ROBOT 1200C MODIFICATIONS
(By Wayne Jamison, W5FJS)

EXTERNAL RECEIVE CONTROL

Note: The latest MMSSTV Ver. 1.05E_B has the ability to do this with it's software.
Mako has the RTS COM port signal going high while receiving and sending a picture.

The latest modification that I made to my Robot, provides a set of contacts to close
when the Robot goes into the receive mode. When running vox, this allows you to
mute your mic any time that you are receiving or transmitting a picture, providing
you use your Robot, as a monitor while transmitting from another SSTV system.
You can also mute your speakers so you don't have to listen to the sstv audio.
What a pleasure it is to set back and watch a picture come down without that
annoying audio. This is done by monitoring the voltage to the receive LED.

The receive LED is not activated by a dc signal; therefore, it took several components
to pull in relay contacts, however the circuit is very simple. It is isolated with a photo
coupler and uses a 14-pin dip relay as an output. The circuit is on a small 2" X 2"
perf board . I mounted it just behind the power transformer using 1/2" stand off's
on the lip of the back panel circle #1 . I found +12V next to a regulator. circle #2 .
An eyelet marked CR9 on the power supply board. I used chassis ground for -12V.

I used the RCA phono jack, circle #3 (To Tape) for the relay output by clipping
the wires and replacing them with the relay wires. You will need to disconnect and
remove the Linear board to connect the photo coupler to the receive LED, circle #4 .
The positive is on top, the one the 470-ohm resistor goes to and the negative is on
the bottom.

I use the above dip relay to operate a Potter Brumfield DP/DT relay. (KRPA-11DG-12)
I use one set of contacts to mute my mic and the other to kill my speaker. The ones to
mute my mic, operates a dip relay like the one above. The contacts short out the mic
audio in my 8 band audio equalizer. The Potter Brumfield is one that was available,
any similar relay whose coil current doesn't exceed the contact rating of 0.5 amp. of the
dip relay would be suitable. Be sure to put a back EMF diode across the coil to protect
the dip relay contacts.
 
 

VIDEO PEAKING MODIFICATION

A noticeable picture improvement may be had, with a simple mod. to the linear board
of the Robot 1200C.

After removing the top cover, you will find the linear board, the smaller one on top
and to the front.

Tack solder a 220 pF capacitor across the emitter resistor (R11) of the first video
amplifier (Q4).

R11 is in the center and about one inch from the left side of the board.

This allows more amplification of the higher video frequencies and will enable
video entering the camera input of the Robot 1200C to give you sharper pictures.

This mod would only be of interest to those using the camera input of the 1200C.

I presented this one along with "Tweaking The Linear Board" at Dayton in the
mid 80's. Anyone interested in the Word Doc file, drop me a line and I will
e-mail it to you. This will allow you to shift the gray scale up or down, change
the amount of color and adjust the tint of your picture. This is done while
maintaining perfect color balance.
w5fjs@myway.com