Filter Board Resistors

Hi all,

I’m close to having my first Shruthi build up and running, only I have been hit by a problem while trying to tune my filter board. As a few others have found I am only getting a rather loud plop sound, rather than lovely digital awesomeness. I know my digi board is fine as bypassing the filter board results in clear output audio (osc pin of digi board jumpered to tip of audio output)

I have checked component locations, values and tidied up all the dull looking solder joints on the board to no avail. I then began checking resistor values with my multimeter and found a couple of surprises!

I will post a pic shortly, but I found that several of the resistor values were massively outside of tolerance. For example the bottom 10k resistor from the group R20,21,22,30 (10k,1k,220,10k) was showing only around 5k. Similarly the bottom 6.8k in the group of four resistors below the DC trim was showing below 3k. Looking at the schematic these resistors aren’t paralleled with any others so the multimeter readings should be ok.

I havent finished a complete check of all resistors - but could this explain the poor filter board performance? Obviously its a simple job for me to swap out a few resistors which is my task for later on. Has anybody else ever experienced this with the kit resistors?? I’m sure I have never seen resistors so far out of tolerance, especially with the actual values being lower than stated!

Any help anyone can offer on troubleshooting the filter board is much appreciated. I will proceed by swapping out the duff resistors, and then tracing the audio path through the filter board if I still have a problem.

Regards,
Ben (Shruthi Noobie)

>> Looking at the schematic these resistors aren’t paralleled with any others so the multimeter readings should be ok.

They are. ICs are not pieces of insulator.

Pissing in a large swimming pool full of overexcited kids, taking a sample and giving that to the doctor is not a good urine test procedure :slight_smile:

I’ve never, ever heard of wrongly labelled or far out of tolerances resistors.

Well thats the last time I go to a public swimming pool!
Apologies, you are quite right of course, slight case of brain going to sleep there.
I guess I was looking for the easy answer, when in fact I will have to debug this thing the old fashioned way. Don’t get me wrong I am not here looking for a quick fix, or somebody to magically solve my problem, this is DIY after all :smiley:

My next move then is to trace the audio path through the board and see where the problem lies… Would running a jumper wire between the audio path and the tip of the audio out seem like a sensible next move to you. Just want to exhaust all possibilities before dusting off my scope!

Cheers,
Ben

>> My next move then is to trace the audio path through the board and see where the problem lies… Would running a jumper wire between the audio path and the tip of the audio out seem like a sensible next move to you. Just want to exhaust all possibilities before dusting off my scope!

Yes, this trick works, this is how datune here found a guilty bad solder joint. The points you can probe are those highlighted on the troubleshooting page.

Well, there ya go, 2 minutes into testing the board and I find the problem, the leads on the header were trimmed too short, and so the ‘F’ feed was not reaching the filter board!

Thanks for your help, will be adding my name to the list of completed shruthi’s very shortly! Its been a fun AND educational project, and one I will recommend to anybody looking for a decent DIY synth project. Now, what to order next is the question… mmmmmmmm polychaining…