Wayne from Boulder, Colorado USA - Mellotron M400

[wseltzer]

6 minutes ago
Hi!
I run the Boulder U-Fix-It Clinic [boulderufixitclinic.org] - free events where our volunteers help people fix their broken stuff.
We're looking to improve our ability to help musicians repair their gear.

Right now, I'm helping someone diagnose a Mellotron M400 (#1452) with the original CMC-10 control board.
The fuse on the supply (with the full-wave diode) was blown.
I'm checking out the power supply and am confused by the schematic on /http://www.outofphase.fr/wp-content/uploads/2018/11/m400_owners_manual.pdf.
The schematic shows SKT.2, pin 11 as "-30V." However, this is connected to chassis ground from a black wire on the 2500 uF capacitor.
Is the schematic incorrect? Should pin 11 be GND?
Thanks for your help!
- Wayne

 

[JMan]

Hi Wayne,

Welcome!

With regards to your question, someone more knowledgeable will likely chime in, but on first glance it seems like a mistake on the schematic, especially if the unit itself has pin 11 wired to chassis (assuming this is stock and not someone else's mod). If this were a bipolar +/-30 supply, I would think there would be a 0v reference for those voltages, no? Maybe I missed something, but my uneducated guess is that your suspicion is correct and pin 11 should be ground.

Edit to add: does -30v appear anywhere in the rest of the schematic, or only at pin 11 in the power supply? That would be a good hint.

J

 

[ruffrecords]

Before the advent of split supplies, old schematics would often label the 0V end of a supply as negative. Neve used B+ and B- for their 24V supplies. B- was actually 0V.

I think in your case -30V simply means the negative end of the 30V supply and it is quite rightly connected to ground.

Cheers

Ian

 

[wseltzer]

JMan:
Thanks for your response. Yup, that's got to be an incorrect "-30v" label in the schematic.
Or, maybe the author was thinking "Here are the two pins for the 30 volt power supply, +30 and -30"? That's reasonable, but confusing if you're thinking this is a notation for a bipolar power supply, which this is not.
On to troubleshooting the CMC-10 motor control board; alas no schematic.
- Wayne