EHX Small Stone Phasor Pedal LFO

[bobloblaws]

Hello, I have a vintage EHX Small Stone Phasor pedal here that passes signal but there is no oscillation/phase shift. I went as far as replacing the electrolytic caps and IC's and transistors, but to no avail. What I've done now is recreate the entire circuit on a breadboard, but I have the same problem, its passing through the signal but the LFO is not working. Any troubleshooting ideas are most welcome, thanks. (schematic below)

[mozz]

Have you posted to diystompboxes.com? I know there are a few different versions of this pedal. They should be able to help you troubleshoot this.

[bobloblaws]

Have you posted to diystompboxes.com? I know there are a few different versions of this pedal. They should be able to help you troubleshoot this.

Ok, I'll give that a try. For the record, the one I'm trying to fix is identical to the schematic.

[g1]

What voltage are you getting for Vbias ?

[bobloblaws]

What voltage are you getting for Vbias ?

Hey g1, Vbias is 2.8V.

[g1]

Hey g1, Vbias is 2.8V.

That seems low, depending on V+. I would say either V+ or Vbias has an issue. The voltage divider of the 15K and 10K off V+ sets the Vbias. Going by those values, Vbias should be 40% of V+. For 9V V+, that would be 3.6V for Vbias.
If V+ is a good 9V, check those resistor values. Other than that, I would guess that something running off Vbias is pulling it down.

[bobloblaws]

That seems low, depending on V+. I would say either V+ or Vbias has an issue. The voltage divider of the 15K and 10K off V+ sets the Vbias. Going by those values, Vbias should be 40% of V+. For 9V V+, that would be 3.6V for Vbias.
If V+ is a good 9V, check those resistor values. Other than that, I would guess that something running off Vbias is pulling it down.

The 4 IC's that each have Vbias on pin 3 are cumulatively pulling it down from 3.6V.

In any case, it looks like Vbias is not involved with the LFO (please correct me if I'm wrong). I would still expect to see oscillation at pin 6 on the LFO chip shown at the bottom of the drawing, although I'm not exactly sure how to check that. Should I expect to see the DC voltage rising and falling on pin 6?

[bobloblaws]

The 4 IC's that each have Vbias on pin 3 are cumulatively pulling it down from 3.6V.

In any case, it looks like Vbias is not involved with the LFO (please correct me if I'm wrong). I would still expect to see oscillation at pin 6 on the LFO chip shown at the bottom of the drawing, although I'm not exactly sure how to check that. Should I expect to see the DC voltage rising and falling on pin 6?

I made some progress! It turns out the replacement IC's I bought are junk (scam?) despite having the same CA3094E marking. I plugged the original IC's into the breadboard project and now at least I have some oscillation. However, the pot's range is fast fast to hyper-fast and there is not a lot of depth, so I need to figure that out. (I'm seeing DC volts on pin 6 of the LFO chip fluctuate between 5 and 6 volts). Also, the signal is getting to the output even with the 30K resistor that carries the "dry" signal lifted, so the 4 phase shift IC's are working to an extent, but I am not hearing any phasing even with the dry signal mixed back in.

[bobloblaws]

I made some progress! It turns out the replacement IC's I bought are junk (scam?) despite having the same CA3094E marking. I plugged the original IC's into the breadboard project and now at least I have some oscillation. However, the pot's range is fast fast to hyper-fast and there is not a lot of depth, so I need to figure that out. (I'm seeing DC volts on pin 6 of the LFO chip fluctuate between 5 and 6 volts). Also, the signal is getting to the output even with the 30K resistor that carries the "dry" signal lifted, so the 4 phase shift IC's are working to an extent, but I am not hearing any phasing even with the dry signal mixed back in.

It's working! It turns out there was an issue with the pot, changed that out and now the speed is where it should be. Also, the reason I wasn't hearing the phase shift is because I had a sine wave going through it (duh!). Plugged in a guitar and it sounds bona fide!
Now I'll take note of the voltages around the circuit for reference, then figure out why the circuit on the pedal wasn't working. Thanks guys!