Re: K6100 T901

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
http://www.biltronix.com
  ----- 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 12:48:46 2011