I had suspected the triple nickel (555), and indeed it was fried.
It also took out Q905, AND T901.. I verified this by trying T901 in
a known good HV unit. I got smoke, and another blown Q905..
So if anybody knows where I can get a replacement T901 thatd be
swell!
> I'm not saying that you don't have a bad T901, but can you
verify that
> IC901 (555 timer as a simple oscillator) is running. A scope would
show a
> nice square wave coming out at pin 3. If it is stuck in the high
state,
> it would keep Q904 and Q905 turned on constantly and that would
certainly
> cause your problem. Of course in this state, there would be no HV
output
> at all. Check ZD900 for 13V across it. ZD900 is the voltage
regulator
> for the 555 chip. If the zener is partially shorted and there
isn't
> enough voltage across the 555 chip, then the 555 chip output might
be
> stuck even if there's nothing wrong with it. If the zener is
> open-circuit, the 555 chip will likely have blown.
>
> You should remove R917 and leave it out until you can verify that
IC901 is
> running okay and that the transistors Q904/905 are both switching.
To
> functionally test Q905, you can add a temporary 10k (to 47k)
resistor
> (1/2W to 1W) from +Vsupply (+28V) to Q905 collector. If scope shows
a
> good square wave, remove the temporary resistor and return your
focus to
> T901.
> If you do not have a scope but you have a voltmeter, connect your
meter
> (-) to ZD900 anode or IC901 pin 1. Use the meter(+) to measure
voltage at
> ZD900 cathode and IC901 pin 8. Both should be about 13V. Measure
IC901
> pin 3. If it is running, it should be around 6.5V plus/minus around
0.5V
> roughly. The point here is that pin 3 should be outputting a square
wave
> having roughly a 50% duty cycle so the voltmeter should show
approximately
> one half the chip supply voltage.
>
>
> William Boucher
> ----- Original Message -----
>
From: akswanson@ntelos.net
> To: vectorlist@vectorlist.org
> Sent: Sunday, August 28, 2011 9:20 AM
> Subject: VECTOR: K6100 T901
>
>
> One of my 6100s had a fire on the HV board, took out R917 and Q905.
I
> replaced those, and got smoke again.. Comparing the resistance of
the
> primary of T901 to a known working T901 I get 9.5 Ohms on the
suspected
> bad one and 17 ohms on the known good one. I get the feeling I
melted
> the varnish on the wiring and shorted them together.. Does anyone
know
> where (or have) and extra T901? Also, Is there a secondary cause,
such
> as a flyback failure that would cause this kind of failure? I
dont
> want to blow the dang thing again..
>
>
>
> Andrew
>
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Received on Sun Aug 28 21:48:54 2011
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