Model D Refurb in progres... (new guy)

[Revtor]

UPDATE New bushings are installed, bell crank caps cleaned and greased, I leveled the keys, gave them all a bath. Its looking great and feeling great. The buss bars were wiped with alcohol and I did remove a good bit of funk with that treatment but two/three squirrelly notes still remain…. maybe those springs are bent a bit? gotta give it a look. maybe a stray hair or something. There was evidence of someone working on the buss bars previously with some deoxit or similar , I can see tarnish spots etc. Hopefully I can nail those few bad notes.

I've got the 3 knob trim disks I need coming in the mail, that will get the dials all up to snuff. When they arrive its time to pull the knobs, bathe them and clean the panel dust/funk.

It looks like my Fiance is the crazy Ebay sniper I was worrying about. She bid on that authentic front panel Minimoog badge and won it! Happy birthday to me . . I think a thin double-stick tape is the way to attach this and I found a 3x5" piece of 3M 9690 tape on ebay for $3. I'll let you know how it works out. Could someone bother to measure where it is located from the factory for me?


#11279 is coming along nicely. Man is it fun to play. Every time I turn it on I'm like Rick Wakeman journeying to the center of the earth and when I get there I meet Sun Ra. I've got it paired with a fender twin Reverb. Sweet combo.!

~Steve

[megavoice]

.....Could someone bother to measure where it is located from the factory for me?

~Steve

I`m happy with you man, .

When Ive got home after I can do this for you, but I think other ones are faster till in the next days when I`m back here...

[megavoice]

Sizes of the badge: 7,64 cm x 3,8 cm

Distance from the right end of the badge to the nut: 3,82 cm, and to the outer edge of the right side plank: 5,63 cm
Distance from the rear end of the badge to the rear edge of the cross plank (close to panel): 1,735 cm
Distance from the front end of the badge to the front edge of the cross plank (close to keybed): 2,15 cm

Hope this will help ...............

[Revtor]

Megavoice, thanks for those #'s..

So I installed new caps and diodes on the rectifier board today, nice Nichicons from Mouser. 2x 1500uF 35v, 1x 470uF 63v and 1x .01 630v film. 4 new 1n4004's too.

I noticed when I opened it that the PS was regulating at +/- 9.64v. A bit low. After the parts replacement the #'s were the same, but even after trimming it all the way up, it would only reach 9.97v. Negative followed as expected. I read a KL post about absolute value not being as important as them matching, so that's what I went for and settled at 9.90v. I don't really like how I had to have the + trimmer (R21) almost fully CCW and it still wouldn't reach 10v. Something running out of spec. Any one have any ideas? R39 and or R34 are 1% so pretty specific values, they may have drifted off over time? ? (click on the pics and they rotate to normal)

Tech Doc 1

Regulator Schematic

Tech Doc 2

Cleaned some dust out of the enclosure, brushed dust from the backside of the filter board and cleaned the gold fingers w/alcohol a few times. She's back up and singing!

~Steve

[Revtor]

One more pic, of the board for my fellow geeks out there.

minimoog regulator board

~Steve

[thealien666]

Voltage stability (regulation) is much more important for analog music circuits than getting exactly +/- 10 Volts.
+9.97 Volts is pretty damn close to +10 Volts IMHO.

After a 30 minutes of warm-up time, my perfectly working Minimoog D PSU outputs "only" +9.95 V and -9.96 V (couldn't quite get +/-10 V with those old wire wound trimpots), but the variation is less than +/-0.008 V (measured at the external DC power outputs for accessories).

[Revtor]

I know the regulation is more important then the absolute value. But I'd still like to see if I can get this to sit at 10v, because thats where it should be... Its something I'm going to take a look at in the future. The synth is all back together singly nicely now. I was just wondering if the people here had seen this before and had any ideas what to check based on the schematic. The zener and all the 1% resistors will be renewed eventually.

thanks
~Steve

[noddyspuncture]

Increasing the value of the main PSU smoothing caps would lift the DC voltage to the regulator IC's, possibly allowing more volts to be dialled in by the circuit..!?

Just a thought...?
Cheers,
Tom

[Revtor]

Good thought Tom.. I did raise the big caps to 1500 from 1000 since I was in there, but the voltage is locked. It's well regulated at least! My understanding is that the 6.2v diode is the "reference" for the rest of the system and R21, the +v trimmer sets a voltage for the transistors to "lock on to" via feedback. I'm sure that my description is childishly inaccurate but I don't have many EE friends who I can discuss this with over coffee.

It looks like R21 (the trimmer) in combination with a few 1% precision resistors of very particular values forms a voltage divider. My thought is that those resistors have drifted out of spec over time, so I can't trim the divider up to where it has to be. I also had the thought that the zener diode may have drifted over time but apparently these don't really do that unless abused, and I'm sure that keeping the voltage reference happy was a design priority. There are three other precision resistors, one that feeds the zener and two that live around the -v trimmer that I will check out too when I get in there again. Also two 10 ohm guys that feed the sense rails. These will be checked too.

I ordered replacements for these specific resistors, plus the 8 or so electrolytic caps that live on this board and I found the correct diodes on ebay. All told $23. I bit the bullet and got a nice bench top meter with frequency counter too, which is something I've wanted for a few years now. (I'm a ham and a frequency counter is huge when building/tuning radios) It will be very nice to actually know whats going on in there (and will help in tuning it precisely)


When these parts and tools arrive I'll dive back in and get this baby right on 10v... And of course obsessively report it all back here!

~Steve