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Re: OT: Classic Macs group?
jsnospam@cimmeri.com wrote:
Thank you very much, Schmidtd, BenYates, SimonWilliams.
Hi Michael!
Ticking can be caused by an overload (does it still tick if the
power supply is disconnected from the load?) or by an internal
fault.
I have two Mac Pluses.. which apparently have a nearly identical
analog board to the 128k + 512k machines. Both have the same symptop
-- a faint ticking sound that occurs irregularly about every second or
so.. I stress irregular.
One Mac Plus is the spare power supply / logic board for the other.
Don't you hate it when your spare system develops the same symptoms
as your primary system. ;-)
Yes, I have a voltmeter, oscilloscope, etc. I also have schematics
for various sections of the analog board. This is an interesting
board to learn switching power supply and video circuitry from....
which I think will be my goal now.
The analog board consumes a lot of power, so I'm wondering if the
Mac can be operated without the analog board being powered--or vice
versa--at least enough to verify whether the power supply can power
one or the other (but not both)... My first objective would be to
try to localize the problem.
For example, if the analog board, when powered separately, causes
a blank raster (for appropriate settings of the intensity control),
then you can assume that it is functioning at least at a minimal
level. If the digital board functions correctly when separately
powered (I'm assuming that it's safe to do so with a Mac Plus), then
that would show that it is functioning. If the power supply voltages
drop significantly when both are powered, then insufficiency in the
power supply is strongly suggested.
An SMPS is a non-trivial diagnostic problem, since it contains
a feedback loop and a non-isolated "off the line" HV supply--a bit
dicey to probe on the line side.
Commonly failing components in older electronics are electrolytic
capacitors--not highly reliable to begin with and often adversely
affected by hot environments. Since their most common failure mode
is low capacitance (not shorting), the usual diagnostic method is to
parallel them with a known good capacitor of similar capacitance and
same or higher voltage rating (watch the polarity! ;-). If the circuit
works with the good capacitor in parallel, then that cap needs to be
replaced.
I note that there are 3 spark gap capacitors used for transient HV
surges.. I wonder if one or more of them is sparking?
Good question. I was assuming the "ticking" came from the high-
frequency transformer in the power supply. If it's coming from
someplace else, that would be very significant.
Is the system at all responsive in other ways? For example, does
its speaker make any sounds? Do any LEDs light?
-michael
NadaNet file server for Apple II computers!
Home page: http://members.aol.com/MJMahon/
"The wastebasket is our most important design
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