Small Stone Crackle & Pulsing

[rundgrenrules]

I have a newly aquired 1980 Small Stone that has some noise issues:

• The white noise floor is pretty high.  I figure thats par for the course.
• There is also a bacon-frying or crackling noise in the sweep, not really like the white noise but similar to the sound of noisey plate resistors in a tube amp
• 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.
• Tested all pads, wires, jacks, switches for continuity and have re-done some iffy looking solder joints on the board and off.
• 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.

I suppose the next step will be measuring each resistor on the board for drifted values, but I am starting to suspect one or more of the EH1048/CA3094 might have been exposed to some excess voltage or some other environmental stressor. 

My understanding is due to the OTA style of the circuit, the normal methods for controlling LFO ticking or pulsing are not possible to implement. 

Schematic that seems to match my unit:



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Noise Sample (White Noise/Crackle/LFO Bump)


The first half of the clip demonstrates the white noise level and crackle, and I ramp up the LFO speed to demonstrate the pulsing for the second half.

[rundgrenrules]

Well, after replacing the ICs I was able to get rid of the sandpaper crackling noise in the sweep.  Overall noise floor is now lower as well.  Problem is, prior to replacing the ICs (to fix the sandpaper-y sweep) I had already implemented two additional capacitors from the Tonepad Small Stone project to effectively completely remove the LFO ticking.  After replacing the ICs, the ticking is back with a vengeance.  I have found a better schematic and marked up the changes I have made in red.  Here are some things I have tried that have NOT gotten rid of this newly stubborn LFO ticking:

• 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.

Are any lower power op-amps possible to sub out for the CA3094 in the Small Stone for just the LFO opamp, perhaps?

Everything else is good now but this ticking!

[Kevin Mitchell]

Are any lower power op-amps possible to sub out for the CA3094 in the Small Stone for just the LFO opamp, perhaps?

Nope. They're discontinued OTAs. Any replacement or workaround will have the same issue.

Question:
I see green capacitors. Those wouldn't happen to be bipolar electrolytics would they?
Low ESR polarized is preferred for decoupling/filtering.

[rundgrenrules]

I see green capacitors. Those wouldn't happen to be bipolar electrolytics would they?
Low ESR polarized is preferred for decoupling/filtering.

Good eye!  I had those bipolar caps sitting around extra after a hi-fi receiver re-cap.  I went ahead and replaced them with polarized caps that read about 0.3R on the ESR meter.  Unfortunately the ticking is the same.  Here is a picture of when I had the LFO OTA off the board on a separate board away from everything else, which didn't fix it either.  You can see the 10uF to ground from V+ and the 100R between V+ and the V+ pin of the OTA, which also didn't remove the ticking.



The shielded input and output cables didn't help either!  A head scratcher...

[Chillums]

While you have it opened I would try moving some of the wires away from the back of the PCB and see if that helps.

[rundgrenrules]

While you have it opened I would try moving some of the wires away from the back of the PCB and see if that helps.

All manner of wire dress approaches and no change!  I might have to try shielding not just the input and output wire from the switch to the jacks, but the input and output wire from the switch to the PCB as well. 

[rundgrenrules]

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.