vintage Technics amp. The old ones were the twist lock kind that will grind off strands of copper each time you connect or disconnect the speakers, and I installed new banana plug connectors instead.
As far as I can tell, that replacement went well in itself, but for some reason the amp's protection circuit doesn't go into operation mode when I turn it on anymore. According to the manual, this could be a short circuit between the speaker cables, or DC voltage in the terminals. I've checked both and come up with nothing.
On the advice of a more experienced vintage Technics owner — I'm a complete newbie myself — I tested for voltage at a pair of output resistors, and they were way off. This lead me to replace two pairs of transistors (pre drive and current stabilizer), but the problem persists.
I realise I'm way out of my depth here, and although I have a cheap replacement amp, I miss the beautiful sound of my SU-V4X. Any advice is welcome.
Just wanted to bring closure to this.
I spent a couple of months switching out the same four transistors (if you're here for a model specific lead on similar repairs, it's the Q501-502s pre drive and current stabilizer Q503-504s), and freshening up 40 y-o solders on the power amp PCB... until finally, finally, all the little fixes came together and everything worked again.
But of course it wasn't quite done yet. I'd done most of these repairs with the amp upside down, but when I mounted the bottom chassis panel and flipped it right side up — stuck in protection mode again.
Another week's sweating and resoldering went by. In the end I was sure something connected to the metal bottom when the amp was turned over, so I started looking for loose components. Sure enough, when I had had the back panel off to replace speaker connection, I hadn't screwed the input RCAs back on tight enough. And the input selector board is mounted on that. So when I would flip over the amp, the input PCB sunk a couple millimetres, enough to cause a ground loop.
So my ears were burning with embarrassment when I tightened those screws and then could fasten the chassis without issues.
My amp plays like a dream again, thanks to the patience of a fellow enthusiast halfway around the world who guided me through the repairs. And a little bit of my own wiggling on back panel screws...