Technics SU-V1X – Volume Board Short

I spent ages trying to resolve the loss of signal on one channel so traced from input to volume pot, getting a slow decrease in signal on on side. I removed and replaced EVERY component on the volume board after checking the pot as fine also, but still couldn’t find why the signal dropped at the pot! It turned out to be a tiny wire from the link cable had shorted across the link cable pins! As I had re-soldered the whole amp for dry joints I can only assume it happened while using a brush to clean the boards and the metal part had nicked a wire and flattened it! What a F*&(*!

Least I got really familiar with the Vol and Balance pre amp board. I removed and tested the tone pre amp to find one side had failed – probably during the nasty mains noise period of the short – so replaced it with an LM358 after building the breadboard 5:1 gain test circuit from a prior Post.

There is still some – I assume – transistor noise somewhere, but now both channels are working fine and the headphone outs sound good.

This was an expensive repair costing me my Minimus 26s speakers burned out with the nasty low mains noise – this explains the transformer overheating before I’d realised the speakers were short!

I will continue to replace the 2SA1123, 150V, 750mW transistors to try and remove the odd frying noises in this amp, as most of the capacitors and some resistors have been replaced on the pre amp, volume and main boards.

Lessons Learned

At least it has helped components replacement skills, and made me think to check re-soldered connections with a magnifier.

Driver transistors Q401-2 PNPs are quite high spec. so not replaceable by many common transistors – the 150V and 750mW power handling being the requirements.

A shorted speaker will cause large current to be drawn by the PT – IF you get away without damage to it or the SVI 3002 – at least it shows these components are over rated for the loads on them.

It has cemented the block chains for these amps in my mind more – passive inputs and controls from phonos – selectors – link cables – mute/loudness switches – volume pot – link cable – balance control – tone pre amp (M5219P is compatible with LM358N so can be used as replacement)