does anyone know what thermistor is needed for a th-1?

hi, i'm new to this forum been working on a maestro theremin th-1 for a bit now and mine is missing the thermistor and i can't find a value to replace it. any help is greatly appreciated.

My first guess is the original design was using a thermistor for thermal drift compensation and it was found to do more harm than good so someone removed it from your box. If one is used it would be very specialized and just the correct resistance value is not going to work the same,  maybe better? (-'

Schematic

I can't answer your question but I hope that helps.

Christopher

It appears you can get a full service manual for the TH-1 :

Maestro TH-1 Theremin Service Manual
Contents: Schematics, Product Picture / Illustration, Parts List, Semiconductor Generic Reference Numbers (Dated 1975) ($15)

http://www.musicparts.com/products.asp?Company=Maestro

As to whether theres any more in this than on the Moog site, I couldnt say*..  They do say "Schematics" not "schematic" so it may be a full service manual, but check!

Looking at the volume circuit, I suspect that the thermistor could be quite a critical component related to the audio leakage you will hear - the mica capacitor with parallel NTC capacitor leads me to guess that the thermistor and these components werent just put in there for fun ;-) ... But its only a guess - This isnt a theremin I am inclined to spend any time studying or simulating.

If you did buy the service manual and sent me a scan, I would be happy to advise on replacement component/s if there is sufficient data and the original thermistor is unobtainable.

Fred.

*Just noticed this on Musicparts.com so I advise you check with them before spending $15 to be sure of what you will get:

Q: Is the document a complete service manual or just a schematic?
A: If the item is a "green button", the title will say what it is and below it, what it contains. 
Note however that many of our "white button" items are complete service manuals, but we only list them as a minimum "schematic"  since we haven't yet pulled the files and listed their contents.  If it turns out it's the whole manual, you get it, and at the schematic price.

The Maestro TH-1 Theremin Service Manual is a Green Button! - So it looks hopeful, but I would be cautious! It may just be what you get for free from Moog

There sure are a lot of negative tempco caps in there!  Hmm...

"There sure are a lot of negative tempco caps in there!  Hmm..." - Dewster

Yeah -

But we know nothing about the inductors. I suspect that this theremins topology, and the plate antennas, make this beast a thermal nightmare!

This theremin should probably be operating with minimum volume / silent set at hand away from the plate, because the plates coupling to the body of the instrument is going to vary hugely with temperature I think.. Well - actually.. its going to vary hugely on both plates...

Oooohhhhh - its horrible! ;-) Dr Bob Moog on a REALLY bad day, I think! (Just been looking at Bob's patent on frequency to voltage conversion - and his clever waveform peak detectors.. Beautiful simple clever analogue stuff - He was certainly capable of much better than this "Maestro" even back when he designed it)

  I must admit to being really puzzled by the volume coupling though - I cannot see how the volume doesnt affect the pitch! - Must be looking at something the wrong way - really need to look at the IC data sheet, but cant be bothered! ;-)

ok - I was being silly.. It was a MFC6040 not a 3080 OTA ! ;-)

Fred.

BTW - Welcome Electricchurchrepair ;-)

The image I have in my head is of a Maestro as a pulpit, preacher waving his hands about - ;-)

A spiritualist (or scientology) church by any chance ? You wouldnt be adapting a Maestro so as to hide the plates in a pulpit with perhaps hidden controls would you ?

Hell, that would be a fun commission!  ;-)

What I see, is the good old MFC6040 as a VCA. And I see the complementary transistor pair Q2/Q3 which form an exponential voltage controlled current sink on pin2 of the MFC6040 to replace the potentiometer between pin2 and ground in the standard application. I suspect that the thermistor is there to compensate the exponential temperature drift of this voltage/current converter.

Thierry, you are right!

I am embarassed to disclose what I first saw ;-) But think along the lines of Q2/Q3 being a shunt and the MFC6040 being a OTA requiring a sourcing control current, and you might if you are in a really stupid head state get a glimpse of what I imagined briefly ;-)

Fred.

hi, i'm new to this forum and I have a problem with my TH1 !

I can modify the frequency with the pitch antenna but nothing with the volume !

I opened it trace the signal with my scope on the VCA but I have no signal on the output of the MFC6040 ( pin 5 ) !

any idea ?

Thank you

regards from France ;o)