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Re: OT: Classic Macs group?

jsnospam@cimmeri.com wrote:

Michael Wrote:


Perfect--finding out exactly what's ticking is clearly the next step.



Hi.   I found what's ticking.  Some background.

The power supply is made of two modules... a 120VAC->170VDC converter,
and then the switchmode section.   The switchmode section has 3
subparts: feedback controller, switching converter, and an array of
filtered output rectifiers.

What's buzzing / ticking is the big transformer in the switching
converter section.


That's what my original hypothesis was.

But you say that the computer is working fine with this ticking
going on, and that seems very unusual to me.

The "ticking" is the usual sound of the switch-mode power supply
starting up and shutting down immediately, so it usually indicates:

1) shorted output (also sometimes no load at all),
2) overloaded output,
3) internal short fault in one of the output filters, or
4) special case of 2), in which the "overload" is actually
   a normal load, but the off-line supply (120VAC-->170VDC)
   is delivering insufficient power to the switch-mode
   converter.  (This is often the result of a low-capacitance
   filter cap in the off-line supply's filter, but could also
   be caused by an open diode in the bridge rectifier.)

There are other possibilities, like trouble in the feedback
circuits, but I expect those are the most probable.

If your machine continues to operate with the ticking going
on, then the ticking is something quite different from repeated
power supply shutdown--for example, the switching transistor could
be periodically failing and recovering or otherwise "dropping pulses",
which seems very improbable in a self-excited oscillator of this type.

If you can observe the outputs with a scope, do you see anything
on any output that is synchronized with the ticking?  (Avoid probing
around the transformer, since there may be very high peak voltages.)

-michael

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