15 Jan
2002
15 Jan
'02
9:11 a.m.
Pete Turnbull wrote:
They used Philips or Microvitecs. I think that model
is a rebadged Philips
There's no indication of who really made it that I can see, just several
labels saying "Made in Taiwan Manufactured for Acorn Computers Ltd."
VGA. There's a stock LOPT fault on one of those,
but I'm not sure if it's
the AKF40 or AKF30. One of them also has a stock problem with cracked PCB
under the LOPT, due to poor mounting design.
The PCB looks sound. I've no idea how to test the transformer itself.
"Check Horz O/P transistor for S/C.
I tried that; in the circuit I was getting 18 ohms in both directions
across E/C, so suspecting I was actually measuring other components in
the circuit I desoldered it. It appears to have (as far as I can tell
with my meter) infinite resistance between E and C, 4 M ohms from B to
C, and 15 M ohms from B to E. Would I be right in thinking that this
transistor is dead? If so, where could I find a replacement? It has
written on it an M in a square box, then "D1577", then "PV 19" with an
underscore under the 9.
If OK suspect EHT/LOPTX assembly. Test by
substitution.
I don't have a substitute. Is there another way to test it with just a
multimeter?
A less common fault is a problem with the over voltage
protection circuit. This is a crowbar type across the
28V supply and diodes
6452/6454 type PHF15 (15V 300mW Zeners) and thyristor 7452 type BT151 can
fail causing a short circuit. These can fail for no apparent reason or a
fault in the regulator circuit causing excessive voltage. If these have
failed remove them from the circuit and disconnect the following. Collector
of the horizontal output transistor to avoid the possibility of excessive
EHT, R3563 feed to frame O/P and R3512 feed to horizontal drive circuits.
Connect a meter across the 128V rail and switch on the unit. If the voltage
is excessive and does not respond to adjustments with R3414 switch off
immediately and examine the regulator circuitry. Transistor 3470 BC558B can
also fail and is best tested out of circuit using a transistor tester. If
in doubt replace it."
If this had failed could the overvoltage have caused the other faults?
Sorry for asking newbie questions.
--
------------ Alex Holden - http://www.linuxhacker.org ------------
If it doesn't work, you're not hitting it with a big enough hammer