NAD "Thup Thup" - and dial noise

[Lioneye]

Hello all you smart people.
2 things, one probabaly easier fix then the other.
I have a NAD 7155. I know, oldie, but it's been a good amp. Anyway after it starts to warm up I get a "Thup Thup" at a low level in both speakers. It doesn't increase in volume as I increase the volume, But within about 5-7 minites it increases tempo from about once per second to twice that or better.Can't be heard over the music except at low levels. I can see my mid and base driver move once per thup. Any ideas? Happens on all speakers Ive tried and both A and B

The other issue I have is dial noise, mostly in balance. Can I clean these pots with some electrostatic cleaner?. Hose em down from the outside or try to dissasembe the pot?
Thanks for any tips!!

[cliffforrest]

The first thing is probably what we old timers call "motorboating" and was common in battery receivers as the battery got flat and went high impedance. Similar smptom with "high impedance" reservoir caps. *

The cure is almost certainly replacing the dried-out main reservoir caps with some more more modern ones. Same or greater voltage rating, same or greater capacitance value.

I don't think i have ever used the balance pot on my NAD!

You can try a squirt of switch cleaner but it has to be directed inside the pot metal casing.

NAD never used very high quality pots or switches and they are the main cause of failure on the great number of NADs I have repaired - or tried too as spares are not available on older amps and they used quite a lot of non-standard configurations.

Good luck!


EDIT: * motorboating 'cos that is what it sounds like"! The high impedance cause the whole amp to oscillate at very low frequency.