Re: G05-801 Deflection Board question

<snip>

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

That's okay then - they're so hot that I'm convinced if I could find a
small enough egg I could fry it on them. :)

>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.

I'll double check those then, thanks very much for the pointer.

>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.

I see, thanks for the clear explanation. Certainly sums up the problem nicely.

>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 it's cold then it takes about 12 seconds before the glow is visible, so
have tried it when warm. I can't see any sign of any drift at all though,
the glow just expands slightly (and that's with a 1 second time from power
on to glow).

>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.

Right you are, will try and verify that.

>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.

Sound advice, thanks.

I do though seem to have TIS98 transistors, not the TPS98 variety (I
understand that the TPS98 is of a similar specification to the TIS98, but
it has a higher breakdown voltage).

Here's something that might be useful to some people: someone I know has
the ECA databooks and checked the transistors on my board to see what the
modern day equivalents were, here's the results:

TIS98 = TIS95 = 2SC2240 or 2SC2459
MPSU03 = BF466 or BF468 or 2SC2384
MPSU07 = BD529
MPSU57 = BD530

These can be ordered from CPC in the UK (tell a lie, CPC don't seem to have
the BD529).

The TIS98 replacements have, IIRC, the base and emitter leads in different
locations, but that's easy to resolve when soldering them to the board.

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

I'm sure you're right Andy, your knowledge of these things is several
magnitudes greater than mine. :-)

<snip>

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

Oh, well that's something.

>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 see, I won't use the continuity test again then, it has only served to
confuse me. :)

>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 :)

I'm afraid my multimeter isn't exactly perfect - I bought it from Maplin
about 7 years ago and it won't even measure half the caps on my boards.
Seems okay for diodes and resistors though.

Maybe I can find a Fluke 12 cheap on Ebay. :)

Many thanks for the excellent info and advice, I'm sure that will also help
others who read this mailing list. :-)

Cheers,
Phil

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Received on Sun Jan 6 07:27:36 2002