I already covered that in the original post. I tested the CRT in a
known good Mac and it is flawless. I have traced it down to the no
power making it to the T1 flyback transformer. Unless I get some good
pointers, I'm going to have to systematically test every component
in-circuit backwards from the Q3 transistor in a good Mac to find the
bad part or series of bad parts. Power is making it to everything else
in the Mac, except the flyback. How do I know this? The flyback of one
Mac will power the CRT of another. The 128k's flyback will not power a
good Mac's CRT. The reverse IS true, to a point -- with the 128k's
flyback out of circuit, there is nothing to power the Horizontal Video
circuit that comes after, so I get a Vertical line, but the CRT powers
up. The only odd thing is that, other than the CRT not powering up, is
that the voltages all read correctly throughout the Mac when using a
400k disk drive. When I replace it with an 800k drive, suddenly I get
"flupping" as if the 800k drive is pulling more power than the 128k can
provide (though I didn't know an 800k drive drive pulled more power
than a 400k). If there is a connection, then a diagnosis of no power to
the Q3/T1 and not enough voltage to the rest of the system should pin
it down pretty good, but damned if I can find it on Prof. Lee's well
documented schematic. Hence the need for Expert help!
On Jun 25, 2005, at 3:31 PM, Antonio Rodríguez wrote:
Maybe it's a silly question, but... are you sure the CRT is in working
order? Even if the glass isn't broken, it's possible that a blow
breaks the heating resistor. You can check this by powering on the Mac
with the case opened and looking at the end of the neck, near the
circuit board. It should light with a orangeish glow. If you don't see
the glow, it may mean that the resistor is broken, and thus, the
tube's anode won't heat to the temperature needed for it to emit
electrons and no picture will be shown in the screen, even if
everything else works perfectly.
[EMAIL PROTECTED] escribió:
Well, I tested the 128k's T1 Flyback transformer in another Mac and
worked
perfectly. SO that's good. But installed in the 128k it does not seem
to get
power at all. That leads me to suspect the next prior component in the
circuit, the Q3 (or HOT). I tested a known good Q3 transistor in the
128k
and still no power to the T1 flyback. Using Prof. Lee's schematic,
tracing
the circuit backward
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