Re: Fix/replace an Applecolor RGB (1987) ?

[rubywand@aol.com (RUBYWAND)]

In article <E2qynx.Fv8@info.physics.utoronto.ca>,
neufeld@caliban.physics.utoronto.ca (Christopher Neufeld) writes:

>
>   Well, my one and only computer, the ][GS, just lost its monitor. When
>I turn on the power the LED doesn't light up, though there is the usual
>startup surge hum. No images are produced.
>   This is a 1987 monitor, the kind with the faulty flyback solder
>points. I resoldered them about five years ago when the monitor started
>to flicker, and it worked fine until a few weeks ago. Then, it started
>flickering again, so I redid the points, even though they looked fine.
>Now, after two weeks, it suddenly doesn't power up. The points still look
>fine, no obvious cold solder joints and no bridges, but I wonder if the
>flyback is smoked. Hmm, no smell, either.
>   Anyway, are there any replacement components or repair procedures that
>anybody knows about? ....


     Know? Unfortunately, no. There are a few things you can try.

     The flickering you were getting may have been due to arcing between
the black HV module (encapsulated flyback with two slotted controls) and
the nearby metal brace. This kind of flicker is different from "loose pins
flicker" because the HV is, briefly, shorting and the display kind of
snaps out of focus and shrinks for a fraction of a second until the HV on
the CRT is recharged.

     If arcing is not fixed-- e.g. by slightly bending away the metal
brace, cleaning, and applying HV dope to the brace and module-- it may be
that, eventually, something will blow.

     Since, evidently, the basic power supply still starts up, it may be
okay. If it is, then the 'no indicator light' symptom makes it look like a
power connection has been cut-- possibly a fuse has blown or a circuit
breaker has tripped. (I do not know where a fuse, fuses, "fuse resistor",
or a breaker may be located in the GS monitor; but, a blown fuse/breaker
looks like a good bet.)

     A blown fuse would, also, shut off power to the HV module. (You may
notice that the usual high-frequency whine associated with the horizontal
sweep oscillator is missing.)

     Of course, the big problem would be fixing whatever caused the
fuse/breaker to blow. It could be that a line surge led to a brief
over-current condition which caused the fuse to blow. In this case,
replacing the fuse would not cure the flicker; but, at least, you would
get back your display.

     A short in the HV section (e.g. from arcing) is another possibility.
When a TV or monitor HV supply bombs, a decent chance exists that the
flyback power transistor has been zapped and shorted. This can lead to a
blown fuse. (If a replacement fuse blows but, then, another fuse does not
blow after the flyback transistor is removed, this is an indicator that
the transistor is shorted.) The flyback transistor should be located on
the brace/heatsink near the HV flyback module.

     Since going from tubes to transistors, toasted flyback transformers
seem to be fairly rare.

     Another cause of flicker is a dirty control (e.g. Brightness,
Contrast, etc.). If a dirty control were to fry it might short and lead to
a blown fuse. A fired control should have an odor and/or feel 'crunchy'
when turned.

     As you might guess, there are other possibilities. If we take flicker
as a pre-blow symptom, and there is no bad smell/cruncy feel coming from
any control, then a blown HV flyback transistor looks like a good
candidate.