Re: Quantum fix

On Tue, 11 Jan 2000, Steven Zeuner wrote:

> I had some success today in getting the amplifone monitor HV unit working.
> I wanted to post this cause I really don't know the direct fix. I had
> gotten to the point where I put in a new Wintron flyback, capped the board.
> New regulators, 3904's, replaced the W1 jumper. I also ordered some
> SK3199's just in case.
>
> What I finally did was in one swoop replaced the red mystery can, the driver
> transformer assy (why? Who knows...there was nothing left!), and the 555
> oscillator chip.
>
> I now have heater voltage and a nice spot killer. Boiard repair is up next.

        ...and you have HV, I assume? (You can do the "Do the hairs on the
back of my hand stand up when I put it near the CRT" test to see if you
have HV.)

> The board simply plays blind and trips the spot killer.

        Are you sure it's the board? It could be the deflection board. I
forget if you said that you had a tested good anything in there. It DOES
seem like your HV board is working fine, though (assuming you have HV.)

        The board playing blind and tripping the spot killer USUALLY means
that there is some problem in the AVG section of the board. If you have
access to a scope, get it, because this is one of the few times a scope is
REALLY useful for board repair (in my experience.) First, try to play the
game on the scope (oh yeah, the scope should have X and Y inputs) and you
might be able to see which part of the AVG circuitry is causing the
problem (i.e. vertical or horizontal.) Then, you need to trace through
that AVG circuit with the scope and see where your waveform disappears (I
usually compare the waveform at each point with the "good" version on the
other channel. They look different enough when something is bad.) If you
have to do this without a scope, you MIGHT be able to tell which channel
is bad by looking at the outputs with an AC voltmeter. No AC is
bad. This technique is nowhere near as reliable as using a scope. Then,
once you've guessed which channel is bad, start replacing chips one by
one and testing the board until you get it. I'd start with the Op-Amps,
because they are the cheapest and most readily available. They are also
probably the most common failures too. Then, go for the analog switches,
multipliers and then the DACs, which is roughly the order of their
availability. Oh yeah, before you do any of that, check your supply
voltages to the AVG section (+5 and +/- 15, I believe) those regulators
go bad every so often, too. The +5 to the AVG is different than the +5 to
the digital section of the board, I believe (the +5 to the AVG is called
+5V DAC, if I remember correctly.)

        Good luck.

Joe
Received on Tue Jan 11 18:08:21 2000