[sdiy] Help calibrating Emulator II power supply... Thanks inadvance !

[Mike Pepper]

> I was recapping the power supply in my E-II and mistakenly fiddled with
all four trimpots on the PS circuit, trying to set the correct voltages. I
found that only two of them affect the voltage rails (-15v/+15v/+13.2v) but
the other two don't affect the voltage at all.
>
> I now have no idea how these other two trimpots (marked as R109 and R110
on the schematics) should be set and am afraid that being set wrongly I will
damage the E-II.
>
> Does any of you have any idea what they do and how they should be set, or
have an access to the calibration procedure of the E-II ?
>
> here are the two trimpots as appearing on the schematics:
> http://www.fileden.com/files/2006/10/13/285304/E-II%20PS.JPG

A few thoughts:

This is a line power switch mode, and the two pots are on the line side of
the high freqency transformer. Be careful, a sort here, or a finger on the
wrong bit is dangerous.

The switchmode gets it's chopping frequency from the Unijunction transistor
Q101, this is used to drive chopper transistor Q103. All this section is
live to the mains! It looks like R109 sets the startup frequency,
independently of the feedback loop. R110, I guess, sets the baseline of the
feedback from the opto in the 5V output, (A101) and the transformer winding
7--8.

You definitely need the manual, and you might need an isolated scope probe
too, or a 1:1 mains isolating transformer.

My advise is to proceed with care, and not to use the Emu until you've
sorted the supply. I'd do my testing with some dummy loads, the Emu is too
precious to risk killing if the supply goes wrong.

There appears to be an over-voltage crowbar circuit, (around A103) in the
12V output. If this goes off, check the mains fuse, and the chopper
transistor.

Hope this is of soem use.

||\/||ike