Re: IIgs Monitor Repair

[Polymorph <polymorph69@hotmail.com>]

On Dec 27, 9:53 am, ferd...@gmx.de wrote:
> 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...
>
> > 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

Ferdinand,

Thanks for the reply, you certainly sound like you know what you're
talking about. Unfortunately for me, I'm still trying to learn my way,
and I didn't understand a lot of what you were talking about. Maybe
when I read the monitor repair FAQ in detail (I did scan over it
briefly) I might have a better understanding of what you were on
about.

I will be initially concentrating on getting the AppleColor monitor
working properly (mostly due to my study being filled up with semi-
deconstructed monitors), then I will return to the old Amiga monitor
once that is done. I'll post as much detail as is needed, because it
was a good monitor and I'm sorry to see it out of action - one thing I
definitely miss is the monitors inbuilt speaker with volume control.
Because I was using a SCART interface, audio information was also
being sent to the monitor and the volume control on the monitor was
*much* easier to use than having to go into the IIgs control panel.

thanks,
Mike