Total recall - crackling delay pot

[madOti]

Hello all! I recently built two total recall with vintage ICs (mn3005, ne570, 4558, 4047...) they sounds quite good but weird cracklings turning the delay knob on both. I have a '78 mmd with the same ICs which is darker sounding (has different and older caps) but has a smooth sounding delay pot so I suppose there might be something wrong in my builds. I used mc7915actg regulators instead of regular lm7915s, voltages seems good to me but who knows...any advice? Any suggestion will be very appreciate, thank you

[madbean]

I start with a little contact cleaner or Deoxit on the pots. Crackling pots can come from a little dust/debris buildup in the resist track in the pot itself. There is DC on the delay pot by design but it's not in the audio path.

[madOti]

Thank you Brian, as you suggested I tried with some deoxit d100 but the noise is still there, is some kind of whooshing with some crackles increasing or decreasing in pitch turning up or down the delay pot...they're all new alpha pots and the noise is present on both my TR but my mmd's delay knob is dead quiet, on it just some common little clock whine increasing the delay time. The noise in the signal path feeds the delay circuit so turning the knob I can hear the noise repeating. Feeding the circuit with a 250hz@500mVpp sine wave I can clearly see what happens on my scope, turning the delay knob the sine gets bigger and smaller with some dents here and there and varying is shape quite a bit before stabilize and return to its original and regular shape. I'll try to post some pictures of the TR and the mmd I tried to backup and I used as reference while looking for caps and ICs.

[madOti]

Here some voltages using a european 24v EHX positive center supply measuring 33Vdc. Nothing looks strange to me except pins 2,5 and 6 of IC9 fluctuating around 7Vdc, just some mV, around 100mV...it could be normal, isn't it?

[madbean]

Here some voltages using a european 24v EHX positive center supply measuring 33Vdc. Nothing looks strange to me except pins 2,5 and 6 of IC9 fluctuating around 7Vdc, just some mV, around 100mV...it could be normal, isn't it?

Since IC9 is the LFO some voltage fluctuation is completely normal.

The noise is pretty concerning. Obviously it should not be there at all. Possibly something in the build is acting as antenna and picking up that noise somewhere then amplifying throughout the rest of the circuit. Or, maybe it actually is some clock noise bleeding into the circuit. The biggest drawback in the DMM is lack of any clock adjustment (although they did include it in some later circuits).

My advice would be to try and narrow down exactly where the noise starts. This will be somewhat tedious but shouldn't take long.
Start with feedback all the way off. Audio probe the outputs of the op-amps in a linear fashion. So, first check the output of IC1 pin1, then IC1 pin7, then IC2 pin7 and so forth. Work your way from beginning to end to see if there is an actual point that it first becomes detectable.

Only thing I noticed from the pics is that you 571 and 4047 are unusual looking. I don't know when RCA made 4047 but I get the impression it could be ancient. Do you have any other ones you can swap in? And, I've never seen a 571 compander like that. To be clear - I'm not questioning your judgement on parts used here, just when things go wrong it's worth evaluating every unusual component.

[danfrank]

That's an old school Signetics 571, the original manufacturer of the 57X companders. Philips bought them out in the late 80s or early '90s.
His 4047 is from the 80s. If they are known good ICs, they will work well in that circuit.

[madOti]

Thank you for your replies and suggestions, maybe I found something probing around...
I have many other and newer ICs that I tried few weeks ago but they all seems to respond in the same way about the noise I hear so I left the ones you see beacause they are the ones I found in my reference mmd from august '78 (this is the production date of the internal transformer) so I suppose the signetics ne571 and the rca4047 are from that era (wiki says Signetics was sold to philips in 1975).
I found that the noise I hear starts quite early in the circuit on one side of R9...turning the delay knob I can clearly hear a high pitch clock whine appearing at very long delay time on the r9 side between C6 and pin6 of ic2 (ne571), on the other side of r9, to pin5 of ic2 and r10, I hear the whoosing noise I'm searching for, no clock noise on this side, just that lovely noise I hear all along the delay pot...
The resistor is right, a 68k, so what do you think could be?

[thomasha]

Resistor sounds right.
Is the noise only on one side of C6?
Could it be the capacitor?
I though the green caps are kind of inductive, and try to avoid using them, but it could be just misinformation...

[madOti]

The clock noise is on the two sides of C6.
C6 is before R9, on this side I can hear a clock noise at high delay times, with an audio probe 5cm away from the pcb I can hear that clock noise everywhere on the half pcb where clock ic and bbd are located, without touching anything. The whoosing noise i can hear with a regular use of the pedal turning the delay knob is something like a zipper sound that sounds "zip" or "zop" turning thedelay  knob ccw or cw...and is after r9, on that side it seems to be everywhere. I used green mylar caps because they're everywhere in my reference mmd and because I used them in many other pedals without any problem...but maybe you're right, there's a cap that intercept and leaks clock noise all over the circuit...I don't know...It's quite frustrating...

[madOti]

What kind of caps do you suggest? I found kemets often darker and rounder than others, maybe they can fit good in a bright delay...I usually don't like how they sounds, I prefer the brighter and crispier sounds of pana ecqs and greenies when possible...wimas sometimes sounds very good on bass too, grittier than kemets imho...is there a consensus about what sounds better in here?

[madOti]

I'm changing some parts, begun with the wiring cause moving the output wire solved the clock wine before r9 but the zipper/stepping noise is still present after r9.

[madOti]

Problem solved, avoid ti rc4558p for ic4. I found that in both pedals thouching with a finger (don't do this at home) pin2 of bias2 trimmer the delay pot noise disappear leaving intact the sound of the effect. Looking the scheme I saw bias2 pin2 goes to pin5 of ic4(b, +) so I replaced the ICs with something I had in my pocket, two tl072...and the zipper noise is gone! It's stange cause my reference mmd is full of rc4558p...maybe the new ones are crappy, maybe the circuit has something different, I don't know. I'll try some jrc4558d and dd to see what happens. Thak you for the support, have a nice day