Gibson Maestro theremin

Posted: 7/1/2016 8:07:40 PM
lotharpc

From: colorado, USA

Joined: 7/1/2016

Help!  Does anyone have technical information (schematics, documentation) on a late 1960's Maestro theremin made by Gibson, the guitar maker?

This is a solid state instrument with two slanted metal plates for the antennas.

It no longer works and I can't find any info on it to aid in its repair.

Thanks,

Paul (Lothar and the Hand People)

Posted: 7/1/2016 10:02:12 PM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

The only reason I can think of that someone would invest in repairing this antiquated theremin design is because they have developed some good playing skills using the unorthodox approach of metal plates? Moog Archive

Christopher

Posted: 7/2/2016 8:20:53 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

I believe the diodes in series with the tank drive transistor collectors are to allow the LC to swing below ground (without reverse biasing and clipping).  The tanks seem to have a lot of capacitance, but it's hard to tell for sure without knowing the actual inductance values.  I wonder what the thermistor TR1 is doing?

Posted: 7/2/2016 10:31:51 PM
oldtemecula

From: 60 Miles North of San Diego, CA

Joined: 10/1/2014

Everyone's original theremin design is interesting to me. Why the extra capacitor on the tanks? If my circuit interpretation and math is correct the RF oscillator transistors might be passing about 50 ma? What affect could this add to the final sound I wonder. Most theremin oscillators pass 1 or 2 ma.

Christopher

Posted: 7/3/2016 2:22:51 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

C16 and C24 in the pitch tanks are N750 (-750ppm/C) which is temperature dependent, as is C8 in the volume fixed tank.  C4 in the volume variable tank is an N2200!   Mica caps tend to be positive tempco (around +50ppm/C) so this is clearly an attempt at temperature stabilization (have the two tempcos multiplied by the two values duke it out). 

Tempco stuff like this should be a last ditch effort, as it often requires fine tuning for each circuit produced in order to be less worse than no tempco at all.

Posted: 11/30/2016 1:31:09 AM
mikebuffington

From: AZ

Joined: 11/25/2005

Hey everyone. I've got a Maestro HT-1 theremin in for a tune up, and it's having two issues: 1) the volume field is super compressed, acting like the action is reversed and 2) the sound becomes distorted at loud volumes (possibly due to issue 1). I'd appreciate any advice or thoughts on the the issues.

I have probed the 5 test points, and identified what each one is:

TL1: Volume oscillator (clean sine wave but having trouble reading frequency with my meter, 9.0 V DC)
TL2: Volume control DC voltage
     2.75 V — hand away from volume
     0.9 V — hand just above plate
     4.2 V — hand touching plate
TL3: Fixed pitch oscillator (clean sine wave but having trouble reading frequency with my meter, 9.21 V DC)
TL4: Variable pitch oscillator (clean sine wave but having trouble reading frequency with my meter, 9.21 V DC)
TL5: Pitch waveform before volume control

A+: 9.35 V DC
B+: 9.02 V DC
C+: 9.18 V DC

C10, C32, and C17 were recapped before I got it with modern, correct-value electrolytic caps (but oddly C28 was left original...)

Here's how the volume control is behaving: Volume increases as hand approaches antenna and then drops off quickly when only a millimeter or so above the plate and goes silent when touching the plate. Looking at the voltage on the scope, when I place both hands (larger mass) very close to the antenna, it's working correctly but in a very compressed field (see video below). This makes sense looking at the voltages measured at TL2 and the datasheet for the MFC6040, Figure 3 Attenuation vs. DC CV. So I think the MFC6040 is working correctly but the voltage range is offset (possibly due to the oscillators...? too much capacitance in the volume oscillator circuit...? or thermistor...? or...?).

The volume oscillator coils' tuning slugs are still in their factory state as the thread locking glue is still intact, so I'm hesitant to tune the volume oscillators just yet. While I can't get a stable frequency reading with my meter, the oscillators look like nice sine waves on my oscilloscope. The pitch section is working fine.

Here is video:
http://mikebuffington.net/maestro/TH-1_TL1_Scope.mov Volume Oscillator (0.5 V/Div, 1μ Sec/Div)
http://mikebuffington.net/maestro/TH-1_TL2_Scope.mov Volume Control DC Voltage (1 V/Div)
http://mikebuffington.net/maestro/TH-1_TL3_Scope.mov Fixed Pitch Oscillator (0.5 V/Div, 1μ Sec/Div)
http://mikebuffington.net/maestro/TH-1_TL4_Scope.mov Variable Pitch Oscillator (0.5 V/Div, 1μ Sec/Div)
http://mikebuffington.net/maestro/TH-1_TL5_Scope.mov Pitch waveform before Volume Control (0.5 V/Div, 1 Sec/Div)

I've also created fabrication drawings for the antenna, if anyone is interested: http://mikebuffington.net/maestro/TH-1_Antenna&Base.PDF

Posted: 11/30/2016 11:59:00 PM
dewster

From: Northern NJ, USA

Joined: 2/17/2012

Nice movies Mike (is there any reason they play back upside down in VLC?).

I think you will probably end up messing with one or both of the volume side slugs.  I'd mark them somehow so I could put them back where they were.  And use a plastic tool so as not to damage the ferrite.

It's not clear to me where the front panel "sensitivity" control is in the schematic?  There's something that seems like it near TL4 on the schematic but I don't recognize the symbol.  What does this control do on your Maestro?

Can you adjust the pitch side so that the null point is farther away from the antenna?

IMO the physical design of this particular Theremin is kinda poor, the antennas seem way too close together.  I imagine it's another example of the choice of enclosure dictating the playing ergonomics.

Posted: 12/5/2016 4:00:08 PM
mikebuffington

From: AZ

Joined: 11/25/2005

Thanks, Dewster.

The videos might be playing upside down for you because I filmed them with my phone and maybe your VLC settings are ignoring the auto rotation. They play fine on VLC on my Mac.

Zero beat can be set at a reasonable distance, such as arms length. The antenna are strange, and not ideal for precision playing, but I find that the volume and pitch controls don't interfere.

The slugs are represented on the schematic with the rectangles with arrows. The second volume oscillator does not have one in the schematic, and indeed the slug is locked down at the end of the threaded plastic tube, not interfering with the coil.

The pitch sensitivity control is also a rectangle with an arrow on the schematic. It adjusts a rod of metal into the coil, acting as a slug. The rod is directly connected to the knob and is located under the coil, aligned with a hole in the PCB. It has a stop in the threaded base which engages with the L bracket to set the maximum depth.

sensitivity control

 

Maestro PCB with test leads

Posted: 6/9/2017 1:23:40 AM
mikebuffington

From: AZ

Joined: 11/25/2005

So, I finally got around to this theremin again, and I can report that I did have to adjust both volume slugs to fix the compressed volume field. I got it so that there is about 3 or 4 inches around the antenna where I can get some noticeable volume control, and just above and touching the plate silences the theremin.

I still think it sounds pretty bad though. Are there any other recordings of it besides the demo disc video on YouTube? https://www.youtube.com/watch?v=7de357Bnw4o

Can any owners out there make a recording for me of their instrument?

 

Mike

Posted: 6/13/2017 3:04:30 PM
mikebuffington

From: AZ

Joined: 11/25/2005

I've recorded this Maestro theremin directly to computer.

http://mikebuffington.net/maestro/Maestro.mp3

I first do a general sweep from low to high and back down, then add a little vibrato on some random tones up the range of the instrument. The volume control knob (just a pot controlling max volume output) is set low and the line adjust on my record is set mid-range. The next sweep is set with the volume at maximum and the line adjust on my recorder set lower so as not to peak the recording. The sound is clearly muddier and distorted. Space control of volume at this setting can improve the tone by reducing volume by hand.

How does this compare to other Maestro theremins out there? I find the low end to be quite poor. Is this the factory sound quality?

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