Author Topic: cd4049ube has a hole in it.....  (Read 847 times)

otomo

Re: cd4049ube has a hole in it.....
« Reply #20 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

anotherjim

Re: cd4049ube has a hole in it.....
« Reply #21 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



"The passage of my life is measured out in shirts" - Eno

I've only done it once, so now I'm an expert.

mcknib

Re: cd4049ube has a hole in it.....
« Reply #22 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

« Last Edit: May 02, 2020, 11:31:57 AM by mcknib »

anotherjim

Re: cd4049ube has a hole in it.....
« Reply #23 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.

"The passage of my life is measured out in shirts" - Eno

I've only done it once, so now I'm an expert.

mcknib

Re: cd4049ube has a hole in it.....
« Reply #24 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