Sansui au 70 Stereo Tube Amp noisy volume pot

[unparalleled]

Just thought I would share a repair trick.
This is a common issue with these Amps.
The pot is a Dual gang 500K with tap and a long splined shaft and is not obtainable.
I cleaned it with spray, this improved it but when the amp was first switched on from cold the problem persisted.
The pot also serves as a grid leak for the following stage so I added a 2.2M resistor from the wiper to ground.
This stabilised the grids and drastically lowered the pot's noise to an acceptable level.
Alan.
Anvil Amplifiers
New Zealand.

[Jon Snell]

DC on the grids can be caused by faulty or gassy valves. That in itself will cause scratchy pots, whether cleaned or not.

[Mars Amp Repair]

Hello Unparalleled,
I would assume you already swapped the tubes involved, correct? Either way this is good info as when these old amps/receivers get older, values & other parameters you can't even explain change, and then require weird countermeasures to correct that you can't always explain. We've all run into this type of thing.
Thanx, Glen

[unparalleled]

Hello Unparalleled,
I would assume you already swapped the tubes involved, correct? Either way this is good info as when these old amps/receivers get older, values & other parameters you can't even explain change, and then require weird countermeasures to correct that you can't always explain. We've all run into this type of thing.
Thanx, Glen

Thanks for that.
Yes I substitute tubes during analysis.
The Pot was very intermittent before cleaning, cracking and banging.
I was almost going to replace it with a NOS one with no tap but that would have meant losing or altering the Loudness filter.
It sounded like a little DC present when turned but the crackling reduced as the amp warmed up and that made me think of the grid leak.
I Build/ Repair a lot of Tube stuff but no one is infallible!
Alan.
Anvil Amplifiers (on F/Book)
New Zealand