Vintage Synth Repair

First of all, the circuit is a bit unusual in as much as the feed for the 5v
regulator circuit is taken
from the +15v rail.  This is ok to a point but if Q2 fails, 15 volts could
and will feed back through Q1. This would only be limited by R2 giving about
11 volts on the 7 volt rail.  You could try removing Q2 as a start to see
what happens.
OK, Can you do a few things to track the problem down.
1) Is the 5 Volts correct? Is the +15 and -15 correct?
2) have you checked D4.
3) What is the AC voltage between ~ and ~?

Ignore for the moment the 'AC' reading you are getting.  This could be a red
herring depending on the model test meter you are using.
Best regards.
John. (U.K., where are you?)

----- Original Message -----
From: "gil_we" <gil_we@...>
To: <vintagesynthrepair@yahoogroups.com>
Sent: Sunday, August 20, 2006 12:22 AM
Subject: [vintagesynthrepair] What causes bad unregulated voltage on the
power supply ?


> I got a Roland MKS70 of which the Power Supply unregulated voltage rail
> is too high. The schematics says there should be +7v but I read 11V
> when the DMM is set to DC and 28v when set to AC !
>
> This fits with the fact that the display of that unit is overbright,
> showing all characters at once....
>
> But what can cause the unregulated voltage to be incorrect ?
> could this be bad regulators in the PS unit ??
>
> I already replaced all capacitors in the PS.
>
> Thanks all !!
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