Chroma PS Question

[Don Tillman]

   From: "|||| Philip ||||" <thelab at sprint.ca>
   Date: Wed, 7 Jun 2000 20:14:03 -0300

   I got a DOA Chroma on a bad trade :( The unit was sitting at a
   friend's for 3 years. I finally got to see it last week. We checked
   the synth and the problem seems to be with the power Supply. With
   no load (boards disconnected) it pumps the appropriate +12 -12 +5
   -5 and +24 but with the boards connected, the ps's +12 and -12
   outputs drop to about +5 and -5v. The circuit board that causes
   this does not seem to be sinking that much current. The problem
   seems to be that the ps is not regulating. Does anyone have
   experience with this ps?  

The Chroma power supply is well known as a trainwreck of a design.
There was apparently some goal to save money in the transformer
department, so the +5.0 V digital regulator has this awkward 120Hz
switching circuit that doesn't really work.  Generally Chroma power
supplies fail with something related to the 5 +V digital output.

Check out my ARP web page for various Chroma resources, including my
article about replacing the supply with a modern switcher.
http://www.till.com/articles/arp

(There's also a link there to Chris Ryan's wonderful Chroma site and
Chroma mailing list.  Check it out.)

The service manual is still available from Fender last time I checked.
Call'm up.  You may have to remind them that they used to make this
great synth.  ("No really, this is a keyboard synth you guys used to
make.  Wait, do you have anybody there who's been around for a really
long time?  Maybe he'll remember.  Or check the dusty shelves way in
back.")

   There is only one common component shared by the +12 and -12v
   sections, an LM-358 op amp. 

It's a little more involved than that.

The Chroma supply outputs +5.0 V digital, +5.0 V analog, +12.0 V,
-12.0 V, the tapper supply and a microprocessor reset line.

Besides the +5.0 V (digital) regulator with the goofy switching
circuit there are also three "conventional" regulators built with
opamps and pass transistors to supply +5.0 V (analog), +12.0 V, and
-12.0 V.

The +5.0 V analog output is used to set the voltage for the +12.0 V
regulator.  And that voltage is used to set the voltage for a -12.0 V
regulator.  So if the thing is up and running and something happens to
the +5.0 V analog line, the +12.0 V and -12.0 V lines will follow the
disturbance.  Likewise, if something happens to the +12.0 V line, the
-12.0 V line will follow.  

Good luck!

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com