Odd fader problem on Yamaha Analogue Mixer

[paulears]

I have a Yamaha GA32 mixer. It's 6 years old now, and been very reliable, but a while back one fader failed. Nothing happens at all for about an inch of travel, then it comes on at full, and stays there for the rest of the travel. No noise, no crackle and nothing in the 'feel' of the fader. With 32, I just stopped using that channel. Now another is doing exactly the same - one of the groups this time. Again, I've just moved one along, but this doesn;t to me sound like it's a fader fault in the old fashioned sense. I can't see quite why a fader would fail in this way. The group is not a VCA - unless all the faders are actually VCAs, but I doubt this. I have no service manual - as I had to import this mixer as Yamaha in the UK would not supply it, or offer any backup service or parts for it. It's discontinued now, but if I can, I'd like another years use out of it. If it's a case of swapping the faders, I might have the time to do it - but I can't see how a fader could fail in this way and become a switch?

 

Anybody got any ideas, or had this happen to them?

 

Yamaha link to the item

 

http://medias.audiofanzine.com/images/normal/yamaha-ga-32-12-98200.jpg

[DrV]

If the carbon track is cracked then at the bottom end you have the wiper connected through a resistor to ground only. Above the crack you have the wiper connected thru a resistor to the signal. If the input impedance of the following stage is high enough, which it will be, then the resistors become negligible. Result - exactly what you are seeing.

 

Scruffy sketch here https://www.dropbox.com/s/aic1s5uzkqwt89d/20160502_234402.jpg?dl=0

[dbuckley]

'Tis something of a common occurrence with faders.

[MarkPAman]

If the carbon track is cracked then ....

 

Yep.

 

 

I have managed a pretty good temporary fix with a drop of conductive paint in the past, but you really need to replace the faders.

[paulears]

Thanks for that - something I've never come across before , just noisy one! Cheers!

[david.elsbury]

Are carbon faders common for new kit these days? Is there better tech out there? I always thought carbon pots were pretty old school- am I wrong? :)

[DrV]

Are carbon faders common for new kit these days? Is there better tech out there? I always thought carbon pots were pretty old school- am I wrong? :)

I was using "carbon track" in the generic sense meaning the resistive element. Low cost general purpose pots use a carbon loaded compound, higher quality ones use a conductive polymer and expensive, long life high stab ones such as those found in servo systems use cermet tracks (metal loaded ceramics)

[GaryNattrass]

Have you tried a spray of servisol to clean the track?

[paulears]

Not had time to even take it apart yet Gary - although I'm not a great lover of Servisol.

[jamesperrett]

Not had time to even take it apart yet Gary - although I'm not a great lover of Servisol.

 

When I first bought my large A&H desk it needed a number of faders to be serviced. The engineer replaced the worst ones but then completely disassembled the other noisy faders and cleaned the tracks using cotton buds soaked in isopropyl alcohol. They're still fine 20 years later.

[paulears]

I'm going to have a go as soon as I get a chance James - It's big enough to make servicing with big fingers not too much of a chore (says I, BEFORE taking the bottom off!)