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Tuesday, 8 November 2016

Technics SU-V40 amplifier repair.

It's time for this thing's turn on the bench.



It's a rather tatty looking Technics SU-V40 amplifier, dating from about 1986-87

It should be a great performer, but this one isn't...

It works after a fashion, but all is not well. It has a "rustle" on the left channel. Probably a dirty pot, I thought.

The amp has a slightly unusual look to it internally, as the main output stage has a small heatsink, coupled by a "heat-pipe" to a large heatsink to it's left.

This is the current amplifier....









...It's a large "Class AA" IC...

I'm not quite sure what Class AA is, but it's really Class AB1!








This is the driver, or voltage amplifier..













I cleaned up the volume pot, which, whilst noisy, wasn't the source of the rustling sound ...

Then a clue, as the amp warmed up, the current drive LED flickered along with the rustle, indicating the amp was trying to go into "protection" mode and cut out...










Sure enough there was a small DC offset occurring on the left hand output... I cleaned up the two bias pots. This didn't help.

It turns out Q409 is thermally poor and noisy. It starts off bad when it's cold, and gets worse as it warms up! A quick squirt with freezer spray on the cheap proves it's guilt. It's a 2SC2631 in the voltage amplifier. Amazingly, I have one..






Replacing it provides a complete cure. Another saved from landfill, even if it looks like it was already there!

Soak tested with Debut by Björk, a fine album I'd forgotten about...


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