Hi, my 838 has been annoying me lately. It has a niggly problem where if I switch the stereo selector from mono to stereo or biphonic, the treble cuts off on one channel.

With the unit in stereo, I move the balance to the problem channel and move the treble slider up and down. It makes no difference to the sound. When I move the balance to the other channel and I move the treble slider, it makes a big difference as it should do. When the unit's in mono, the treble works fine on both channels.

I've had the unit apart and cleaned every control, but it's made no difference. I don't know what to do next.

Can anyone suggest anything?

I remember a issue in my 838.
When I bougth it, bass control didn't has effect.
As you say I clenaed the slider of the unit.
The bass control didn't work after the cleaning.
When I openned the slider, I notice that one contact was mising because one part of the slider broke (a very litle and thin piece of plastic).
In mono thing change ... I guess the BBX sums-up both signals in one, so both channels get all the information. Dependign on the system that could occur after the tone control, so the trebled channel sums up to the flat channel giving an overall perception that the treble control works ... but really it most work by the half ....
Look like a slider issue. Remeber that the slider (a good one) most read 0 at max o min position. If you cant read 0 ohms with an ohmeter .. it's definitely open.
In my case read about 20K ... (that's the leackage impedance of the circuit seen by the slider, because I did the measurement with the slider soldered).

This one is form ALPS, and it was used in many Sharps happily.

Pedro

Thanks for the reply, Pedro. Using an Ohmeter is beyond me yet, I've never seen one and wouldn't know where to probe it or how to use it. So that option is out of the question for now. I'll have to just put up with the problem. It sometimes works as it should, but it's very rare and on & off. Thanks anyway.

well it sounds like the slider may in fact is wearing down the carbon compound contact on the board which after years of useage, but the best way was to replace with a new one as its not any component related just the slider pot

It's easy to replace in the 838, you only need to take the mian board appart, unsoldering some wires first (I hate that from JVC).

And the slider was used in other boxes like sharp ... so it wouldn't be difficult to find.
In fact, the APLS replacement that I put came in a "sharp parts" labeled bag !!!. It was for the GF-300 (could be)

Sometimes, like the record bar, cleaning using spray isn't enough ... you need to dismantle the piece.

I'm No techno Geek Ra,But i had a similar prob with mine. I was going to suggest a through cleaning but you said u did that allready,That worked for me! Good Luck!