8 Jun
2004
8 Jun
'04
3:35 a.m.
--- Tony Duell <ard(a)p850ug1.demon.co.uk> wrote:
Anyway, what I would do is figure out what runs off
the 12V line
(probably some of the motor drivers, etc). And then
disconnect the 12V
line from them (disconnect the power cable from the
drive PCB, etc as
appropriate). Try to get the fuse to hold _and the
12V line correct_ with
no load or a dummy load.
-tony
By pulling boards I was able to isolate the problem to
the regulator board which contains three 12V supply
circuits, one each for the tape, logic and drive
spindle. I was able to identify high power thyristors
and op-amps on each 12V supply suggesting some sort of
crowbar arrangement for overvoltage protection (no
schematics available and this is a four-layer PCB
which makes it hard to derive the actual circuit). I
haven't checked the thyristors for shorts yet but I
believe the problem is either a defective thyristor or
some defect in one of the voltage comparators, maybe a
dead zener or something.
I need to keep the fuse from blowing long enough to
figure out why the overvoltage protection is
triggering. What would be the risk in trying to defeat
the overvoltage protection without any load connected
to the regulated 12V lines ? I assume I could do this
by disconnecting the gate of each thyristor.
Thanks,
Dave
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