Re: G05-801 Deflection Board question

[snip]

> I hope not to have to replace them but am currently testing all
transistors
> as the monitor won't display any graphics, just a 'glow' taking up most of
> the screen that slightly varies in intensity when things are moving about,
> especially evident when the ship fires (the game is Asteroids). I know the
> main game board is fine. From reading on the Vectorlist this 'glow'
problem
> invariably points to either a bad transistor OR a problem with the two
> large black 10W resistors at R620 and R720. These two resistors get
> EXTREMELY hot when the monitor is switched on. In situ their resistance
> checks out as it should (5.1 ohms for one, 4.5 for the other) - I would
> test them out of circuit but they don't want to come off the PCB. They
> seems to have extra casings around the legs and will not de-solder.

the big ceramic 5ohm resitors do run real hot anyway..

I have come across the 'glow' problem several times in the past and all have
been down to small signal transitors early in the deflection stage.

The reason you only see a 'glow' on the main screen is because the beam is
being deflected extremely, drawing the picture on the inside of the back of
the tube. The electrons are scattered, and in a general direction to the
front of the tube where they hit the phosphor and glow. Because of bouncing
off the rear of the tube, they don't hit the front of the tube in any
recognisable pattern, just a random-ish glow that changes with the nmber of
vectors being drawn at that point in the game.

Usually the 'glow' can be seen to jump about a bit, or drift.. if the tube
has warmed up quick enough, when you first power the monitor up you can
usually see the game screen marching off to one side of the screen, the more
it marches off, the more of a glow you get until eventually all you see is
the glow. This is commonly known as 'extreme beam excursion'.

If the picture marches off to the left or right, look at the first
transistors in the X amplifier circuit. If the picture marches up or down,
check the Y deflection circuit.

I've only seen this symptom 3 times before, and everytime it was marching to
the left or the right, which was the x-circuit (from memory). Just remove
the transistors one at a time and test them. If you can do a HFE check on
them, and they appear wildly out of spec, replace them anyway. They are
TPS98 and 2n3906 transitors. i couldn't find any TPS98 transistors at the
time, however i have used 2n4401 (much more common) in the place of the
TPS98's with no problems whatsoever. In the schematics you will see an NPN /
PNP matched pair. If one of these is bad, say the TPS98 and you need to
replace it with a 2n4401 then make sure their HFE's match up evenly or they
will in effect be fighting against one another and cause one to work harder
than the other, and lead eventually to another failure.

That's my interperetation of it anyway... seems to be right as i've fixed 3
deflections with this problem before :)

> The four accompanying wirewound green resistors near them also get pretty
hot.
>
> Curiously enough, both the two black resistors (supposed to be black, not
> scorched! :-) AND the four green ones, if tested with the continuity
> tester, all show as a short - or is this 'normal' for these type of
resistors?

Resistors never short (in 99.999% of cases unless a freak carbonisation of
the pcb occurs)..

Don't test with a continuity meter. of course there is a path for the
electrons to travel from one side of a resistor to the other.. thus giving
continuity.. and the continuity settign on your meter will tell you there is
continuity thru the resistor.... because there is...

> I'm puzzled - can anyone help please?
>
>
>
> Thanks,
> Phil

Andy

PS - make sure you have a decent meter to check transistors on, or you'll
likely miss a bad transistor because of the phenomenan known as 'transistor
leakage'... I reccomend the Fluke 12 (4 button type) for this :)

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Received on Sun Jan 6 06:31:57 2002