Re: C128 issue

From: Mia Magnusson <>
Date: Tue, 3 Oct 2017 01:52:09 +0200
Message-ID: <>
Den Tue, 3 Oct 2017 15:05:26 -0600 skrev Terry Raymond
> Well,
> My fiddling with my flat NTSC C128 this summer I found
> The power regulator (located to right of MMU chip)
> On the output voltage- nothing, but reading OHMS resistance using a
> digital Multimeter that has the OHMS Buzzer reading:
> Power Regulator to GND reading the output voltage should have about
> 12VDC nothing

Does the regulator get hot?

> Reading just resistance on regulator with buzzer, the buzzer barely
> tries to beep indicating a dead short.

A very strange buzzer if it barely tries to beep to a dead short. All
buzzers I've ever seen or heard of beeps as much as they can on a dead

> The SID and VIC Video uses the 12VDC!
> So both SID and VIC circuits have shorted components.

Why do you assume it's on both circuits?

You could try to ohm measure at some different points in the 12V
supply, preferable on each capacitor, to se where you get the lowest
reading. This is a bit hard as the wires to the multimeter and the
measurement pins themself usually have a rather high resistance in
comparision to a short circuit. Preferable you should use an ohm meter
with "4-wire measurement" but that's something few people have among
their measuring equipment at home.

If you remove the SID and VIC chips, which usually are socketed, then
there is mostly some capacistors on the 12V supply. Desolder one
electrolytic capacitor at a time until you get the 12V supply to work.
(I assume that it's safe to turn on the power on a C128 without SID and
VIC chips, it might even display text on the 80 column output). Maybe
the RF modulator also uses 12V DC? If so you should check if there is
some electrolytic capacitors in the rf modulator that could short the
12V supply.

> I think a cranky power source may have caused the issue.

Unlikely. If the 12V regulator is of the 7812 kind, it can withstand
35V more the input than the output, so the regulator itself should be
able to handle 47V DC (or atleast 35V DC) on it's input. For that to
happen you need to feed 35 (or atleast 26V) AC on the 9V AC input. That
would happen if you feed a bit over 300V AC to the 110V AC input on the
transformer, if it werent for that the transformer would get saturated
before you reach that voltage.

So no, it seems virtually impossible to destroy anything on the 9V AC
AC / 12V DC part of a C64/C128 by feeding something incorrect on the
power input, as long as you don't just grab any random irrelevant
equipment and connect to the power input socket. Highly unlikely on a
C128 as the only other stuff that fits the socket is an Amiga power
supply and a C128 should be able to withstand such supply.

> I trying to find enough components from a parted out 128, for parts.
> Fun project for the winter months.
> Terry Raymond

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Received on 2017-10-04 00:00:03

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