cd4049ube has a hole in it.....

[otomo]

so ive been pulling hair over this for a few months. i wanted to use electric druids tap tempo board to make a tremolo, but instead of a clean sound, have a overdrive circuit instead. so, i decided to combine the chrome dome / bloody finger with the tapflo chip. its pretty much working, and i got around lots of issues with lfo noise etc, but, on my tone stack, theres a hole?!!?

from left to right, when i get to say about 9 o'clock there's a crackle and then all distortion goes, and a very faint dry guitar sound. then, at about 12 o'clock it cuts back in. if i turn the tone to say 11, while not playing, i can hear everything is ok, but the slightest touch of the strings, and the sound will cut out again.

ive followed the gtc schematic and the chrome dome pcb schematic in terms of values and components, and my layout is retty uch the same. ive even built a new pcb discluding all the components that make up the tapflo tremolo, and still the problem exists. ive tried changing the inductor/tranny, different cd chips, different transistors, and still i have the space on the tone pot that cuts nearly all sound and distortion. what puzzles me is the tone control is before the 3 gain stages of the cd, and an audio probe on the transistors seems fine.

it almost sounds like a starved transistor, but power is all getting to them fine.

anyone had a similar experience? can post voltages if anyone could possibly be of a massive help to myself!

cheers, tom

[tonyharker]

Sounds like a dry joint somewhere.

[PRR]

Dry joint. Bad pot. (Not talking about dope.)

Supersonic oscillation. When pot is trimmed very near the edge of cut-out, have someone else wave their hand near the circuit, poke a "lead" pencil between wires and parts.

[anotherjim]

...and sing it to this tune...

[otomo]

ha, loving the song choice! cheers!

but, unfortunately, this is the 4th attempt on a new pcb, all new components, and exactly the same problem.

when i get to the 'hole in the bucket' or the gap on the potentiometer on all 4, its the same.

so i tried moving my hand around, battery supply, pokey pokey all over and nothing changes the situation.


this is my schematic, and board layout, please ignore the air wire on r9!




pin readings for the cd4049

1-15.4
2-4.9
3-5
4-5
5-5
6-5
7-5
8-0
9-1.6
10-15.2
11-1.6
12-15
13-0
14-1.7
15-15
16-0

q1 is e-170mv b-0.7v c-3.4v and its a 2n5088
q2 is e-190mv b-0.7mv c-5.8v a 2n5210
q3 is e-4.7v     b-5v      c-14v  a 2n5210


and its a 42TM013 tranny im using as well. is perhaps q3 looking a bit suspect?

[otomo]

i've jumped the gap where the vactrol should be for testing, so i know its not to do with that, and im running it off a power supply board, with lm charge pump switchable between 9 and 18 v. these readings were on the '18v' setting, although i do get a bit of current loss through protection diodes, hence the 15 volts ish readings!

and the pads at the bottom are to connect to my footboards, but again, this isnt connected atm, ive just jumperd again across the pads.

[StephenGiles]

...and sing it to this tune...

You beat me to it!!

[duck_arse]

you must, must, MUST connect the unused invertor inputs to a logic level. can be ground, can be supply, can be a mix of both, but EVERY cmos input must be tied to a logic level. can be high or low. leave the unused outputs floating.

[otomo]

thank you duck.

i think that was what i tried about 3 months ago, but ill pull the pcb out the draw and tie them all to gnd.

fingers crossed!

[otomo]

so, unfortunately connecting pins 9, 11 and 14 (unused inputs) to ground didn't solve the problem.

its so strange, tried 3 more pots, and still the exact same thing. from far left, its fine, once it gets to 9 o'clock, that's when the sound is all cut out, then from 12 o'clock its fine again.

if i mute the strings, then turn the pot into the dead zone, i can strum the strings, and get a millisecond of sound, and then it abruptly cuts out. it sounds like the power is just turned off. really cant understand whats happening as i never experienced a transistor or ic almost being overloaded?! and when im approaching the dead zone, just before and after it sounds like a starved transistor with that flappy clippy sputtering decay on the notes. but yea, in between 9 and 12 o'clock, there's just nothing.

can anymore information be of any use to anyone to see what might be occurring?! really want to get to the rout of this problem as whole in lock down its just sat there staring at me!

cheers

[otomo]

just and update, ive checked the voltages of all the transistors while sweeping the tone potentiometer. the voltages dont change while the sound comes in and out.

and, if i strum the open strings and sweep the tone pot, the sound wont cut out, its like the sound only cuts out with the first attack of the strings, like in that dead spot something is being overloaded by input signal?!?!!?

[anotherjim]

Could really do with a schematic of what you made. Do all the used inverters have feedback resistors connected? Any electro caps in the signal path fitted reverse?

[duck_arse]

do you really have a 22uF at the input? and with no pull down?

+1 for your circuit dia, and photos of the real world build.

[otomo]

hi guys, sorry, thanks for all your help so far. been busy with a parent who came down with the covid, been stressful!

so here's an image of my schematic. its the tapflo chip from electric druid, where the led is linked to the ldr at the end of the chrome dome distortion on music pcb (i think).

so a mosfet overdrive mixed with a tremolo. also here is my layout, ive used 2 grounds until the daughter board to try to keep digital and analog separate, so -vc is digital GND, and foot switching is also on a daughter board. i have also tied all unused inputs to gnd on the cd4049, but that still didn't solve the issue.




any help would be AMAZING!!! and any more info you need i can send. cheers, hope you and yours are all well. tom

[tonyharker]

Q2 base is the voltage really 0.7mV or 0.7v?

[otomo]

hi tony, sorry, 0.7v

[mcknib]

I built the pedalpcb version of the Chrome Dome using their pcb

Exact same problem contacted him after tearing my hair out and he said a few people have had this problem which they rectified by changing the 2N5210s Q2 and 3 with MPSA18s,
it didn't work for me

I'm not technically gifted but when he said MPSA18 I thought crybaby, maybe a modded value in the fixed wah section could be causing the problem?

I've already tried changing the tone/wah pot to various tapers and values it merely shifts the dead spot eg with C taper as you'd expect it moves to the opposite end,  I've tried the lot A,B,C,W taper dropped the value to A50K even tried a wah pot nothing worked from that point of view aside from that I didn't do a lot being busy at the time

Pedalpcb have a forum I've not checked for a while so you might find someone's sussed it on there

https://forum.pedalpcb.com/threads/chrome-dome-distortion.765/

[otomo]

dude, sorry to hear that, but also really relieved!

im not an expert in this stuff, but pretty competent in understanding a circuit, and this problem just has me completely stumped!

so there's some sort of problem in the sweep of the frequencies, but yea, definitely sounds like a transistor is starved, even though its not!

glad to hear im not the only one thought!

[otomo]

so, changed Q2 and Q3 over to mpsa18, tied the middle leg of the tone pot to gnd through a 120k resistor, and still the same.



however, when i used a charge pump to supply 18v instead on 9, cleared it up! still some static noise when your finger makes contact with the pot in the 'dead' position, but goes away when you release. though the sound i had sounded like a starved transistor, just not sure why the power draw would be more at that specific point when its a passive tone control?!?!



wierd!

[anotherjim]

Could be a power rail limiting thing. That is somewhere an output is sitting at as high or low a voltage as it can go so when signal tries to wiggle it, it's jammed out.
In your last set of readings, some of the 4049 outputs are at 15v or 0v. If they still are - it's a problem. Paradoxically, current consumption will be lower when they are jammed up like that, but it makes it harder for signal to wiggle! However, this design deliberately pulls the inverter amplifiers off-centre by way of the input resistors going down to 0v in order to lower current consumption. 

I have to ask if you are really, really, absolutely certainly sure that the 4049 you have is a 4049UBE type and not a 4049BE. The BE will be very wrong for this circuit.