Dead Varitone

Mike58

Hi looking to see what to test for in a dead Varitone circuit from the ? late 70s.
This is in a guitar that has been stored for 20yrs+ and whilst there is output and all pots work, there is no tone change when the Varitone selector is moved around. it did work pre storage .

It looks to be the classic Gibson style circuit with caps and resistors wired in parallel with ? 100ohm final resistor and small inductor producing the notch filter arrangement.
Any advice in bringing the circuit back to life would be appreciated.
guessing this is the circuit , with Gibson’s “accidental” labeling of the inductor value as 1.5h

ICBM

100K, not 100 ohm... big difference .

I would guess the problem is corrosion either in the switch contacts, or the choke.

Try touching the terminal where the grey wire goes between the rotary switch and the choke, with the guitar plugged in - if you get a buzz, which changes with the varitone position, it's the choke. If you don't, it's the switch. It could also be a failed solder joint but they do look OK - unless it's where the ground wire from the choke goes to.

Mike58

Cheers John, I was hoping you would reply. There is a LOT of corrosion going on, so either way I’m probably going to replace everything with a simpler modern varitone design with less of the useless tones, my plan would be fairly low cap values and ending up with a brighter and darker cocked wah.
It’s a guitar I co-designed for my 20th birthday and had made in USA when I was there… it’s not great at all ( hence the storage) but I’ve dug it out and I’m determined to get it as good as it can be.
if you know of a design of Varitone ( with or without choke ) that would just give me some subtle variances from sweetening to cocked wah without mud, let me know.
regards
mike.

Mike58

ICBM said:

100K, not 100 ohm... big difference .

My bad ( as the kids would say )

ICBM

I don’t know, I’ve never found increasing degrees of nasal honk useful. I would probably re-purpose it as a tone control cap selector, but it would need two poles and sets of caps if there are two tone controls. On the other hand, if the switch is knackered you’ll need to replace it anyway, so you could then get a 2P6T.

Mike58

Manage to see a De Lisle varitone in the flesh today, very nicely put together , includes a small inductor , no cap resistors as switch is a modern Alpha MBB so no pops. Still a bit confused re inductor values… original varitone allegedly used 15H value ( purposely mis written in their schematic as 1.5H to avoid copying ( similar “ errors in PAF drawings too) ). 

Judging by the weight of the inductor in the DL version I don’t think could be anywhere near 15H and it’s covered in epoxy to avoid prying eyes.
i love a bit of counter espionage from Gibson back in the day  .

Mike58

IT LIVES !!! I had another look at the control cavity today and sprayed some pot cleaner ( WD contact cleaner) into a scratchy pot, then decided to fire some into the switch…. Bingo! The circuit sprang back into life, I reglued the inductor that had detached itself over the years.
So happy to get a neglected old guitar back to full electrical health.. now the hard work begins with the actual playability