Re: OT: Classic Macs group?

["jsnospam@cimmeri.com" <js@cimmeri.com>]

> > 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.)

A few more clues:
- if I lower the input VAC via variac to about 100vac, the sound
diminishes into a softer hissing (ticking so fast and and soft, it's a
hiss/sputter).

- if I adjust the single trimpot that provides for fine adjustment of
the dc output voltages (+5,+12,-12), I can change the frequency of the
ticking or make it go away almost completely... but only if the input
VAC is at 100v.  This trimpot is in the feedback control circuit.    I
can also get the hissing to go away but then the output voltages are a
tad too high.

-------------

As an aside.. I'd just like to say how grateful I am for resources
like those of you who reply to my posts.. especially Michael who has
spent so much time explaining things to me.   You guys make life
better.

~ J