4PM Assembly Testing / Calibration


Today is holiday here in Basel / Switzerland :slight_smile: so just finished both DCB and Filter board assembly.

My Multimeter is in BUZZ Mode

I am trying to do testing;

red/blue point testing are OK
green/red - I hear a short buzz but not continuing
red/blue - I hear a short buzz but not continuing

1 - I did not understand the problem. Can you guide me ?

2 - For the power distribution test where I am supposed to be inserting LT1054 Filter or Control Board ?

3 2 Out of 3 trimmer screews are on top instead of their side. When I stacked the boards together, How am I supposed to the calibration as I will to be able access them from their top ?


1. I don’t understand why you are using the continuity testing mode while you’re supposed to measure voltages. It would indeed be a serious problem if there was continuity between the power supply rails.

2. Where would you want to insert the LT1054 on the control board? There’s a slot for it on the filter board only!

3. You can connect the two boards with jumper wires.

Thanks @pichenettes

Test procedure suggest

1 - Without any IC inserted and the board powered off, check that there is no short between a red and green pad ; a red and blue pad ; a green and red pad.

This is what I was trying to test so I used buzz mode to see whether there is short

1. Then if you have no buzz it’s good!

My concern is this short buzz so if this is not a problem ? Can I continue with Power distribution tests

Put the meter in resistance measurement mode to be sure… Which resistance do you measure between the rails in which you can hear a short buzz?

It seems my first report was wrong because now I also hear buzz between red and blue.

When I take first red on left as reference and go all green points shows 15.2k
When I take first red on left as reference and go all blue point shows 17.3k

Is this normal that BOM lists ceramic 220p cap. instead Polystyrene cap ?

Do you make these measurements with the ICs inserted or not?

It is not surprising at all to find a high resistance like that between the rails - these do not qualify as shorts!

Without ICs.

Does that mean I am OK to continue with inserting LT1054 and do power distribution testing ?


It all worked fine both control and filter board passed the tests and lcd also assembled easily.

Now I am stuck with V/Oct Tuning as what ever side I turned the trimmer I can not keep same two note one octave appart on keyboard

I think my problem was tuner I used on iPhone ( Tuna Pitch ) as it listens the tune from my speakers.
When I tried Tuna Pitch on my mac and listen the tune via my sound card I was able to tune it.

Range calibration is not easy to understand to me so I left it where I like the tone of the patch.

I might have a problem with the resonance though so I will check tonight and record couple of notes and posted here to ask your help

@pichenettes: Do you know a source of the jumper wires in germany? I can not find them at Reichelt…

@annu I think this one is the right one from reichelt.de

Hello @pichenettes

I played C3 with init patch with cutoff at 64, env 34 , lfo 0 , and resonance at 0, 16, 32, 48, 63.

Here is the recording

Is everything ok with my resonance ?


Sounds good.

why you don’t use the filter calibrator from TubeOhm


@tubeohm I did realize your tool but it was too late :slight_smile: but I will definitely use it for my next Shruthi-1 because 2 of them is not enough :slight_smile:

@cereyanlimusiki thank you!