More parts I need to replace on the VT100 - test-drb@ccmp.vtda.org

2 Dec 2005

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 From: ard at p850ug1.demon.co.uk

 Hi
  I looked at the schematic and the parts you'd indicated.
 It looks like one of them was one off and was R55 and not
 D11. From the looks, it might be that you have other failures
 in the supply. You've indicated R11, R21 and R55. These
 are all part of the circuit that drives the primary switcher
 transistor through T3. 
Having looked at the printset, the first thing that struck me is that 
these are all high power resistors (2 or 3 watt rating). They would 
naturally run hot, therefore, and may look discoloured without there 
being anything wrong with them.

Before we dive into this PSU, I would be interested to know if the thing 
produces any output voltage, and if those 3 resistors test bad on an 
ohmmeter.

-tony 
Hi
 I agree. But as noted, this should be done without
connecting the rest of the terminal.
 My only question is what toasted R21. From the
schematic, it should on be supplying current to
only R55. If we assume that R55 is running at 100%,
( which it shouldn't ) R55 will have 43 ma flowing
in it. This current is only a wattage of 150 milliwatts
in R21. This is well below it's rated value of 3.75 watts.
 I find it unlikely that anything less than a short
connecting the mains to the winding of T3 pins 6 & 5
could bring R21 to a current level that would
burn it as well as R55. The Diode D11 being short
might only wipe out R55. C6 shorted might take both
out and increase the power on R11 enough to do
in R11 as well.
 He might check to make sure that C6 isn't shorted.
What do you think? How else can I get R21 hot enough?
Dwight