Help! My Melodia stopped working!

Posted: 1/12/2018 1:44:11 AM
roricka

From: Peaks Island, Maine

Joined: 12/18/2017

As a twelve year-old I purchased the six Melodia coils from Bob out of the Jan. 1961 Electronic World magazine. I ordered the rest of the parts from Allied, successfully put it together following Bob's instructions, and entered my Theremin in the science fair. I won and Bob included a picture of me in his Moog Music catalog that year. Eventually I took the instrument apart, saved the coils., knobs, and switches and 30 years later my daughter and I rebuilt it. After showing it off and playing it for about a year, it stopped working (no output). I put it aside, moved several times, and now, at sixty-nine, I've gotten it out again and am trying to troubleshoot it.

In the original magazine article, and the instructions that came with the coils (and thanks, by the way, for posting the pages from that manual) Bob says to do two tests after assembly prior to turning it on. One is to verify 500 to 2500 ohms resistance between ground and the red wires (the transistor bases). The other, after inserting the battery and turning it ON, is to measure D.C. for 0.3 v. and 6 v at two points listed on the schematic.

So the 6v measurement is fine. The 0.3v point is giving me about 5v! The resistance from the V1, V2, and V3 bases is checking out at 2500 ohms. The resistance from V4's base to ground is closer to 8K ohms.

So why is the one voltage measurement too high? (And why is the resistance on the red wire to V4 too high?)

I don't know where to begin to troubleshoot. I tried re-soldering all the joints (except on the xistor sockets and coil terminals) but it didn't help.

Is it possible the glass diodes have gotten damaged? I may not have always been careful with heat-sinking when I soldered their lugs. When turned OFF, the diode over by L3 measures about 5K ohms in one direction and 54K ohms in the other. Obviously the diode is working. But the one by T1 only measures 2.4K ohms in one direction and 2.2K ohms in the other. So I guess it's working?

Do you have any suggestions? My alternative is to carefully take it apart again, and start over with new resistors, capacitors, diodes, and transistors. I hate to do this if the bug is some silly minor problem. But I have no clue.

 

Thank you for any help.

Rory  Sellers

Posted: 1/12/2018 5:00:06 AM
senior_falcon

Joined: 10/23/2014

Don't do anything too hastily on this.  I will check the voltages at those points on my Melodia, which works, and report what they are.  One thing to remember on the Melodia is that it may be positive ground.  I believe that all the ones that came from Moog used PNP transistors and were positive ground, although the magazine article used NPN transistors and a negative ground.  So check what the transistors are (PNP or NPN) that the battery polarity is correct for those transistors. The 4 AA cell battery holder is a good replacement for the original one that uses a 6 volt battery that is no longer made.

Posted: 1/12/2018 5:47:48 AM
roricka

From: Peaks Island, Maine

Joined: 12/18/2017

You are right, the NTE-100s I'm using are PNP. 

Thank you very much. Did YOU order a set of Bob's coils 57 years ago too?? 

After I moved in 2012 (we live in Portland, Maine) my Theremin file went into storage and I'm afraid it's temporarily lost. In semi-retirement I've finally decided to fix the Melodia so I bought another copy of the Jan. 1961 EW from eBay. When I rebuilt the thing twenty years ago I had the original assembly manual but with everything being (temprarily) lost (including my cherished copy of Moog Music with me and my "Music From the Ether" Chicago Area Science Fair pic.) I only had the magazine article to rely on until I found the manual on this site a few days ago. I wish that was easier to download -- once I have mine back I'm going to scan it into a .pdf and post it here!

So, yes I was confused about the polarity (trying remember why the power supply seemed to be backwards -- which you've now cleared up for me!) But it looks like the manual didn't have a schematic -- is that right? And the parts list there used PNPs? I've twice (1961 and 1990) ordered all the parts (except for the 6 coils) and I did it from the manual. I don't think I ever knew the info in the EW article reversed the polarity. Any idea why he did that?? (Btw -- thanks for the advice about the battery holder, but obviously I'd already done that for the re-assy. back in '90. This time I actually did try to find one of those old square 6 v guys -- I can still visualize the original battery holder! But they're all antiques now, of course.

Anyway, it's late (after midnight) so I may not get back to this until the weekend. I'm embarrassed to admit I'd put the battery in according to the mazagine schematic (and also, I think, bolixed up my description of the test above -- those red wires don't go to the bases -- they go to the collectors, is that right? It's too late for me to dive back in and figure that all out, but I did find something a few days ago that reminded me which wire the tab on the TO can indicates!) But, back to my embarrassment -- did I damage anything by putting the battery in backward? When I measured that voltage just now with the power polarity correct it actually measured .4v, much closer to the .3v I want than the 5v I got before! So maybe it's working?? I didn't attach it to an audio system yet. All I know is it failed suddenly 20 years ago and I'm going to get it working! I sure appreciate your help. I will keep you posted.

Rory

 

Posted: 1/13/2018 12:33:28 AM
mpampouras

From: Epalinges, Switzerland

Joined: 3/15/2005

What a wonderful story! I hope it brings some good memories ;) The first contact with the theremin is always magical!

Posted: 1/13/2018 5:30:38 AM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

roricka,

Here is some extra melodia info that senior_falcon also shared.

Reversing the battery polarity temporarily will not harm anything.

Christopher

Posted: 1/13/2018 6:02:29 AM
senior_falcon

Joined: 10/23/2014

Chris, there are a few extra pages to the manual that I was able to scan from a sale on ebay.  Now there is only one page missing which probably doesn't contain anything very important.  Probably something like "Congratulations on your purchase of the Moog Melodia theremin kit.  etc. etc."  You are welcome to download them from the site here or I can send them to you.

I don't think reversing the battery injures the melodia.  I clearly marked on my battery holder what the polarity should be, but when I replaced the batteries a few years ago I used the magazine article as a reference and put them in wrong.  Needless to say it didn't work, but when I got the actual manual out I realized my mistake.  When the batteries were installed correctly it worked fine.  The previous owner used a 9 volt battery to power it and that too did not seem to hurt it.  I would guess yours will work fine.  If not I would suspect the transistors.  Replacements can be found easily on the internet.  Although I have had no problems with the transistors, just to be sure I bought 4 NOS transistors for $10 and also have germanium 1N34 diodes.  All other components should be available in the foreseeable future, so this instrument should be playable for another 50 years at least.

The manual I posted here under pictures (Media>Photo albums>page 2) does have a schematic and Christopher's info above has the same scans.

 I live in Ithaca, NY which is only 10 miles from where Bob Moog set up his factory.  My Melodia was found by a colleague at a garage sale in the '70s or '80s, and to make a long story shorter, he traded the Melodia to me for a bottle of good Scotch Whisky.  This was back around 2004.  Take good care of yours; I saw that one sold on ebay recently for something like $1400.  

When you are testing out your Melodia, remember that the tuning knob is very sensitive, so be delicate in your adjustments.

Posted: 1/14/2018 4:28:38 AM
roricka

From: Peaks Island, Maine

Joined: 12/18/2017

IT'S WORKING!!!

You guys are the greatest. For the third time  in my life I've got the Melodia working (when I was 12, when I was 41, and now at 69.)

I'll post some pictures, once I clean her up some. I don't have the fancy escutcheon -- the only thing I got from Moog was the coils and I believe the variable caps and switch. Anyway, now that it works I have a lot to do to make it "presentable" but I do have a few comments/questions:

First of all, thank you so much to senior_falcon. Once I really started studying the forum postings here I probably would eventually have figured out the P-N-P/N-P-N thing but you sure saved me a lot of time. (And of course, besides the power-supply reversal, a bunch of the component values were different (resistors) so it was very confusing just trying to get oriented inside the chassis. I'd built the damned thing, of course, but it was decades ago. And as you all know the schematic reverses the positions of the antennae.

So unfortunately, since I started messing with it with the battery in backwards, and then decided to re-solder the terminal strip lugs, I really have no idea what actually fixed the original problem. The kit-builder in me wishes I knew what it was that failed twenty-eight years ago, but that disappointment lasted a nanosecond once I realized it was going to work.

Senior_falcon -- your comment about the "tuning knob"... are you referring to "C1" (or "V" as it is also referred to)-- the variable capacitor? It actually doesn't seem to be that sensitive.

Finally, now that I've read so much at TW about the Melodia, can someone fill me in on the coils? Mine came from Bob in 1961. Did everybody's originally come from Moog, or have people been "winding their own"? Mine look a lot different from those I see in the photos.

Thanks again. I'm going to play at this year's "Sacred and Profane" inside one of the tunnels at Battery Steele here on Peaks this Halloween (it's an enormous partially buried abandoned WWII bunker). That's what inspired me to get that old germanium lady up and running again. I have a feeling it will be, literally, awesome.

Rory ("roricka") Sellers, Peaks Island, Maine

 

Posted: 1/14/2018 5:19:31 PM
senior_falcon

Joined: 10/23/2014

Senior_falcon -- your comment about the "tuning knob"... are you referring to "C1" (or "V" as it is also referred to)-- the variable capacitor? It actually doesn't seem to be that sensitive.

Yup, it is C1.  Maybe you have better fine motor control than I do. :-) 

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