Due to numerous requests I am posting modification instructions for the Paccom NB-9600 G3RUH modem to convert it to TTL input/output to drive a Kantronics D4-10 at 19.2Kbaud. Don't forget to change the radio speed jumpers in whatever TNC you are using to 19.2K after these mods are done. I mention this only because I spent one hour trying to figure out why my mod wouldn't work, only to finally realize I never changed the TNC SPEED to 19.2K to drive the blasted modem! (what an idiot huh?) It's the simple things that getcha! :-) ----------------------------------------------------------------------------- 5 Feb. 92 G3RUH Modification to convert to 19.2K and drive a Kantronics D4-10 through the TTL port. All part numbers given are for the Paccom NB-96 modem. 1. Receive audio in to receive data in (conversion to TTL input) mod. A. Locate U10 pin 2, and circuit board trace going to U5 pin 2. 1. Disconnect output of U10 pin 2 by either lifting the lead leg on the integrated circuit or by cutting trace. Lifting the leg on the IC is the easiest way to go about it. 2. Connect TTL receive data input from the D4-10 to trace going to U5 pin 2. Note: On Kantronics 19K2/9K6 modem, they actually put in a jumper that lets you either drive U5 pin 2 directly from a TTL source (such as the D4-10) or when the jumper is installed, the input to this stage reverts to normal G3RUH operation with U16C acting as the Rx filter for the analog input. 2. Transmit audio out to Transmit Date out (TTL output) modification. A. Locate U18 (74HC164) pin 3 and trace going to Jumper-1 pin 1. Please note that Jumper-1 normally is jumpered from pins 2 to 3 with a header jumper. 1. Connect wire from from Jumper-1 pin 1 to D4-10 transmit data input. Note: On Kantronics modem, they do the same thing. They disconnect the output of U18A (after C34) with a header, and allow you to drive the output with U18 pin 3 or the analog output from U18A determined by the position of a header jumper. If it is desired to disable the audio output of the G3RUH modem, you can cut the trace after C34 or remove C34. I did neither and just let it run since I was no longer using the audio output, and I saw no harm in just leaving it run. 3. Lock Detector modification. A. Locate resistor pack RS-2. It is a 100K resistor pack and its only purpose is to give 100K of resistance between pins 1 & 2 and 3 & 4. The other resistors in the pack are unused. The goal here is to change this resistance to approx. 50K, and there are a couple of ways to achieve this as listed below: 1. Cut traces to RS-2 and install two 50K resistors. One between pins 1 & 2, and another between pins 3 & 4. 2. Leave everything intact and place a 100K resistor across pins 1 & 2, and another across pins 3 & 4, this will form two parallel resistor networks with each one offering 50K to the circuit. I used this method. Not much room to work on the board for this step and soldering is in very tight quarters. 3. Use the unused portions of RS-2 to do the same thing as #2 above using shorting wires. Note: The value of 50K for the lock detector was chosen from notes given in the Paccom NB-96 manual. I have been experimenting with these values and have achieved what appears to be better results with values different from those reported. However since I can not document WHY that is happening, and since it might be limited to my application, I will not go into details on what values I am using. I merely point this out so that if you happen to notice what appears to be poor DCD detection on what appear to be good signals, this might be a good first place to look. Note: There were some suggestions given in the past to modify a Kantronics DVR2-2 to work with the NB-96 modem at 9600 baud. These suggestions consisted of bypassing one resistor and changing the value of another IN the radio. The net result was to increase the audio output level of the radio to the modem, and to lower the transmit audio input level from the modem to the radio. I have found that increasing the audio output of the radio to the modem often- times results in WORSE performance and would recommend keeping it stock. However, lowering the transmit drive level is required but it seems to make more sense to do this in the modem rather than in the radio. If a 47K resistor is placed in series with transmit audio output (INTERNAL to the modem) then it will duplicate Kantronics design, and will offer easy adjustment of transmit deviation whether you are using a DVR2-2 or a D4-10. Of course this is only necessary at 9600 baud, as when the above modifications are done, the output of the modem is TTL and not analog. Mark Bitterlich wa3jpy@wb4uou.nc.usa.na (packet) mgb@tecnet1.jcte.jcs.mil (internet)