Bulletins from the Pacific Packet Radio Society - page 124

73 es cul
John, VE7ADE


To: All Packeteers
From: Bill Danielson and Ron Brown

Here are the errors/changes that Ron and I made to get our VADCG modem boards operational:

In the instructions, the pictorial of the PC board contained the following errors:

1. The jumper wire above C11 is not shown .
2. The locations of the resistors R27 and R19 is reversed.

The values of the resistors R15 and R17 are reversed. R15 should be a 33K resistor and R17 should be a 16K resistor. In my schematic R15 is the resistor connected to pin 7 of U3 and R17 is connected to pin 8 of U3. This error causes the two modem frequencies to be reversed.

The 1uF capacitor C4 is not listed in the parts list.

In addition, I made the following modification:

The board contains a CMOS switch (4053) to automatically switch the transmitter input between the modem and a PTT microphone. it also can use the receiver's squelch output as a transmit disable. I did not want either of these features so I disabled them and used the parts on the board for a CTS indicator. To do this:

1. Remove D2, D3, and D7.

2. If you cannot find a 4053, you can replace it with a header with pins 13 and 14 connected since it no longer needed as a switch .

3. Connect the "squelch in" line to the "CTS" output line. This can be done either on or off the board.

PIN 3, the Squelch LED driver output, is now the CTS LED driver output!

Misc comments:

After tuning the modem as described in the instructions, you can check it out by temporarily connecting pin 7 (audio out to xmitter) to pin 12 (audio in). \Make sure nothing else is still connected to these pins!} This puts the modem into what's known in the modem trade as analog loopback mode. Adjust the output level using R20 until the carrier detect lamp comes on. Now if you toggle the TXD input between +12 and ground, you should see

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