RS 232 Modifications for the AEA CP-1 Computer Patch For FSK RTTY

by Pete Rimmel, N8PR

The AEA CP-1 is an excellent older style terminal for use as an interface
between computer and tranceiver for Baudot RTTY and possibly Pactor. I have
not tried any Pactor programs, but once the RS 232 mods are made, there is no
reason that the proper Pactor program would not work with the CP-1.

At present I use the WF1B contest program with the CP-1. I have used other
ragchew programs which allow type ahead buffers to make the QSO more

The CP-1 is excellent on receive, since it uses two detector circuits to
decode the mark and space separately, combining the results before sending
data to the computer. Calibration of the mark and space filters is a simple
matter, using the system I will outline later in this article. Data taken
from the computer in transmit originally could be sent as either FSK or AFSK.
I have chosen to disable the AFSK circuits, since FSK is my only interest on

Originally, the CP-1 was designed to be used with the C-64 types of computers
and I/O was at TTL levels through a five pin plug on the back of the unit.
Some of these circuits run in parallel with the inputs to the optional RS 232
circuits which could be purchased and added by the user, or as a factory
option. Most of the CP-1's found today do not have the RS 232 mods, and that
is the purpose of this article. Unfortunately, the two IC's which were
designed to be used specifically with the CP-1 are no longer available and no
longer in production. Therefore, an alternate means of interfacing the RS 232
levels of modern computers is required.

This is easily accomplished using inexpensive parts and a few modifications to
the CP-1 circuit board. The 1488 and 1489 Quad line receiver and driver IC's
are used in place of the original proprietary AEA chips. Unfortunately, they
will not directly plug into the U12 and U13 plugs on the board. In order to
make two circuits which will plug into the sockets on the board, it is
necessary to purchase a 14 pin and 8 pin wire wrap plug which allows 'bread
boarding' a circuit from two 14 pin sockets mounted above the wire wrap plugs
into which the 1488 and 1489 will eventually be plugged.

Figure 1 shows the modified circuit for the I/O portion of the CP-1. The
original circuits can be obtained form the owners manual and are not
reproduced here. Figure 2 shows the circuits taken from the AEA service
bulletin indicating the required 1488 and 1489 connections. Figure 3 shows
how to piggyback the 1489 socket over the 14 pin wire wrap plug and Figure 4
shows how to piggyback the 1488 socket over the 8 pin wire wrap plug.

Note that the 1489 replacement circuit must plug into a 16 pin socket. BE
SURE to plug this unit into pins 2 to 15 and leave nothing plugged into pins 1
and 16 when installing the unit into the CP-1. Wire up the plug to the wire
wrap as follows.

Be sure when constructing the two substitution units, that leads are kept
short enough so that, when plugged in, the lid will go back onto the CP-1. I
will identify the pins on the socket by preceding the numbers with an "S" and
the pins on the wire wrap plug by preceding the numbers with a "W". Be
careful to use minimum heat so that the wire wrap pins to not melt the plastic
mounts and move into a position which will prevent plugging them in properly.

1489 replacement circuit--14 pin socket to 14 pin wire wrap plug

1. Prepare one 100 ohm resistor by soldering a .001 uF capacitor
to one leg of the resistor. This capacitor will be soldered to
the jumper which is soldered to S7 and W8 (leading to ground).
2. Wire the 100 ohm resistor from S1 to W5 with the cap at W5.
3. Wire a jumper from S7 to W8 and connect the free end of the
.001 cap to this jumper.
4. Wire a jumper from S3 to W2.
5. Wire a jumper from S14 to W3.
6. Properly orient this unit and plug into Socket U-13.

1488 replacement circuit--14 pin socket to 8 pin wire wrap plug

1. Prepare one 100 ohm resistor by soldering a .001 uF capacitor
to one leg as above.
2. Wire the 100 ohm resistor from S6 to W6 with the cap at W6.
3. Wire a jumper from S7 to W4 and connect the free end of the cap
to this jumper.
4. Wire a jumper from S1 to W5.
5. Wire a jumper from S4 to S5 and connect to W2.
6. Wire a jumper from S14 to W8.
7. Properly orient this unit and plug into Socket U-12.

Now to modify the CP-1. Carefully remove chip U14 and bend pin 9 out so that
it will not make contact when the chip is replaced. Reinsert U14 into its
socket. This disables the AFSK circuits. Place a jumper between pins 1 and 3
on SW3(a). This half of the switch is the side to the right when viewing the
switch from the front panel. Pin 2 of this switch was connected to U14 pin 9
which we just disconnected.

Install a right angle female DB25 socket onto the circuit board, being careful
to not bridge any two pins with solder. Check that the cover will go on and
allow a cable to plug into this socket. You may have to take a nibbler and
cut away part of the top case. Install wire jumpers JP7 and JP10 immediately
behind the socket.

The following modifications and additions will depend on which circuit board
revision is in your CP-1. The revision is the last letter in the series
013-023* on the front right corner of the board by the power on LED. If this
letter is B, C, F, G or H the following components will have to be added;

1. Install Q12 a 2N3904 NPN transistor (note orientation)
2. Install D29 a 1N4448 or 1N4148 diode (note polarity)
3. Install R 99 -- 820 ohms } or 1K ohms } Option
4. Install R 100 -- 1.6K ohms } or a Jumper} Option
5. Install R 123 -- 10K ohms
6. Install R 124 -- 47K ohms

For Revision D, these parts may already be installed.

For Revision E circuit boards, the mods in Figures 5 and 6 must be

While the board is removed from its bottom case, the following will make the
FSK output modifications easier.

1. Cut the trace leading to the base (center) connection of Q13
This trace is on the bottom side of the board.
2. Cut the trace leading from J6 pin 5 (far right when viewed from
the front.
3. Run a jumper from J6 pin 2 to the trace just cut leading to the
base of Q 13.
4. Relabel the case at J5 FSK OUT over 'Demod out'

Reassemble the board onto the base. Only one further change might be required
depending on the load placed on Q13 by your FSK circuit in your rig. I found
that by changing R 126 from 4.7K to 1.0K ohms the unit worked better with my
Icom rigs. This may be a cut and try for each user depending on the voltages
to be switched at the back of your rig. Trying this out now, will save time

Connect pins 2 (high) and 3 (ground) of J11 to your PTT circuit, connect a
DB25 cable to your computer COM port and J7, plug in your receive audio into
the RX audio IN jack, and connect a 12-16 Volt AC source to the power input
jack. You are ready to go.

Tune in a signal and see if you have text on your computer screen. You
should. Tell your rig to transmit (alt-K on WF1B). The rig should go into
transmit. If you can monitor your signal, hit a few keys and you should hear
the data shifts from mark to space. If successful, put the unit together and
enjoy. If you want to calibrate your receive filters, proceed as follows
before closing the cover.

Adjusting the CP-1 Receive Filters

You need a good source of audio to use as calibration frequencies 2125 and
2295 Hz. These can come from an audio generator or your rig. If your rig has
a monitor mode, you can use it to check your mark and space filters. I
suggest that you turn your transmit RF output to minimum, run the Rf into a
dummy load and then key up the rig. If you can hear a shift in tones when
shorting or opening the circuit at the end of the RCA plug which would
normally be plugged into the FSK OUT jack in the CP-1 you are ready.

Using the lower tone, calibrate the Mark filter as follows: Connect a
digital voltmeter to the SCOPE OUT center conductor J3 (mark) and ground in
the 0-2 volt AC range. Adjust trim pots R18 and R39 for maximum voltage on
the meter. Adjust each several times to assure maximum filter response. Note
that R39 is vertical and R18 horizontal on the left side of the board. Both
are marked 2125 on the board.

Now short the RCA plug to shift the tone to 2295 Hz. Connect the digital
voltmeter as above to J4 (space) and ground. Adjust trim pots R12 and R34 as
for the mark. They both lie flat on the board. It's that simple. Close up
the case, reconnect the wires and have fun!

NOTE: If using the WF1B program, on the cover page set the TNC to BAUDOT-DRV
and the baud rate to 110. These will work fine.

73, Pete Rimmel N8PR Email: Pete Rimmel <>