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MOTM Dotcom Power Supply Help
MUFF WIGGLER Forum Index -> 5U Format Modules  
Author MOTM Dotcom Power Supply Help
Hi friends,

I'm having some issues integrating a MOTM module into my Dotcom system.

I've got the Living VCO module in MOTM format, and my plan was to have it alongside my 5u gear.

I'm currently running about 15 modules off of a power-one supply.
It distributes electricity via a speakon cable into the side of my cabinet. From there,I connect the harness for all the 5u modules, and the setup works great.

The issue comes when I try to connect the MOTM module. I wired in a MOTM busboard, and measured the correct voltage at the pins. When I connect in the Living VCO module, though, the power supply starts chirping and the power for the other modules seems to fade on and off, almost as if being run by an LFO. I haven't left it in this condition for much testing out of fear of damage, but any clues as to what I'm looking at?

Thanks for entertaining such a long post!
Hi misterbunty

Yes, sounds like either a wiring mistake or a problem with the Living VCO?
hmmm..... The "chirping" is likely the power supply trying to protect itself from having one of more of the rails shorted/cross wired to each other.

I wouldn't connect it up again until you can determine the source of the problem and are reasonably sure that it is fixed. You might also want to disconnect all of your other modules when you do your initial tests to minimize the chance of doing them any harm. MY ASS IS BLEEDING

What brand and model power supply do you have?
What brand/model MOTM busboard?
Can you post any pictures of your setup?

I'll assume you are aware of the differences between DotCom and MOTM power connector wiring but just in case here's a handy reference chart to study:
Thanks for the quick reply, John!

I did indeed disconnect at the Dotcom modules, but still got the chirping. I've got the rev B busboard by J. Backhaus. With the MOTM module disconnected, the board measures the correct voltage. The problem only occurs as soon as I plug in.

My only other thought was that I oriented the sockets backwards when I populated the busboard back in the day, but going off for diagram, below, they are oriented correctly. I'm attaching a picture, just in case.

The module was built by a very kind and competent gent across the pond, so I'm going to guess the error to be mine.

That said, I've eliminated most of the more obvious mistakes, I believe.

I also tried powering the busboard using one of the Dotcom connectors off of the harness (removed the header and connected the black, white, and green wires). Same problem.

Thanks again.
I didn't see the picture, did you forget to add it? hihi cool
I'd check to see if you have any shorts on the module - test for continuity between the +15v and -15v end connections to the middle (0v) connections. Do this with the module connections first (not connected), and then with the power cord attached (not powered up). You don't want any continuity at these places, this will cause the problem you're having.
The Living VCO is a difficult build project. The connectors are very tight all about the board and the power input is a daisy-chained quad header. The only good build blog I've read put me off to trying this one. There might be a high probability of a short or poor connection along the series headers on the power in. The headers usually have caps covering the terminals where the wires are crimped. Of course be certain to disconnect from the power supply before removing the header caps to inspect these. It could be that a braid of wire is lying across more than one terminal, or even one of the wires is not making contact. It's suggested the wires be soldered to the crimp terminals in the blog I read.

Also there is a bit of confusion about the rotation and position of some of the headers. All of this is discussed in the blog listed below. Since I've never been brave enough to build one of these I have no practical knowledge other than through reading this blog. It seems quite involved and above my pay grade.

I hope it is something obvious. It is a pretty involved job to troubleshoot this one. You could get lucky and find it right away in the power header string. That is most likely where the problem would be considering how hard it would be to get that part right.

sduck is right about checking the boards input headers. I would unplug all of them and check each one first then check the header string as well while unplugged.

Good luck!
Remove the MOTM power board from the existing 5U power wiring. Connect the LVCO module up to the MOTM power board. Measure the resistance between one of the +15V pins on the power board and pin 8 of any one of the many TL072 op-amps on the LVCO board. It should be a low resistance - less than 5 ohms. Do the same for -15V from the power board to pin 4 of any of the TL072s. It too should read a low resistance.

If they are high resistance - check that -15V isn't giving a low resistance to pin 8 of the TL072s. If it is, you may have the power leads wired up wrong.

Thanks so much for the very helpful tips and diagnoses procedures, Gents!

The storm we're getting in New England is preventing me from getting into the studio before Thursday, but I am chomping at the bit to hopefully rectify the situation.

That LVCO module and its mythical sound will hopefully be making its way across my monitors, shortly.

Will report back with findings.

All the best,
I'll be excited to hear your LVCO when it is working too. It's a module I've often thought to hire someone to build. Quite a fine design idea with the linear core.

I am not entirely well versed on the differences between various VCO designs and wonder if the way the tracking on the Living VCO behaves across octaves would be similar to my bank of MosLab 901's. I think the original Moog 901-A had the linear to exponential converter circuitry but I am not so certain whether the MosLab clone does. Supposedly they were redesigned to improve accuracy and may have eliminated this conversion. I know mine track off a bit which sounds pretty good but may behave entirely different from the way the linear/exponential conversion tracks. I bet robotmakers or someone else knows for sure around here.

Let us know your progress.
The couple times something like this happened was because I reversed the polarity of the power leads coming into a module some how. I'd check if the MOTM power connector on the module is soldered in the wrong way around, or the MOTM power cable wiring got crossed during assembly of that cable.
Thanks for the picture of your busboard, it looks correct to me! thumbs up
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