Moog Ladder Shruthi-1 Filter Board Trimmers

Is it possible to break the kind of trimmer potentiometers used on the Ladder filter board (and other filter boards) by turning the screws too far?
I’ve just built the ladder board, and the trimmer pots for volume and resonance level seemed not to do anything, so I just kept turning them. I didn’t notice any obvious click, but now adjusting them in either direction seems to have no effect, and now I’ve lost oscillator sound from the control board (though initially it did work). I can still hear resonance, but it kicks into full self-oscillation at a much lower resonance setting than it should.

a|x

Can you spot anything obvious, tubeohm?

a|x

I guess you would need to drive the screws a bit faster than with you can by hand to melt the inner mechanism with something like this…

Otherwise they are indestructable theres a friction clutch inside which prevents any damage.

Thanks for getting back to me, Frank. That’s odd, then… should I just keep turning the things until something happens, then?? I did try turning them a LOT of times. According to the BOM, they should be 25-turn units. Hmm… I’ll have another try…

a|x

It would be the first dead turned Trimmer of which i know… but luckily they are easy to desolder :wink:

Hi toneburst, i had the same issue, as the “end” of the trimming wasn’t noticeable. To volume: if the volume is loud enough - do not touch it. To the resonance tuning: i had to turn mine about 40 times until i realized a change in the resonance behavior. To be honest, the resonance tuning with those trimmer i bought at the big R served me the hardest time of my life. It took more than an hour to get this right. Compared to my Shruthis, they took 2-3 minutes.

Ah… got the oscillator sound back! I was turning the wrong way, it turns out. I assumed that turning anti-clockwise increased oscillator volume, whereas it of course increases resistance, and lowers volume…

Resonance still seems to kick into full self-oscillation on res value of around 4-5, though. The res pot is now clicking, after turning clockwise for some time, so I assume I now have to turn it anti-clockwise to decrease the resonance feedback level (though this is the opposite way round to the oscillator volume, confusingly). Hmm.

I’ve always found the tuning stage frustrating and time-consuming with all the Shruthi-1 filter boards I’ve built, but this one seems even more so, as you say, nightworxx.

alx

I’ve been turning the res pot for about half an hour now, from the maximum (fully clockwise) value, anti-clockwise, and have seen absolutely no difference in the resonance level. These pots are supposed to be 25-turn, and I’ve gone waaaaaaaaaay over that in an anti-clockwise direction. Am I to conclude that the pot is broken? Or is there somewhere else I should be checking on the board, I wonder? I think I was pretty careful about resistor values (I actually tested a lot of them with my multimeter, just to be sure).

alx

I think it is time to talk to Andre, perhaps some caps or resistor have been swapped/faulty…

Hi, sorry i have read this threat yesterday.
Ok, @ Toneburst, ONE hour ??? nooooooo.
The adjustment is relative simple.
@Nightworxx,
First, yes , i had also one time a def pot. If the pot clicks, sometime the pot doesn’t change the value anymore. ( but normally not )
So, you have the blue PCB ?? Ok, there a 2 Pots for the volume. On pot, POT 3 is befor the filter.
1:)You had set this pot to maximum volume.>>> MAXIMUM ! This means, move it to the end for maximum loudness. And let it so. It is the best balance between the Signal and the reso.
If the volume is too less, the resonance is too loud and via versa MAXIMUM…
2:)Than adjust the resonance. All OCS off, VCF ADSR and LFO to zero. Cut to 50
now screw the RESO Pot until you hear a self oscillation.
In you hear the self oscillation always, you have made a mistake by building.
Look into the schematics and control R45,R46,R19,R37 and R47 . and the reso pot must have 50Kohm!
The reso is controlled by IC6, OTA 2 and get this control current from IC 2B and the TR Q4
If there is no controll current, the Ota 6 closes and normally no feedback is possible.
Maybe you have change 50 Kohm with 5 Kohm ??
First , post a good picture in this forum. And, if nothing helps , and you came from germany,
Simply send it back to me . I will take a look.

Greets

Andre’

@tubeohm thanks for getting back tome.
I have a green PCB- I bought it a while ago, only just got around to building it.

The resonance isn’t always audible. If I turn the res value all the way down on the Shruthi-1, the resonance does disappear. However, it reappears at around a res value of 5. Turning the trimmer in either direction seems to make no difference.

I’ll have another look at the board around the components you identify.

I’m fairly sure I have the correct trimmer installed. The res trimmer is labelled ‘Z 503’, the same as the one for filter linearity. I looked up the codes in the datasheet, just to be sure, and 503 does = 50k.

Incidentally, should there be some way to tell if these kind of pots have reached their minimum anti-clockwise) value. The res pot did click when turned clockwise for a while, but when turned back again, never seemed to do anything to suggest it had reached a minimum value, even when turned more than 25 turns.

alx

Toneburst … though you can’t correctly measure resistance in circuit i think you could get an indication of if the trimmer is working by measuring resistance between the middle pin and the outer pin… Turn the trimmer and see if it changes… if the resistance changes the trimmer should be working,

Do i understand right,
if you set the reso in Shruthi to 5 (display) the reso is there ? Hmm, this means too much feedback signal.
Test R20 (2,2Kohm) and R21 (18 Kohm) and Q 4.
Can be that it is the trimmer.
Greets
Andre’

Thanks shifter and tubeohm, will try both your suggestions.
Yep, tubeohm, with res set to 5 on the Shruthi-1 display, and no oscillator sound, I get self-oscillation. Also, the frequency of the resonance changes slightly when I increase the res value on the Shruthi by turning the knob. Don’t know if this is expected behaviour.

a|x

Yes, the reso moves a little dwn/up, but normally you can with the reso trimmer mute it complete.
My feeling says you have a short in your PCB, or you have use wrong resistors in the feedback way.
Can you post me a picture ??
Greets

Andre’

Hi, sorry for the delay- had a busy time at work.

a|x

Hmm, seems that the resistors are ok.
But look on IC3 Pin 8 ! Can it be that there is no contact ?? It looks like that the IC pin is behind the IC socket.
Can you hear the oscillator signal from Shruthi ??. If not, pin 8 is the output. Than you don’t have a audio signal and also not the 39khz carrier signal.
This can explain , that you only hear feedback.

Remove the IC 3 and look if all pins had contact.
Hope this helps.

Andre’.

Thanks for getting back to me! I see what you mean- it does look like the leg of the IC is bent back. I’m still at work at the moment. I’ll check that when I get back. From memory, though, I think I was getting no oscillator sound for a while, but eventually, managed to adjust the volume pot until the osc. sound started to come through. The Res trimmer still didn’t seem to be doing anything, though.

a|x

Just checked IC3, and it looked OK. I think the metal terminal inside that cell of the IC socket had got pushed up slightly, probably when I was trying to get the socket to sit flat on the PCB. It’s still securely soldered on the bottom of the board, though, and the leg of the IC is making contact.

I think it must be the trimmer. I’ve ordered a new one. I have had these components fail in the past, so I think I just turned it too far, and broke it.

a|x

OK, so I’ve sourced a replacement trimmer, and replaced the old one. The res. is still coming in waaay to early. Before I potentially wreck another trimmer, which way should I be turning the trimmer? And, should I feel it clicking at either end of the range?

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