Re: IIgs Monitor Repair

[ferdimh@gmx.de]

On Dec 26, 7:23 am, Polymorph <polymorp...@hotmail.com> wrote:

> Its just a shame that the capacitors that I installed into the
> recently deceased Amiga monitor were of different voltages and
> capacitances, otherwise I could have re-used them.
>
> Also, upon inspection of the old Amiga monitor circuit board, I
> noticed something that looks kinda weird and maybe possible damage. It
> looks like an inductor, has come away from 2 of its support posts. See
> the following images:
>
> http://www.cirruscomms.com.au/~mike_stephens/Inductor01.jpghttp://www.cirruscomms.com.au/~mike_stephens/Inductor02.jpg
>
> I don't know whether this is normal, but it sure looked suspicious,
> especially since it is very close to the flyback transformer. Could
> this be a possible cause for the problems I'm having? I may try
> desoldering it and taking a closer look. I know very little about
> inductors, but it didn't look "normal" to me. I could be barking up
> the wrong tree here though.
>
This is NOT normal at all. But according to the silkscreen on the
board the two pins which are pulled  out don't seem to be connected.
you can check this from the bottom of the board.
If the other two pins are ok, the coil might  still be fine. A good
coil will always measure a very low resistance. For this coil I'd
expect <5Ω.
With a horizontal deflection problem I first check the Horizontal
Output Transistor for shorts. The Collector (middle pin) should not be
shorted to either Emitter or Base. If it is, there are 4
possibilities:
1) Blown HOT. Note that this may be caused by a blown flyback, snubber
cap or damper diode. If you can't obtain an exact replacement, a
BU2725D should do. It also has an integrated damper diode so it won't
blow if the damper is blown.
2) Blown damper diode. It is connected over the HOT from collector to
emitter (there may be some small resistors/indutors though). Note that
it may be integrated into the HOT. A datasheet will tell.
3) Blown power supply recitifier. This only applies when a switching
power supply is used (which I expect in a monitor).
4) Blown B+ filter caps. These will be rated at >22µF/100V. I'd expect
47µF/200V.
Always unsolder them and check for shorts.

If there is no short, then check for voltage on the HOT collector (use
an analog meter if possible, use the  1000V scale, if available and
fix the probes. Do not tap into there!) The voltage should be between
60 and 200V. If there is not, this is either a flyback or a power
supply problem (may be just a blown fuse or resistor. If you'd make a
photo of the whole horizontal section I could tell you which are
suspect).
If there is voltage, it get difficult. #
The first thing I would do is to resolder all big solder joints (and
all that look suspect). This might be a good idea anyway.

If the HOT was blown, you should check if the damper diode is ok
(measure resistance in both directions with the diode unsoldered-
should be infinite in one and something like 1000 Ω in the other
direction.)
Checking the snubber cap is difficult. It is connected from HOT
collector to ground somewhere. If there's  a pincushion adjustment it
might be split into two caps in series. There will also be two damper
diodes then. If in doubt, just replace it a blown snubber will cause
the HOT to blow instantly and testing small caps is a really tough
job.

For mor information, read the RepairFAQ:
http://www.repairfaq.org/sam/monfaq.htm


Ferdinand Meyer-Hermann