Meat Sphere clipping/gating

[nosamiam]

I built this one a while ago and it used to work fine. Now it's not so fine.

I had to arbitrarily assign component designators since the schematic doesn't have any. But should be pretty self-evident.

Any help? I'd love to be able to use this again.


1.What does it do, not do, and sound like? Nasty clipping/gating on all settings. Passive Fender P-Bass
2.Name of the circuit = MusicPCB Meat Sphere
3.Source of the circuit (URL of schematic or project) = Musicpcb.com

https://static1.squarespace.com/static/54696a3ee4b05a85cbfe56e5/t/552da76fe4b0a0d7241d2da9/1429055343405/Meat-Sphere-build-PDF-updated.pdf

4.Any modifications to the circuit? N
5.Any parts substitutions? If yes, list them. N
6.Positive ground to negative ground conversion? N
7.Battery = 9.47 (after all poking around was done. Seems to drain pretty quickly. Started at 9.62 fresh. 10 mins later... 9.47)

Voltage at the circuit board end of the red battery lead = 9.35V
Voltage at the circuit board end of the black battery lead = 000.1mV


U1 TL074
P1= 4.67V
P2= 4.67V
P3= 4.67V
P4= 9.36V
P5= 4.67V
P6= 4.67V
P7= 4.67V
P8= 4.67V
P9= 4.67V
P10= 4.67V
P11= 000.1mV
P12=3.80V
P13=4.67V
P14=4.67V

U2 LM1458
P1=2.062V
P2=1.937V
P3=000.1mV
P4=000.1mV
P5=0.873V
P6=2.268V
P7=2.268V
P8=1.6mV

D1/D2 1N4148 pair near U2 = 2.060V / 1.934V

D3/D4 1N4148 pair near U1 = 4.61V / 3.77V

D5 1N4001
A  = 1.3mV
K = 9.30V

Opto LED1 (Vactrol)
A = 7.44V
K = 9.08V

Opto LED2 (Vactrol)
A = 7.44V
K = 9.08V

[bluebunny]

Your 1458 has no power?

[nosamiam]

Your 1458 has no power?

Well that's interesting. I'll doublecheck and post back.

Thanks for catching that!

[nosamiam]

Ok, turns out I made a typo or my ground came unclipped or something. Revised readings for LM1458 are:

U2 LM1458
P1=2.079V
P2=1.949V
P3=000.1mV
P4=000.1mV
P5=0.878V
P6=2.262V
P7= 2.262V
P8= 9.06V

Anybody have any ideas?

[idy]

I would use an audio probe. But you will probably find the problem is in the 074, as that is the only chip the clean audio passes through... But it's voltages look normal.

Oh, do you get gating and clipping when the blend control is at full CCW?

The 1458 is what generates the "envelope" for the filter, so if it is bad the sound might not be. A lot of us have had to swap out this chip with various others to find the right fit. But when it doesn't work it means a wonky filter. Which maybe you are describing as gating?

Important that it used to work and now doesn't.

I pulled one of mine open and measured 0(or very low) on all pins except 8. With sweep set for down I found about 1v on pins 6 and 7.

[Govmnt_Lacky]

Wondering why you are seeing 0V on Pin 3 of the 1458??

Having ~2V on Pins 1 & 2, you should see something other than 0V.

EDIT: Check for a bridge between Pins 3 & 4 on the 1458. Also, check the 22K resistor between Pin 3 and Ground (make sure it isn't actually a 2.2K or 220 ohm)

[idy]

Pin 3 goes 22k to ground. Odd, if the designers had understood the (bipolar) Mutron they might have sent it to VB... But that's how it is and it works. So yes, that non-inverting input is referenced to ground.

But yes,  check for solder bridge there, funny both pins are at the same not-quite-zero. But also remember, it worked before...

[nosamiam]

A ha! It was a cold solder joint on that 22k resistor going to the 1458 supply pin! No more clipping/gating AND it explains why it worked for a while and then stopped.

Now I'm finding that not all the settings are working (I think. Haven't had time to really play it), but I guess it's probably another cold joint(s). I'm gonna reflow all the joints and see if it clears up. Luckily my soldering technique has improved since I built this!

I think it's safe to close this ticket. Thanks for the help!