mu-tron II gut shot and newbie questions

[MovingInSloMo]

for anyone interested here's a guy shot of my non AC mu-tron III opened for the first time (had to drill a screw out) I was wondering if some kind people could point out the non basic  parts to help describe how it works.. is the metal tube the light/photo resistor? what kind of light source does it use?

[Govmnt_Lacky]

Hate to say it but...

It usually helps when there are actually pictures to look at.

You may want to try re-posting the pics 

[MovingInSloMo]

DUH hahahahah I'm a dumbass.

[Psychopath]

There appear to be transistors (with the white dot) on these first two boards.  But not on the third.  I don't remember any transistors on the schematic.  Is it a voltage regulator?

[dwmorrin]

for anyone interested here's a guy shot of my non AC mu-tron III opened for the first time (had to drill a screw out) I was wondering if some kind people could point out the non basic  parts to help describe how it works.. is the metal tube the light/photo resistor? what kind of light source does it use?

Required reading is Mark Hammer's article on geofex.com: http://www.geofex.com/article_folders/ecftech/ecftech.htm
It contains an analysis of the Mutron III.

The metal tube is a dual optocoupler.  The light source is a LED.  It lowers and raises the resistance in 2 spots of a state-variable filter to achieve a sweeping filter effect.

[digi2t]

Really nice gut shots. Thanks!

[Psychopath]

http://www.talkbass.com/forum/f36/interesting-insight-into-late-70s-mutron-iiis-1047289/

Roscoe East Roscoe East is offline
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Join Date: Aug 2011
interesting insight into late 70s Mutron III's
Mike Beigel just posted this on his Facebook feed, I thought it was pretty fascinating and worth sharing:

==============================

SOME INTERESTING NEWS ABOUT
MU-TRON III PRODUCTS PRODUCED BY ARP INSTRUMENTS
BETWEEN 1978 and 1980.

By the time ARP was making Mu-Tron IIIs, I was already out in my own (Beigel Sound Lab) consulting and product design business again, and i never paid much attention to the ARP Mu-Tron products. (They are the Mu-Tron III products WITH the AC line cords, and labeled as produced by Mu-Tron INC. (not Musitronics Corp. which made them from 1972 to 1978.

Last night (January 17) while working with my colleague Richard Lingenberg on some new product design work, we got to talking about the ARP (Mu-Tron INC) units, and I got a BIG and DISAPPOINTING surprise. I had always figured that they were competent enough to make the product the way I had originally designed it, but that was WRONG.

To make this brief, Richard (who gets all my Mu-Tron Vintage repair work and his own repair work from other sources) has fixed up and calibrated a lot of these ARP units.

Apparently they changed the inside POWER SUPPLY VOLTAGE but in doing so, they apparently DID NOT UNDERSTAND to change some component values that were designed for the ORIGINAL power supply voltage.

What Richard noticed about (ALL?) the ARP units coming in for repair, was that the FREQUENCY SWEEP was WAY TOO HIGH.
And also that the DOWN DRIVE never really worked to drive the filter down "all the way" as it was supposed to.
Both of these symptoms had to do with changing the power supply voltage but not compensating for the change by changing ALSO some very critical component values.

So Richard (who knows what a Musitronics Mu-Tron III unit SHOULD sound like), changed the offending parts values and recalibrated the SWEEP range and DOWN DRIVE to what they were designed to be.

I never knew about this before, and am sharing it now.

if you have an ARP Mu-Tron III and don't like the way it sounds or works, now you know why.

if you want it to be fixed (even though it's not officially "broken"
and recalibrated, please send me a note at <info at mu-tron dot ORG (NOT .com), and I will forward the information to Richard, who will contact you and give you a quotation on fixing the parts values and re-calibrating it to the frequency sweep range of the original factory Musitronics Mu-Tron Calibration.

(But if you are perfectly happy with your ARP-Mu-Tron unit, then by all means USE IT AS IS. Richard also noticed enough variation in the calibration of the ARP units that it's just possible ttyat YOUR vintage ARP-MuTron III unit is accidentally calibrated "just fine".

There weren't all that many ARP units made, but I thought it appropriate to inform the owners of these vintage (and AUTHORIZED by Musitronics Corp) i.e. LEGITIMATE reissues, that there might be issues with the units that (I) didn't know till last night.

Thanks,
Mike Beigel

[Liquitone]

I have one of these boards with a transistor on the workbench right now. The owner managed to get it for a good price because it was broken. It was in a bad state and has two huge 22mm holes drilled right above the footswitch.
I got it without the transistor and the LED side of the (newly placed NSL32) optocouplers wired in series instead of parallel and a 9v to 9v+/9v- circuit added. The owner mentioned it was a BC107B transistor.
After restoring those parts it sounded way off; It only worked in the 'Down' drive and similar to the post above the frequency sweep was way too high. I managed to get it sound right by replacing the 3k9 from LED to ground with a trimpot set to about 30K, which seems a bit high to me, but it sounds good in every setting now.

[Psychopath]

Maybe those holes allowed the operator to control the light hitting the ldr manually

there was no transistor when you received the unit?

[Liquitone]

No, the owner got stuck trying to fix it and send it to me with the transistor removed and the traces bridged from A6 1 to the red dot side of the optocoupler like it is on the known Mutron III schematic, but he mentioned it was the BC107B.
The transistor traces on this unit are; A6 1 to base, collector to 9v+ and emitter to red dot (LED+) of the optocoupler. The board has traces and holes for putting in either 1 dual optocouler or 2 single optocouplers, just like on my Q-tron has. Also the broken optcoupler had been replaced by 2 NSL-32's, which with the trimpot adjustment are working properly.