26 Aug
2010
26 Aug
'10
12:34 p.m.
From: ard at p850ug1.demon.co.uk
---snip---
=20
WHile cleaning the dirt off this PCB (HV attracts dust electrostatcally=
=2C=20
of couse)=2C I noticed that the ground pin of
this IC never made it throu=
gh=20
the board. It's folded under the package=2C
but has been effectively=20
surface-mount soldered to the pad when the board was wave-soldered. It=20
works=2C but I can't beeleive it's as good as a proper through-hole=20
connection=2C and it's certian=3By not what was intended.
=20
I could trivially desolder the IC=2C straighten the pin=2C and solder it =
back=20
properly. But should I? What would others do?
=20
-tony
=20
Hi
First=2C can you explain how it works as is? Being one shots=2C such a fix might effect the
timing. Is it adjustable?
The 2 one-shots are cascaded. The first one is driven by the Hsync signal
from the logic board and delays it. The output of that
triggers the scond
one which probides a constant-width pulse to the horizontal
drive transistor.
The first one-shot is adjustabnle. The adjustment is a 'customer preset'
control -- that is a preset that can be turned witha screwdriver through
a hole in the tope of the terminal casing. It's used to centre the
picture horizontally. The second one-shot is not adjustable. But I can
see no evidence of there being any select-on-text compoennts here.
Much the same circuit -- with the same component values is used in the
HP120 and HP150 comptuers, for example. Totally different layourt,
though. And the adjustments (or lack of them) are the smae there.
At a place I once worked=2C we had a board where the
+5 was missing from
a CMOS part. It was running from the inputs that luckily always had
at least one high.
More ofen the circuit works until all the ipuuts go high at the same
time. A nasty fault to find if you've not seen it before.
-tony