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
Received on Tue Jan 15 2002 - 09:11:16 GMT