Phil Crosby (a man of rare talent even *before* he owned an Amiga!) found
some design flaws in his Amiga 1080 monitor.  Since he has been involved
in television test and measurement for at least 20 years, it was second
nature for him to dive right in and fix the problems.  He passed the
information on to me, and I am passing it on to you.

A set of four files contains the information needed to update Amiga 1080
monitors (at least, those made by Toshiba) to eliminate audio crossover
distortion and significantly reduce interlace errors.  The files are:


  1080Mod.uue
  -----------

      This is a uuencoded Notepad file which contains Phil Crosby's
      instructions for modifying the monitor.


  1080Mod.info.uue
  ----------------

      The uuencoded icon for the instruction file.


  1080Fixes.uue
  -------------

      A uuencoded high-resolution monochrome picture file which contains
      the schematic for the 1080 monitor audio section.  This file was
      created by DPaintII, and can be viewed with DPaintII or with
      viewilbm.

      I tried to look at the file with my copy of DPaint (the
      original. . .), but it complained that the file's format was wrong.
      However, I found a copy of viewilbm in the c directory of Fred Fish
      Disk #121 (I think -- any of the recent Fish disks with pictures
      should have it).  Viewilbm worked just fine.


  1080Fixes.info.uue
  ------------------

      The uuencoded icon for the schematic file.


You will need to uudecode each of the files and place them together in a
directory before you can use them.  You may want to edit the "tooltypes"
entry of 1080Fixes (using the Workbench "info" menu item) to point to a
viewing program other than DPaintII.

Please read Phil's instructions before you begin!





Before you start, a warning:


                          ***  W A R N I N G  ***

      In order to make these modifications, you must open up the case of
      your monitor and replace and add parts.  This exposes you to the
      danger of electric shock!

      There is approximately 25,000 volts on the picture tube when the
      monitor is in operation.  A significant fraction of this voltage
      will remain with the monitor off!  The audio output stage has a
      +100 volt DC supply.  And there are other dangerous voltages inside
      the monitor as well.

      If you are not familiar with working on electronic equipment which
      contains dangerous voltages, talk someone who is into doing it for
      you, or take your monitor to your local Amiga dealer or TV repair
      shop and ask them to put in the mod!






In my monitor, the capacitor that was replaced for the interlace modification
(C304) looked like this:


                     -----------------
                    |                 |
                    |                 |
                    |                 |
                    |                 |
                    |                 |
                    |                 |
                    |                 |
                    |                 |
                    |                 |
                    |                 |
                    |                 |
                    |                 |
                     -----------------
                       |           |
                       |           |
                       |           |
                       |           |
                       |           |
                       |           |
                       |           |
                       |           |



The lead spacing was 0.3".  The original part was a wound mylar or
polypropylene (or some such).  I replaced it with a 0.01 uF +/- 20% ceramic
capacitor rated at 250 V.

If you can find a mylar 0.01 uF at 50 volts or more, that should be fine.
However, don't use a 50 volt ceramic part (ceramic capacitors can change
value as a function of applied voltage -- the effect is reduced when the
applied voltage is less than 20% of the rated voltage).  And don't use a
capacitor that is rated 0.01 uF +80% -20%!  This indicates that the
capacitor is probably made of a material referred to as "Z5U", which
undergoes dramatic value changes as a function of temperature and applied
voltage.

Whether you use a ceramic part or a mylar part, make sure that it is rated
0.01 uF +/- 20% (or better).






Phil made the crossover distortion modification by lifting the collector
of Q605 (see the schematic) and then wiring directly to the collector lead
on the top of the board.  I did it a bit differently.

The collector of Q605 is connected to the base of Q604 by a run (on the back
of the board) that is quite fat for most of its length (perhaps 0.2" wide).
However, right near the collector lead of Q605, it necks down to a narrow
trace (0.050", maybe?) for about 1/4 of an inch.  I cut the run at both ends
of the narrow part and lifted that (narrow) piece of foil off the board.

Then, I removed D601 from the board and cleaned the solder out of the holes
the diode had been in.  I soldered the 33-Ohm resistor onto the cathode lead
of the diode (very close to it), slipped a bit of clear heat-shrink tubing
over the two and heated the tubing to shrink it.  Finally, I slipped the
anode lead of the diode into the hole it had come from (but from the back
of the board), soldered it in place, and wrapped the resistor lead around
the end of the collector lead of Q605.

To prepare the two added diodes, I soldered them together in series, slid
a piece of heat-shrink tubing over them, and heated the tubing to shrink it.
Then, I inserted the anode lead of the diode string into the hole the
cathode of D601 had been in (from the back of the board), soldered it in
place, and wrapped the cathode lead of the diode string around the end of
the collector lead of Q605.

Soldering the two wires to the end of the collector lead of Q605 completed
the the audio modification.  I then dressed the two series strings of parts
away from leads that stick through the board to prevent short circuits.

Be sure you know what you are doing when you make the crossover distortion
modification!  If you put D601 in backwards, you will make the audio *much*
worse, and perhaps blow the diode (in which case you would have no audio at
all!).  If you put either or both of the two added diodes in backwards, you
will most likely blow them out (or perhaps just avalanche them), resulting
in grossly-distorted audio or no audio at all.  Diodes are symbolized
schematically as follows:


                            /|
                       |   / |
           Cathode     |  /  |      Anode
                       | /   |
         --------------|<    |--------------
                       | \   |
                       |  \  |
                       |   \ |
                            \|


Normally, the diode symbol is not printed on the part.  There is usually
a band around one end of the diode which indicates the cathode end.






I found one other problem with my monitor.  At the back of the monitor
there is a board which contains the input connectors.  The RCA phono
jacks had not been seated against the board when it was manufactured.
Thus, every time the audio and composite video connectors were plugged
in, the force was taken by the copper foil on the back of the board.
After a while, it just gave way, leaving intermittent connections.

I unscrewed and removed the metal plate that is attached to the board,
heated up the connections on the phono jacks (there are 3 on each jack),
and pushed the jack body firmly against the board.  Then, I scraped off
some of the solder mask (greenish stuff) covering the foil runs and 
solder-bridged the areas where the foil had pulled loose.





The result?  Much better picture, better sound, and no intermittent
connections!  Thanks, Phil!!!

					Steve Rice

-----------------------------------------------------------------------------
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new: stever@videovax.tv.Tek.com               [phone (503) 627-1320]
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