MOTM-410 Vactrol Failure

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DrReverendSeance
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MOTM-410 Vactrol Failure

Post by DrReverendSeance » Mon May 18, 2020 3:45 pm

I was excited to plug in my new build of the MOTM-410 and listen to it, and OH NO, after a minute of two of normal functioning there was a crackle and now it frequency 1 has stopped working as expected. it seems to be stuck at a specific frequency and neither the pot or the sweep are affecting it

I’ve tried my best at troubleshooting and after removing and putting back the Vactrol it was working again for a few minutes, then back to the same issue.

I thought perhaps someone on the forum might have an idea what I’ve fouled up?

Frequency 2 and 3 are working as expected.

Could the vactrol be broken? Anything else that might cause this?
Last edited by DrReverendSeance on Sat May 23, 2020 8:03 am, edited 1 time in total.

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EATyourGUITAR
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Re: MOTM-410 Failure

Post by EATyourGUITAR » Mon May 18, 2020 6:15 pm

maybe you pooped an OTA? cold solder joint? at least they gave you the schematic so you can start chasing it down. you can do this right?
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Re: MOTM-410 Failure

Post by DrReverendSeance » Mon May 18, 2020 8:58 pm

Thanks EATyourGUITAR,

The solder joints all look good to me.

Perhaps the OTA. Half of the U7 OTA is for Frequency 1 and the other half is for 2 & 3. I am surprised that half an OTA might fail, but I guess that could happen.

I am not sure how I would check if the OTA was at fault? I assume that if the voltage coming out is a constant that might be a sign of an issue?

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Re: MOTM-410 Failure

Post by EATyourGUITAR » Mon May 18, 2020 9:24 pm

too much current to an OTA will make it poop. you could also try a way to send a very small amount of variable current there to see if the frequency changes. or lift the pin to see if less current changes frequency. if you have a resistor value for 2uA NOT mA from +12v try that like poking it with a stick.
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Re: MOTM-410 Failure

Post by DrReverendSeance » Sat May 23, 2020 7:46 am

I’ve traced the problem by checking voltages with the module powered up. OTAs are fine. It looks like the D1 Vactrol is broken as there is no voltage going through it as I sweep the filter.

And of course, this is a rare part that I ordered from Germany. Serves me right for buying only enough for the build (no extras!).

How can a vactrol break? Is this a frequent problem, broken vactrols?

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Re: MOTM-410 Vactrol Failure

Post by EATyourGUITAR » Sat May 23, 2020 8:09 am

Only the diode can break. The LDR side will never break unless it is destroyed by heat or 1watt TDP. All LED's can break if you exceed the maximum current. A vactrol is just an LED + LDR. And LED is a light emitting diode. 10mA maybe 20mA is when that will likely fail. All you need to do is current limit with a resistor. Driving an LED from 3v with no resistor there is not the correct way. You need to drive it with something more but with a 220R at least.
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Re: MOTM-410 Failure

Post by BartBral » Sat May 23, 2020 10:19 am

DrReverendSeance wrote:
Sat May 23, 2020 7:46 am
I’ve traced the problem by checking voltages with the module powered up. OTAs are fine. It looks like the D1 Vactrol is broken as there is no voltage going through it as I sweep the filter.

And of course, this is a rare part that I ordered from Germany. Serves me right for buying only enough for the build (no extras!).

How can a vactrol break? Is this a frequent problem, broken vactrols?
I'm not saying it is not Vactrol D1,
but have you checked:

R54

R53 & Q7

R52 & Q4

Because if something is faulty there,
that could be the reason the Vactrol is acting funny...

And it would be a shame if after ordering, paying, paying for postage and waiting for it to arrive it was something else...

If all is okay with those parts, you could also try to swap the Vactrol and see if the problem swaps with it.

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Re: MOTM-410 Vactrol Failure

Post by DrReverendSeance » Sat May 23, 2020 1:22 pm

Thanks BartBral! That is a good idea. Following your advice I checked those components. They all look OK compared with components doing the same function in other parts of the circuit.

I’ve ordered some more vactrols and I will try replacing D1 to see if it resolves things. In the mean time, filter 2 & 3 work fine so that is something.

I do suspect it is the LDR side. I am getting voltage on D2 and D3 through the LDR side, but nothing on D1.

Ah well, such is SDIY.

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