REDUCING RF INTERFERENCE


The later version rf modulator, the Astec UM1291-1, produces a much cleaner
screen display compared to the earlier version. There are some things you can do
to possibly clean up the TV display. The following list are tips that may reduce
some rf interference:

1.To reduce rf interference on the TV display, do NOT use a TV/game switch.
  Connect directly to the TV's 75 ohm connector using 1 of 3 methods:

  A. Use a "no tool required" rf coax F plug adapter. One is available, at this
     time, at Amazon.com, item B0028MXOF6.

  B. Splice on a spare 75 ohm coax cable having a plug in or screw on connector.

  C. Splice on a "tools required" F connector like Jameco part#2272143 or equal.


  Giggle the connection at the TV to see if the TV display will "clean up". If
  so, inspect the connection to the TV to determine why giggling cleans up the TV
  display. Repair or replace the cable as needed.

2. Make a minor adjustment to the channel 3 or 4 modulated output. Adjust the
   channel output using a small flat edge precision screw driver. This adjustment
   may reduce transparent "waving".

3. Sometimes just moving the long 75 ohm cable, from the modulator to the TV, to
   a different position might reduce interference.

4. Inspect the following motherboard connections/contacts for aging and clean
   them if tarnished or dirty. You can use a new, folded piece of 400 grit
   wet/dry sandpaper for cleaning.

   A. The narrow copper contacts attached on the left side of the bottom metal
      shielding. Make sure the cleaned surface area is making contact with the
      rf modulator and the AC power input shielding.

   B. The bottom surface area of the 8 pin connector that mates with the rf
      modulator contacts.

   C. The rf modulator's RCA type output jack.

   D. The 4 metal pins that the 4 wire AC power transformer connects to.

5. If the bottom motherboard shielding is not being used, connect a jumper wire
   from the rf modulator metal case to the negative (ground) lead of the 1500uf
   electrolytic cap C1 at the right, front corner of the motherboard.

6. Reduce the video line resistor R33 from 3.3k to 1.5K ohms. This simple
   motherboard modification might reduce interference a little, but it will
   sharpen the screen graphics. It might also increase "ghosting" in Checkmate. 
   It is unknown if the custom data chip will run a little hotter by lowering
   the value of this resistor. MCM Design has been using its hi-res Astrocade
   with a 1K ohm R33 resistor since the 1980's with no issues. But, the hi-res
   Astrocade uses a custom black heat sink on top of its custom data chip and
   no shielding on top of the motherboard. There is also a little forced air 
   moving over the top of the motherboard.

7. The 50 pin BalcheckHR ribbon cable connector will likely produce transparent
   vertical lines across the entire screen display. The video output line at the
   custom data chip (pin 24) is also connected to pins 2 and 3 in the 50 pin
   motherboard expansion. One method to eliminate the vertical lines on the TV
   display is to cut the 2 trace lines VIDIN and VIDOUT near the 50 pin expansion
   using a sharp hobby knife. Obviously, this motherboard modification would be
   only for a motherboard that is regularly using a 50 pin cable connector like
   a user RAM add-on that also produces these transparent vertical lines.


End of document
MCM Design
Dec 2018