No PAiA Theremax Volume (banging head)

Posted: 4/6/2008 11:05:46 PM

From: Napa, CA

Joined: 6/9/2007

Ok, so I am really, really frustrated (ie. about to give up) with my PAiA Theremax project right now.

The thing is, I assembled it, and after some minor mishaps (popped capacitor, reversed polarities, etc), I finally got it working -- both the pitch and the volume. But it would crackle and pop a little bit whenever it got bumped or wiggled, and I noticed some of the solder connections were loose (on the panel lugs).

So I resoldered those and... then it didn't work at all. I've contacted Scott numerous times, so I don't want to bother him again (with the tuning advice, he just sent me a FAQ with basic troubleshooting tips), but maybe someone can enlighten me (or narrow down) what on earth I did wrong.

Here's the details:

1) Before "fixing" the crackling problem, my gate/trigger LED would light and flicker as I was using the Theremax (I had velocity knob turned all the way up)

2) After fixing, the Gate/Trigger does NOT light. Could this have something to do with my problem? I tried the "gimmick" ie. using the jumper to bypass the volume circuit; when I do this, I DO get a pitch tone and I can tune it to null.

3) I've made sure I'm not nulling one of the other, more faint heterodyne sounds I hear; I'm sure I have the correct (strong - affected by pitch antennae, not reversed) one

4) I used kkissinger's advice on another thread to tweak L4 and make sure the frequencies weren't interfering or harmonizing (ie. my volume antennae is NOT affecting pitch at all with the jumper attached from D1 to Volume lug 3)

5) When I remove the jumper, I get no sound at all. Not even a faint one. I've tried with Volume full, Volume halfway, as well as Velocity max and Velocity zero. As I turn L3 through the entire range, I get NO sounds and the gate/trigger does not flicker or light in any way at all.

Any possible ideas whatsoever? I'm about ready to ditch the project altogether...
Posted: 4/7/2008 2:18:55 PM

From: Colmar, France

Joined: 12/31/2007

kidelectric, where are you located?

Perhaps you'll find a TW member in your neighbourhood who might help you with this. If you were in Northern France, Southern Germany or Belgium, I would be glad to examine your Tehremax with my oscilloscope.
Posted: 4/7/2008 2:36:43 PM

From: Napa, CA

Joined: 6/9/2007

Unfortunately, I'm in Napa, California (a nice place to live, but a little removed from most people)

there are a few thereminists in the San Francisco Bay Area, but they don't seem to be active on here...

in any case, yeah, it would be useful to have an oscilloscope, but I don't have access to one. I'm going to start with my multimeter and test each of the 5 prongs of the 4 Ls (coils) to make sure they have the right power; I'm also going to go through the schematic and make absolutely sure everything is wired correctly (especially the jacks/pots on the front panel)

It's going to be tedious and painstaking, but I guess that's what I'll need to do.

I'm just wondering... is it possible I could have broken L3 through constant tuning/tweaking? It doesn't seem likely, since I've done a lot of turning of L2 and it's just fine, but I'm just trying to narrow down my focus on what could possibly be wrong...
Posted: 4/7/2008 7:04:55 PM

From: Toledo, Ohio United States of America

Joined: 2/22/2006

Are you sure that your power supply input, (whether thru a jack or straight wired like PAIA wishes), is soldered well?
Do you own a VOM? Check the potentiometers on the control panel for failure--- perhaps from over-heating by soldering.
I am in Ohio. I bet I could fix it if I was on the left coast.

Good Luck!

Posted: 4/7/2008 10:48:25 PM

From: Napa, CA

Joined: 6/9/2007

thanks for the tips...

I do, in fact, have a VOM, but haven't used it yet (at all... ever; just bought it for this project)

looks like my next courses of action are:
1) Test the 5 pins for each coil
2) Test the pots with the VOM

[I'm pretty sure my power supply is okay; it seemed to work fine before rewiring, my LED is glowing pretty solidly, and the pitch works well with the gimmick; I did modify the original design by using a coax-style plug and socket so that the power supply didn't have to be permanently wired in]
Posted: 4/7/2008 10:51:48 PM

From: Napa, CA

Joined: 6/9/2007

oh, one more little piece of advice which might shed some light on the subject:

one of the reasons I "fixed" the solder connections was because, even though the pitch and volume antenna worked before, there was one weird thing I DID notice: the volume knob didn't seem to do anything at all. that's when I realized there was something weird going on with the connections at the velocity and/or volume knobs...
Posted: 4/7/2008 11:16:25 PM

From: Fresno, California USA

Joined: 3/26/2006

My guess would be that there is a bad connection or a bad component somewhere. A wire might be connected incorrectly as well (or broken). And the panel sounds like the first place to check. Some of those panel connections are NOT easy to solder. The 15/30 watt soldering iron I used for the board just wasn't enough to solder three wires into a lug; I had to pull out my old 100 Watt Weller--that did the job, but required a light touch to prevent frying something. Another thing that can happen is (I forget the term for it) an insufficiently heated solder connection, where the flux isn't boiled away and acts like an insulator. The wire stays in place, but isn't really connected. You can usually spot these using a strong magnifying glass (think a jeweler's loupe--they have them at Radio Shack). A good joint will be smooth and shiny, and the solder will have wicked up the wire a bit. The insufficiently heated ones will not have beaded up into a smooth shiny form, and will have dark spots (often indentations) in them. Reheating cures the problem. Ditto for soldered joints that look kind of dull rather than shiny.

I think a bad potentiometer is unlikely.
Posted: 4/8/2008 2:37:26 PM

From: Napa, CA

Joined: 6/9/2007

Thanks... the term for the flux-insulated bad solder is, I believe, a "cold joint" or "cold solder"... that is one of my strongest suspicions; though I don't readily see anything strange, but I might go back and reheat all of the joints (or any suspect ones) anyway just to make sure
Posted: 4/8/2008 3:27:26 PM

From: Escondido, CA

Joined: 2/6/2008

Another thing that can happen, especially around high-impedance circuits, is that the flux can be conducting just enough to be sort of a "resistive short-circuit". The cure is very simple. Just swab the excess flux off with rubbing alcohol. The 91% stuff is best, but even the 70% variety should be able to remove the flux.

I use a "flux brush" (these are extremely inexpensive at Home Depot!) that I have cut back the bristles to about 1/2". This is stiff enough to get into the crevices with enough force to remove the excess flux.

After brushing the area with alcohol, take a tissue or rag and dab off the excess alcohol.

I was very surprised to have excess flux to be the culprit in a guitar amp circuit around the volume control. The impedance there was less than 50k Ohms. There was no output from this circuit, but after cleaning off the flux it worked fine.

Posted: 4/8/2008 4:38:00 PM

From: Napa, CA

Joined: 6/9/2007

It was a real pain to get the wires (ground and hot) soldered to the phone jacks and potentiometer lugs, so I've got some big old globs at some of these points... I wouldn't be surprised if that is the culprit. Thanks for all of the advice, and I look forward to posting pics and sounds from my finished baby once it's all complete... (I actually have 2 more phases I'd like to implement, but want to get the barebones system working first - phase 2 will be to add a few coupled resistors to change the waveform tones, with switches to turn the two extra tone mods on or off; phase 3 will be to attempt using the MIDI Fader kit to convert CVs to MIDI-out CC signals so that I can use it as a MIDI controller...)

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