Re: 6101 Def Board Prob

On Tue, 19 Sep 2000 23:03:36 +0200, Tek <mypearl@dds.nl> wrote:

>Okay guys..
>
>I've just made a small test to see what happens. Crazy I am.
>
>Nothing happened, when I took out the power transistors notihing started to
>smoke. Then I probed around with my finger to see if any of the pre-amp
>transistors or resistors turned hotter than normal. Zilch (this with the AVG
>still driving the amplifiers).
>
>As far as I can see the amplifier is a very classic one. The only fundamental
>difference of your ordinairy everyday audio amp seems to be the feedback - It is
>taken from a sensing resistor (in series wiuth the yoke) through which all the
>Yoke current runs as well. This way the Yoke current determines the feedback
>signal, and not the voltage at the output (ofcourse they're still related). See
>it this way, being the Yoke and the 10W sensing resistor a voltage devider, the
>current through the Yoke and resistor varies the voltage over the sensing
>resistor in a linear way. Therefore feedback is now related to the magnetic
>'strength' of the yoke, since this is linear with the Yoke current. This gives
>you nice linear defection, regardless of the impedance behaviour of the Yoke.
>
>Comments are very welcome, as always!

There are differences in the output sections of the different X/Y monitors, so
this may or may not apply to the one you were using.

On a Cinematronics monitor output drive circuits, the pre-driver transistors
also have their emitters connected to the yoke through a 47 ohm resistor (R118,
R129 and R218, R219). If you unplug the power transistors and try to run this
monitor these resistors WILL heat up and WILL give you a nice tatoo if you touch
them. The pre-drivers will also get very hot, and it will be a matter of luck
which will go first, the transistor or resistor. People that have fix many of
these monitors will know what I'm talking about. Very often after the main
transistor blows (becomes an open circuit -- same effect as unplugging it) this
47 ohm resistor is soon to follow, and occasionally the pre-driver.

Another problem with the Cinematronics monitors are their digital inputs.
Without a CPU controlling the inputs the sample & hold circuits drift and
eventually (in a few seconds) the trace is driven off the screen. With the
power transistors in place, and luck is with you, this will cause the circuit
breakers to pop. With out the power transistors in place, this will cause some
other component to pop :^(

Now if you can keep the trace at the center (or close to the center) of the
screen, you would be able to display a small picture with much less stress on
the pre-drivers. I've often thought a small test routine that would draw a
small something or other at the very center of the screen might be a useful
routine for checking if everything BUT the power transistors were working,
without over taxing the pre-drivers.

-Zonn
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Received on Tue Sep 19 19:30:43 2000