Lowww Trimmer Pot Upside Down

Hi guys,

just spent all night soldering my Shruthi 1 kit, and got up this morning to test it, to discover I’d soldered the Lowww trim pot the wrong way up. I probably stupidly, plugged the SM-4 filter board and control-board together anyway, using the provided wires, and I’m not getting any sound at all out of the synth. Is this to be expected with the Lowww trimmer in the wrong way? The control board appears to be functioning properly, as far as I can tell without being able to get any sound out of it.


Incidentally, I’ve tested the output of the audio out pin by connection to a headphone jack, and there is a faint oscillator sound coming through when I hit a MIDI note, so I’m guessing the control board is working. Also, there is a very faint noise issuing from the audio out of the filter board, when all connection between the boards are made.


That’s sound like your cutoff is too much, have you try to turn the trimmer in the other way, many many turn.,?

Changing the orientation of the Lowww trimmer will not affect the sound in any way - the only thing that will change is the direction in which you’ll have to turn it to increase/decrease the cutoff range. Even if this trimmer is incorrectly adjusted, you should hear sound.

The problem is somewhere else - maybe some swapped resistor?

Hi EATYone,

thanks very much for getting back to me!
So do you think having the Lowww trimmer in upside-down won’t be a problem, then? I remember reading when assembling the control board that the trimmers on that board could be inserted either way up, but the instruction for the filter board made no mention of trimmer polarity.


Hi pichenettes,

thanks for getting back to me.
It’s good to know I won’t have to desolder the trimmer.
I’ll check all the resistors. Is it more likely to be some incorrect resistors on the filter board, rather than the digital board?


Ah… I put two of the transistors in the wrong way round. Perils of soldering at 4:00am…


Might I have damaged the 2N3906s by desoldering them? These are the lefthand pair of transistors either side of a 1k resistor (the ones with their flat sides on the outside).

I tried to turn them around the right way (trashing at least one of the solder pads on the bottom of the board in the process). I’ve tried to test the connections to the transistors, following the circuit-diagram, and they seem to be connected to the board correctly, despite my having to solder them onto the top of the board, rather than the bottom, but I’m still only getting a very quiet, noisy output.

I’m afraid I don’t have a scope, and don’t know how to test the transistor with just a basic multimeter. Bit of a newbie here. Any tips gretly appreciated.


Incidentally, I’ve double-checked the resistors on the filter board against the reference photo, and all seem right, as far as my tired eyes can tell.


Without a scope, the best option is to “probe” points on the board with a cable connected to a mixer/amp/soundcard (ring connected to GND, tip connected to probe). Find at which number the signal chain is broken:

Ah, thanks for the tips, pichenettes!

Well… I made some progress there, and then I think I broke it…
I tested as you suggested, and got as far as determining that I was getting what sounded like a good signal up to point 7, but nothing at 8 (or at least the same very quiet signal there). Then, I accidentally touched my probe onto the solder pad above the point marked 7 on your diagram, and now I get a very loud buzz from all points on the board, and even from the OSC out point on the Digital board. Think I may have done something terminal to both boards now…



Ah, panic over- my connection to GND has unclipped itself. So: everything OK up to point 7, nothing at 8. Check for bad solder joins on the IC socket with points 2 and 8, maybe? Or could it still be those troublesome transistors?


7 is the signal post VCF and VCA. 8 is the final amplification stage.

What could be wrong… Bad solder point on pin 10 of IC3 (which is supposed to be grounded)? Bad solder point for C1? Bad “bridge” for R2?

You signal to the leaders of the R6?
maybe you are too heated the capacitor C1 with a soldering iron and was drilled…

else …
check welds IC3
a drop of pond between pin 9 and pin 10 and the signal goes to ground…

Aha! Working now It was a problem with the jumper at R2. It… ahem… wasn’t there. Sorrrrrrry guys. I feel very foolish now :frowning:
I must remember not to do this kind of thing late at night.

Thanks so much for your help, and sorry for having wasted your time.


Thank God it was nothing serious
: )