V3 EHX Small Stone LFO Ticking Fix

[rundgrenrules]

I have a 1980 Small Stone that has a pretty decent lower frequency "bump" or pulse" in the LFO.  Its not really a high-frequency LFO "tick" like I have heard before.  Of course this bump is much more noticeable at higher speed settings and with the color switch engaged.

I have gone through the pedal and done the following:

• Tested all pads, wires, jacks, switches for continuity and have re-done some iffy looking solder joints on the board and off.
• Swapped out the footswitch for a 3PDT and installed an LED and modern 9V jack.
• All the electrolytic capacitors have been replaced and even the film caps as I started to suspect them.  Neither was the cause of the noise but I am glad at least to have new electrolytics in.
• Pulled the (3) 2N5087 and (1) 2N5088, tested their gains, replaced them, determined they werent the source of the noise, then socketed the transistor locations and put the old ones back in.
• The 3rd lug of the DPDT color switch was not connected to ground as I believe it should have been.  That cut down on some white noise when corrected.
• Lead dress has been tightened up and experimented with in an attempt to cut down on noise.
• Input and output jack wires have been replaced with shielded wire.

Next I went and implemented the LFO tick reduction mods from the Tonepad Small Stone layout (in red):

• Enlarging the values of the additional 100nF & 47uF capacitors to ground from the Tonepad layout.  No higher values worked and only began to slow the LFO down at higher values.
• Breaking the LFO IC (U5) off onto its own separate mini-board away from the rest of the circuit.  No change.
• Adding a 10uF cap to ground at the V+ (Pin 7) of the LFO IC and a 100R resistor betweeen V+ and Pin 7.  This also didn't result in any reduction of the clicking

The 47uF cap to ground has removed all the low-end pulse of the LFO and the 100nF to ground has removed some of the high-end pulse, but there is still a decently loud higher frequency tick after trying all of these methods to address.

Picture of the LFO IC broken off onto its own circuit board (reverted since no change):



I have read that the Russian Small Stone had some circuit changes meant to address the LFO tick but I can't seem to find a schematic for the Russian version(s) online.  Does anyone know where to find one?  I would like to compare the two and see about implementing the fixes.

Anybody have any ideas?  Thanks!

[rundgrenrules]

bump!

[ElectricDruid]

I've had serious LFO bumping in OTA-based circuits when the Iabc input current goes so low as to effectively cut the signal off, or if the voltage goes below the diode-drop threshold of the Iabc input itself. Basically, you have to watch the biasing on that input and not push the range too far. Can you reduce the LFO depth and get rid of the ticking? Can you move the LFO centre voltage up or down and remove the ticking? Those would be the sort of things I'd be wanting to try.

[rundgrenrules]

I've had serious LFO bumping in OTA-based circuits when the Iabc input current goes so low as to effectively cut the signal off, or if the voltage goes below the diode-drop threshold of the Iabc input itself. Basically, you have to watch the biasing on that input and not push the range too far. Can you reduce the LFO depth and get rid of the ticking? Can you move the LFO centre voltage up or down and remove the ticking? Those would be the sort of things I'd be wanting to try.

I have read that there might be a simple resistor value change to address the ticking - a change which i assume could do just that - raise the voltage/current or lower it.  Unfortunately the link to the resistor change modification is dead (2009 is the year I found a post which linked to it) and identifying which resistor would accomplish this is a little out of my depth.  Looking at the CA3094 pinout it looks like the IABC Current pin is connected directly to the rate potentiometer.  Center voltage I assume is going to be dictated by whats coming into pin 7 on a CA3094?  Currently its coming right off the 9V supply.

[ElectricDruid]

I don't claim to understand that LFO built around U5 either since it's a pretty unusual-looking thing. I'd have to really study it to get to the bottom of it. The LFO output is at U5 pin 6 (and it's a current output, since it's coming from a OTA) and that's feeding current to the Iabc pins of the shift stages. So I might try tweaking R31 for starters, since that's the only resistor in that path.
Since it's a current output, I'm not sure how much sense the "centre voltage" concept makes, but I don't understand why you'd assume that it's referenced to the pin 7 +V power supply input.
Anyway, for starters try adding a 1K trimmer inline with that 100R resistor and see what happens.

[PRR]

the link to the resistor change modification is dead (2009

Have you hunted/dug-in https://web.archive.org/ (the Wayback Machine)?