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DIY Stompboxes => Building your own stompbox => Topic started by: otomo on April 02, 2020, 01:46:10 PM

Title: cd4049ube has a hole in it.....
Post by: otomo on April 02, 2020, 01:46:10 PM
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

Title: Re: cd4049ube has a hole in it.....
Post by: tonyharker on April 02, 2020, 02:49:58 PM
Sounds like a dry joint somewhere.
Title: Re: cd4049ube has a hole in it.....
Post by: PRR on April 02, 2020, 04:34:55 PM
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.
Title: Re: cd4049ube has a hole in it.....
Post by: anotherjim on April 02, 2020, 04:43:02 PM
...and sing it to this tune...

Title: Re: cd4049ube has a hole in it.....
Post by: otomo on April 03, 2020, 06:17:59 AM
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.
(https://i.postimg.cc/p5h9B6W5/chrome-dome.png) (https://postimg.cc/p5h9B6W5)

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

(https://i.postimg.cc/2LYjKQZT/chrome-dome-pcb.png) (https://postimg.cc/2LYjKQZT)


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?

Title: Re: cd4049ube has a hole in it.....
Post by: otomo on April 03, 2020, 06:28:57 AM
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.
Title: Re: cd4049ube has a hole in it.....
Post by: StephenGiles on April 03, 2020, 09:04:43 AM
...and sing it to this tune...


You beat me to it!!
Title: Re: cd4049ube has a hole in it.....
Post by: duck_arse on April 03, 2020, 10:06:59 AM
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.
Title: Re: cd4049ube has a hole in it.....
Post by: otomo on April 03, 2020, 10:29:22 AM
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!
Title: Re: cd4049ube has a hole in it.....
Post by: otomo on April 04, 2020, 06:06:36 AM
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
Title: Re: cd4049ube has a hole in it.....
Post by: otomo on April 04, 2020, 06:13:08 AM
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?!?!!?
Title: Re: cd4049ube has a hole in it.....
Post by: anotherjim on April 04, 2020, 07:55:32 AM
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?
Title: Re: cd4049ube has a hole in it.....
Post by: duck_arse on April 04, 2020, 09:37:24 AM
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.
Title: Re: cd4049ube has a hole in it.....
Post by: otomo on April 29, 2020, 05:43:24 AM
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.
(https://i.postimg.cc/xkK9S0ds/tempo-flow.png) (https://postimg.cc/xkK9S0ds)

(https://i.postimg.cc/p9jCPL3n/temp-flow-board.png) (https://postimg.cc/p9jCPL3n)

any help would be AMAZING!!! and any more info you need i can send. cheers, hope you and yours are all well. tom
Title: Re: cd4049ube has a hole in it.....
Post by: tonyharker on April 29, 2020, 08:49:19 AM
Q2 base is the voltage really 0.7mV or 0.7v?
Title: Re: cd4049ube has a hole in it.....
Post by: otomo on April 29, 2020, 10:06:03 AM
hi tony, sorry, 0.7v
Title: Re: cd4049ube has a hole in it.....
Post by: mcknib on April 29, 2020, 10:47:01 AM
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/



Title: Re: cd4049ube has a hole in it.....
Post by: otomo on April 30, 2020, 03:03:17 AM
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!
Title: Re: cd4049ube has a hole in it.....
Post by: otomo on April 30, 2020, 07:09:14 AM
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!
Title: Re: cd4049ube has a hole in it.....
Post by: anotherjim on April 30, 2020, 04:55:56 PM
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.

Title: Re: cd4049ube has a hole in it.....
Post by: otomo on May 01, 2020, 02:53:07 AM
interesting jim, sounds like ive got a bit more researching to do then!

it is labeled as UBE, but im going to order some more from a different supplier to make sure before i go much further into this. cheers
Title: Re: cd4049ube has a hole in it.....
Post by: anotherjim on May 01, 2020, 04:20:04 AM
If it says UBE, it probably is - although I wouldn't rule out fakes, I would be surprised if they were. There are different makes of the chip that have different characteristics and get hot when used like they are here.

Have you used different pins on the 4049 than those schematics? You can use any 3 of those inverters but if we refer to them according to that scheme, you need to translate it to what is actually on the pcb.
So, IC2c output pin6 with the feedback R23 wants to rest at around half of its supply voltage, but R22 is pulling (or offsetting) the input toward 0v. It's an inverter so that input pull "down" gets inverted and the output is offset up to a higher voltage. R22 is bigger than R23 so that offset effect should not be a lot.
Things that can go wrong...
Bad chip
Wrong resistor value
Bad capacitor on input side
Circuit board short circuit



Title: Re: cd4049ube has a hole in it.....
Post by: mcknib on May 02, 2020, 11:24:15 AM
I've de enclosured mine seeing as you got my interest back in trying to find the problem

Initially I thought it was a cap problem or the variable cap from reading RG's technology of the wah pedal

The inductor looks to the second transistor like its far side is grounded, through the 4.7uF capacitor. To the inductor, the capacitor kind of looks like it's grounded because its far side is connected to the emitter of Q2. Q2's emitter has a low output impedance and therefore looks like "ground" if you ignore the signal coming out of the emitter. At the junction of the inductor, capacitor, and 1.5K resistor, the voltage looks like the voltage that would happen across a parallel L/C circuit. However - the current through the capacitor is NOT determined by the voltage across the inductor/capacitor, it is also determined by the voltage driving its "ground" side, and that voltage is increased or decreased by the position of the wah pot. If the wah pot setting increases, the capacitor will let more signal current through because the voltage driving it at Q2's emitter is bigger, so the capacitor has to let in more signal current. If the wah pot setting decreases, the capacitor will let in less signal current. A "capacitor" may be thought of as a special instance of ohm's law by the amount of signal current it lets through. The change in the effective current through the capacitor makes the capacitor look bigger to the inductor and rest of the circuit than it really is! We have a variable capacitor!

With it being a crybaby in front of a 4049 distortion I compared the crybaby part to a crybaby schem noted the value changes and also the 82K and 4u7 had changed places and thought maybe that's it Q3 (Q2 in the crybaby) is not getting enough signal when the tone / wah pot is at a certain resistance setting, I think I'm right in saying most wah pots don't use the whole range of the pot and this tone pot kinda mimics that with the dead spot being near the start and end of it

Anyway was messing with it today and purely by luck when I was taking voltages it sprung into life when my probe was on Q3 base and as long as I kept it there it was fine right through the dead spot

So I traced it back touching it with my now disconnected meter probe and got audio until the junction of R11 470K and C3 the 4u7
 

Anyway perhaps the guys with more technical knowledge than me (that'd be everyone) can figure out what's happening I thought maybe it was something to do with the meter circuit but I disconnected the probe and it still came alive when I touched it

Title: Re: cd4049ube has a hole in it.....
Post by: anotherjim on May 03, 2020, 06:39:36 AM
Quote
So I traced it back touching it with my now disconnected meter probe and got audio until the junction of R11 470K and C3 the 4u7
If you see this as a low-pass RC filter, that junction will be a short to ground to any audio frequencies.
The DC voltage on that 4u7 capacitor will be critical to how the preceding transistor biases (I'm not at all confident we are looking at the same schematic, at least as far as part numbers being the same).
When you touch things you are injecting some noise and changing the impedance. Either effect can make something that is marginal and on the edge do something.
As before, check the transistor pin DC voltages to make sure the bias is away from the power voltages and check they are stable when any pot is moved.

Title: Re: cd4049ube has a hole in it.....
Post by: mcknib on May 04, 2020, 12:20:41 PM
This solution from Chuck D Bones worked

'I suspect that Q2 or Q3 may be oscillating at high freq.

Your finger is damping the oscillations. I think a better fix is to put a resistor in series with C7.

There should really be a resistor in series with C7 (maybe around 10K?)'

As advised I lifted a leg of C7  added a 10K in series between that and tone pot 1 and the dead spot dissapeared throughout the tone controls range 

Jim for some reason I couldn't get the schem to upload correctly looked ok but when you clicked on it, it appeared broken up

It's the pedalpcb chrome dome iffin you want a look

https://www.pedalpcb.com/docs/ChromeDome.pdf


Thanks Chuck