Tone Dead Spot

[Billy]

Seeing as we’re in isolation

I’m getting on to stuff long forgotten about

Built a Chrome Dome (GTC Bloody Finger)

All worked great apart from a big gap in the Tone control

So thought I’d throw it in the ring for the technical guys to give me ideas…I’ve no hair left pulled it all out!

Anyway I was reminded if not glad to see someone with a greater technical knowledge than me over on DIYSB was also stumped by it, so perhaps it’s not staring me in the face after all

So on a small portion of the Tone control audio completely dissapears or at least volume is barely audible and clean unless you hit the strings hard, it behaves like a mis biased transistor although it’s not because the voltages don’t change on the transistors, then you turn it a little and it all comes back

Here’s the schem with added crybaby schem the first part of the chrome dome being a crybaby fixed wah using half of the transformer as an inductor going into the 4049 distortion part, the advice given was to change the 2N5210s to MPSA18s that didn’t work, so I changed the tone pot to the original W taper tried A,B and C taper and also A50K value none of which did anything aside from move the dead spot eg C just moved it to the other end of the range as expected I even in my wisdom tried a wah pot but no ceegar

Anyway knowing the crybaby was sometimes sensitive to loading I thought must be something in the filter section so before I go ahead and try adding resistance to various places or flinging in a wah inductor thought I’d see if anyone had any ideas or experience with this dead spot syndrome, does anybody think the 4u7 and 82K and the 470K 33K  being placed differently on the Chrome Dome compared to the crybaby would make a difference given that some people who’ve built it don’t have this problem

[Cybercow]

Billy – admittedly, I’m not familiar with a Chrome Dome (GTC Bloody Finger) – or – clear on how the two circuits are connected – which order, or if there’s an inductor in the circuit.

Still, with regard to the tone control, I imaging you’ve already eliminated a bad pot?

[Billy]

Sure have a few times! I’ll try some of the suggestions made on that site Mark

I’ll report back if I ever get it sorted

It was a commission for someone that obviously he never got so it’s been sitting forgotten about

Now having more time, not because of covid but because I’m off Facebook! I thought I’d throw it out there

It uses half a 42TM013 as the inductor

[Anonymous]

That looks like a wah circuit that has a Tone pot instead of the wah pot.  The dead spot could be the point where the resistance balances the signal with the LRC filter to create a sharp dip in the response like a notch filter.  I believe that Keen did a great examination of the wah circuit that may shed more light on this.

[Billy]

I was hoping you’d chime in Ray!

I’d noted when I built it many moons ago the resistance on my pot from the beginning of the flat spot to the end, can’t find my notes at the moment, what I noticed is at the beginning CCW it’s low volume clean signal when you get towards the end CW more like a mis biased tranny in that if you hit it hard it comes through distorted and a little clearer and as you get to the very end it sounds more like it should with a kind of intermittent stutter

My next move was to change the transformer with a red fasel thinking I may have got one on the extreme of tolerance range looking at the datasheet the 42MT013 is + or – 10% so I thought perhaps that may cause a problem and could be why a small amount of people get this issue

I thought the problem was either a resistance change i.e the 33K being replaced by the 330K 10 times bigger and then further adjusting the tone pot resistance may cause it or the inductor

If I can find one I’ll look at a fixed wah schematic

I note the guy with the same problem on DIYSB posted that he used a charge pump for 18v and it dissapeared completely, albeit pot lug 2 still crackled when touched so again potentially a resistor change would solve it by allowing more current to flow

Anyway I’ll have a read of RGs wah technology page

Cheers mate hope yer all good

[Billy]

The solution to this with credit to Chuck D Bones he thought it was high frequency oscillation on Q2 or 3 and advised to place a 10K resistor in series between C7 and Tone pot lug 1 which got rid of the dead spot

[Cybercow]

Billy – Yay! Good to know. Thanks for letting us know how it was solved.

[Author]

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