Uglyface advice requested.

[rutabaga bob]

Hey y'all...built a standard-version uglyface for a friend, and have been testing it out today.  Not working right: it does the self-oscillation stuff, but I have only gotten guitar fuzz out of it a couple of times, and not for long.  Guitar is fine when using bypass.  Maybe a bogus Vactrol, or cmos chip?  Using a VTL5C2.  Thanks for any help!  Happy 4th of July!

EDIT: played with the controls some more, but even with the sensitivity maxed and the threshold low, the crackly fuzz doesn't last very long.   Not much use as a 'crazy' pedal if you can't hear it...

[duck_arse]

circuit diagram? voltages? build photos? please?

[nocentelli]

I'd try to jumper the output cap from the 386 direct to the volume pot to see if the chip-amp is the problem.

[rutabaga bob]

Not hooked up to a photo-hosting site since the 'Bucket got greedy.
Voltages:
LM386
pin 1: 1.363 v
pin 2: 0 v
pin 3: 10.4 mv
pin 4: 1.1 mv
pin 5: 4.24 v
pin 6: 9.16 v
pin 7: 4.62 v
pin 8: 1.31 v

CMOS 555
pin 1: 0 v
pin 2: 4 mv
pin 3: 3.7 mv
pin 4: .426 v
pin 5: 6.13 v
pin 6: 1.04 v
pin 7: -392.6 mv
pin 8: 9.1 v

[Marcos - Munky]

Not hooked up to a photo-hosting site since the 'Bucket got greedy.

Use Postimages, it's free and doesn't asks for a registration. You can find an "Add image to post" right under the reply box, that it's a shortcut to Postimages.

[duck_arse]

you used to be able to do anonymous posts on imgur, ?still can?

some observations - yer lm386 - pin 4 is ground, should show 0V, not something volts.
your 7555 - pin 2 is short to pin 6 on the circuit dia, but your volts .......
pin 7 can't be a negative, because it has a 100k pulling it to supply .....
if you work the threshold pot from off to full, you should get pin 4 vary from ~0V27 - 3V. does yours?

[rutabaga bob]

Will check, Mr. Duck.  Just got done doing some checking and audio probing.  Weird things are going on.  Probe on pin 5 of the 386...didn't really get anything from the tone I used as input.  Fooled around with the threshold and sensitivity knobs and got the self-generated sounds from pin 5...which quit after a bit, and I couldn't get them back.  Then I got them back.  Turned the knobs back to allow signal...no signal.  Turned the effect off.  Turned it back on, turned on the test tone and got a split-second of sound.  Set the knobs for self-oscillation, got it for a while then nothing, no matter how I set the knobs.  Is there a 'hair-yank' emoji?

Stephen...I won't vouch for the accuracy/quality of my multimeter, old cobber.  Will that voltage swing you asked about be checkable with the IC removed?

[duck_arse]

yes, voltage on that string should be like I said with the IC out of socket. can you lash-up a test circuit for the 7555 on a breadboard? like a slow led flasher, just to prove good. about the 386, you might try and add an electro across the supply pins, right at/under the socket, as big as you like. many people seem to fix their 386 circuits with bypass cap to kill supersonic oscillation. I seem to recall the badness varies according to the IC part # suffix.

[rutabaga bob]

Okay...for those keeping score at home, voltages, no ICs installed:
386
Pin 1: .5mv
Pin 2: 0
Pin 3: -.3mv
Pin 4: 0
Pin 5: 291mv and dropping (connected to 2 electros)
Pin 6: 9.44v
Pin 7: 2.7mv
Pin 8: .5mv

CMOS 555
Pin 1: 0
Pin 2: 9.08v
Pin 3: 9.08v
Pin 4: 289.2mv
Pin 5: 3.3mv
Pin 6: 9.08v
Pin 7: 9.33v
Pin 8: 9.44v

Pin 4, range when I turn the Threshold knob: .423v - 3.11v

[duck_arse]

I think wiring error around the 7555 [as per email].

[rutabaga bob]

Update: had it back apart...can't see any mis-wiring, for what that's worth.  Rigged up a way to hold the faceplate so as to access both sides.  Worked, then didn't.  BUT: discovered something others have experienced - the 555 chip gets hot.  Will do a search for other posts in this regard.

[rankot]

You can't use regular NE555 or TLC555, it must be 7555.

[rutabaga bob]

It is a cmos 555.

[rankot]

CMOS won't work.

[rutabaga bob]

That's what a 7555 is...

[rankot]

Indeed it is - now I saw datasheets - I thought it is some version of regular 555!
I though only TLC555 is CMOS, however, when I tried it in my Uglyface, it was overheating, so I thought that the problem is CMOS specific, when I put ICM7555 inside, it worked well.

[Marcos - Munky]

Does anybody knows the reason for this circuit to require a cmos 555? They're pretty expensive here.

[duck_arse]

the original bipolar 555 has a string of three internal resistors between Vcc and ground, with the upper//mid node connecting to the CV at pin 5. these resistors are each 5k. in the ICM7555 datasheet for the cmos version, the resistors are specified as 100k +/- 20%. so - if you measure the resistance between pin 1 and pin 8 with the IC out of circuit, and it reads ~15k, you can be fairly sure it is NOT cmos.

I tried some measures, and on 2 RC555NB (1976 datecode), they read 13k4/9k and 13k8/9k2 [pin1&8/pin1&5].
on a single GLC555 [on which I have no data, but was sold as cmos 555], I measure 370k/239k, which figures.
on a single Signetics 7555CN, I measured 124k/85k - which kind of tallies, certainly not 15k.
and for shizzle kicks, I measured one NEC D556C [which is a dual cmos 555 ....] and ..... it didn't hold up, 370k across supply, but 334k between CV and ground. 215k between Vcc pin and each CV pin.

anybody want to add a few more datapoints? the Signetics part works in the face I built, I didn't notice any heat or other funnies.

as for why they would run hot, I could only guess that the Reset flip-flop doesn't like being run in linear mode, or there is something screwy about connecting Output and Discharge via the freq pot.

[rutabaga bob]

FINIS!  Got in the 7555 chips, installed one, and that solved the issue!  Got a couple of extra ICs, in case I ever build one of these again.  The guy who's been waiting on this is very glad to see it completed!

Thanks to you all!

[rankot]

I have also measured my 7555 and it is around 125k. Works fine in my Uglyface.