Moog Ladder Shruthi-1 Filter Board Trimmers

You should feel it clicking at either end of the range.
(And it should be possible to destroy it by turning it to far either.)

@V’cent Hmm… with the previous one, I got it to click at the clockwise end of the range, but never at the other end. I turned it a LOT (a lot more than the quotes 25 turns in the component spec), and it never clicked at the anti-clockwise end.

a|x

The mystery deepens…
I’ve replaced the res trimmer. This time, it clicks at both ends of its resistance range, but again, there’s barely any difference in the resonance level, however I adjust the trimmer. The resonance kicks into full self-oscillation at a value of 4 or 5 on the Shruthi-1 display.

I surmise it wasn’t the trimmer than was the problem, afterall…

Anything else I should try?

a|x

Hi , do you have a scope ?what happens with the normal oscillator signal , can you hear it ?
Greets
Andre

Thanks very much for replying, tubeohm.

I don’t have a scope, unfortunately. I can hear the oscillator signal fine, but again, with the oscillator audible, resonance kicks in at a much lower value than it should. With res turned right down on the Shruthi display, there’s no audible resonance, but there is strong resonance by about a display value of 3 or 4.

I also notice there’s quite a lot of what sounds like white noise when the VCA is open. This noise seems unaffected by cutoff value, so it’s particularly obvious with low cutoff values. Don’t know if this is related, or expected.

a|x

Hi, if nothing helps , send it back to me. I will take a look.
Greets

Andre

Hi Andre,

I may take you up on that kind offer. Am away at the moment, but when I get back, I’ll have another go at troubleshooting myself, and see where I get to.

Thanks again,

a|x

Just to check I’m not making a silly mistake- the Shruthi-1 main board should be set to ‘lpf’ as filter type? I’m assuming there’s no special firmware needed for the ladder filter board. Am I wrong?

alx

lpf is what you need.

OK, wasn’t that, then…

a|x

I think you have a wrong value resistor in the resonance path…
see arrow.

@shiftr …Ah, will check that. Thanks very much!

a|x

Ah, that’s a 220ohm resistor. According to the soldering guide, there is a difference in the value of this component in v1.31 of the board, compared to the 1.3 rev.

“…we replace a 1K ohm against a 220 ohm, so we reduce the output volume”

I have the earlier version of the PCB, so I probably should have used a 1k resistor here. However, the note in the guide suggested that changing the value of this component would just affect the output volume. I’ve found in the past that the output of my Shruthis has been quite hot, so I thought I’d try using the lower-value resistor, to reduce the output volume a little.

a|x

Oh… you are right. I’m wrong. I was looking at another part of the schematics… this probably has nothing to do with the resonance…

I started counting from the wrong side of the IC…

only other thing i can find is this solderjoint on this 2.2 k resistor next to IC3.

Ah, that joint does look at bit rubbish on the pic. I tried remelting the solder there though, and it didn’t make any difference sadly. I’ve already remelted the solder on all joints, so I don’t think it’s a bad joint.

a|x

Could the problem be caused by non-matching filters ladder transistors?
I only mention it because when I received the matched transistors in the kit, one had fallen out of the folded-over piece of paper the others were wrapped in. I was pretty certain I’d worked out where the fallen out one had come from, by the spacing of the remaining parts, but I could have been wrong. If I was wrong, that would potentially mean that none of the transistor pairs in the ladder part of the filter were balanced.

Unfortunately, I don’t have the means to test the transistors :frowning:

a|x

Ah… think I’ve worked out where I went wrong. Looks like I’ve mixed up 47k and 4.7k resistors.

a|x

Hi,@Shifter the resistsor is OK. I have take a look also.
Damn’d now i see it. Yes, the 47K coupling from the Ladder to the diff amp are wrong.
Ok, than the ladder gives more output and the resonance is too high.
I have take a look minimum 0,5 h to see whats wrong but i have don’t see it. Sorry.
Be sure that the 4,7K is in the right place.
4,7k is R50,R35,R37 but i think it looks good.

Greets

Andre’

@tubeohm it’s hard to spot, even with the actual item in front of you (hence me getting it wrong in the first place), so don’t beat yourself up about not noticing it. Thanks very much for looking.

I’ll double-check all the 47/4.7k resistors again and replace the incorrect ones. Hopefully that will fix it!

I don’t know how I got that wrong- the documentation is very clear, and I was also checking the resistors with my multimeter.

Anyway, mystery solved now, hopefully. Now I just have to get the incorrect ones out without damaging the PCB.

Thanks again!

a|x