Casio CZ/ VZ/ FZ - Pro Series

On Fri, Dec 12, 2014 at 03:55:28PM -0800, db0451@... [CZsynth] wrote:
> Hi Daniel,
> 
> Thanks for the ideas. Indeed, it might be a PSU problem. For the full latest details, please see my thread on Vintage Synth Explorer: http://www.vintagesynth.com/forum/viewtopic.php?f=5&t=78014 http://www.vintagesynth.com/forum/viewtopic.php?f=5&t=78014  
> But in brief, the cable at the back of the FDD does not supply +12 V - only 0.05 V, even if the synth is powered off! And the +12 V supply seems to come through the fuse F4, rectifier S1WBA10, a bunch of transistors and other things, and finally an electrolytic capacitor. While I could unscrew the PSU and replace everything, it would be nice to have a better idea of what to test - and which things are worth replacing. :-)

Could be a bunch of things.  Check that you've got AC coming into the 12V rectifier and DC coming out.  There's then a fairly simple linear regulator, where a likely suspect would be the pass transistor.

In order I'd check that there's DC present across the first smoothing capacitor just after the rectifier, and if there isn't I'd check for AC from the appropriate transformer winding.  Check the fuse is okay and the clips are clean and secure (they corrode).  If there's AC going into the rectifier but no output, it's probably faulty in which case you desolder it and check it.  The most likely thing is that it's failed short circuit, in which case you really should replace the smoothing capacitor that follows it too, which will have been given a hard time by all that AC.

See if you get unregulated DC at the input to the regulator *first*.  You should see about 16V across that first smoothing cap.

-- 
Gordonjcp MM0YEQ