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SH-101 Repair
MUFF WIGGLER Forum Index -> Music Tech DIY  
Author SH-101 Repair
Hi all,

I've got a SH-101 with its LFO always routed to the VCF.

The MOD slider changes the amount of LFO affecting the filter but the modulation is always there and at a significant amount; it's like the slider is almost halfway all the time.

So far I've checked IC18 (swapped it with IC4, also a IR9022 ), swapped the LFO rate slider with the MOD slider (they're both 100kA) and also tested TR42 with a transistor tester.

I've also checked the ribbon cables between the boards for continuity and they're all fine.

I may need to desolder and check R191, R192 but I don't think these to be faulty because I get a reading out of them and the ENV and KEYBOARD sliders seem to affect the filter as they should.

Has anyone have any ideas on how I proceed with this?

I'm probably going to check the LFO circuit next but since it's functioning correctly on all the other modes I'm not really sure if there's something there.

The bender board can also affect the filter with the LFO; I thought about disconnecting the whole board but I'm not really sure it's safe; need to take a closer look at the schematics.
is there any issue with the mod effecting the VCO?

remove the mod slider to the VCF and see if there is no longer any mod going to the VCF. its possible the signal is finding another way to the filter than through the slider. if this does make it go away, then the slider is most likely bad - it probably needs to be cleaned.
The VCO modulation works as intended.

I removed the MOD slider and the LFO doesn't affect the filter anymore.

But I've already swapped that slider with the LFO rate slider.
The current MOD slider (the one that used to be LFO rate) is worse than the original in its position as it has visible damage to the carbon tracks.
But I had that problem with the original too, so while both sliders need replacing I'm not sure it's only that causing the problem.
On other hand some leakage on the LFO rate does not really affect anything as the LFO is always on.

When the currently installed MOD silder is at its minimum position I can see the LFO waveform on the wiper.
So there must be some kind of leakage going on.

I may try swapping sliders again as the LFO rate is not important at this point but I'm probably going in circles here..

There's not much going on at the potentiometer board; so that leaves with only the slider malfunctioning..
From what I gather the selected LFO waveform goes to a buffer (IC18A), then to one end of both MOD pots and then the wiper of filter MOD goes to the other half of IC18 which sums (?) the controlling voltages to affect the filter.

What could be another way for the LFO signal to creep into?

EDIT: It was the pot..I've found another 100kA pot put it in there and it's now fixed.
glad it got sorted. its possible the track was busted inside, right near where the pot connects to the +5V rail. this would change the mod impedence from 220k to 320k as the pot slid, changing amplitude from full to -50% or so.
I'll try to source new sliders for it.
If you know a place where I can order from let me know.
I think the slider is 30mm travel with a 15mm (?) shaft.

I've not seen it in stock anywhere right now.Most places I buy from only have regular pots.

I've only found it here which seems kind of expensive tbh,especially since I've thought about replacing them all to future-proof it.
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