TAPR

From: K4DMU@WA4UMR
TAPR DCD mods - a review.


                       The TAPR DCD modifications.

                     A review by Joe Leitsch , K4DMU

        During the latter part of May and early June, 1989, I purchased

  constructed and installed four TAPR DCD modification kits- two of the

  XR 2211 Upgrades for an MFJ 1274 and Heath HD4040 (TNC 1 clone) and

  two of the State Machine Upgrades for a Kantronics KPC2 (which wound

  up in a Kantronics KAM) and an AEA PK-232. The following paragraphs are

  my personal experiences and thoughts on the project.

        I tackled the XR 2211 kits first. The only difficulty I encountered

  was an undocumented change in the pc board. There is an area on the

  upgrade board marked "J" that consists of two holes that are strapped

  with a piece of wire for VHF operation and which are wired to a 100K

  pot which adjusts the noise threshold when used on HF. Apparently there

  was a change in the board artwork after the assembly instructions were

  printed, because the "J" holes are CONNECTED by a printed trace on the

  solder side of the board. Therefore, for VHF operation, no jumper is

  needed, but if the threshold adjust pot is to be installed, the builder

  must cut the trace between the "J" holes for the pot to work. 

        Otherwise, the 2211 mod seems to work as advertized. I am able

  to run both TNCs on an unsquelched FM radio with no DCD action until

  a valid Packet transmission is recieved. This results in a quicker

  decode time for the transmission, and if the mod were made at both

  stations, a reduction in TXD time would certainly be possible.


        Brimming with success, I next tackled the State Machine Mod

  Kits for TNCs using other than XR 2211 Demod circuitry. No problems 

  were encountered with assembly - everything dropped right into place.

  After the first unit was completed, I began to install it in the KPC-2.

        The installation instructions provide information about installing

  the unit in TNCs containing the more common generic single-chip 

  modems. Required inputs consist of +5 Volts, Ground, Recieve NRZI data,

  DCD output from the modem chip, and a clock signal source (19.2 khz or

  38.4 khz for 1200 baud). I discovered that the KPC-2 did not have a

  clock source available, so I went to plan 2 -  my KAM.

        The instructions covered installation in a KAM, so I thought I

  had it made. All the connections were made and checked. All looked

  fine. Then, I put the audio to it. It worked - backwards... The

  KAM needed positive DCD out and the instructions used the negative

  DCD out. I lifted the green wire and put down the yellow wire and...

  EURIKA !!!

        The KAM seemed to perform as well as the MFJ. Encouraged, I 

  built the second State Kit. This one was going into my AEA PK-232.

  Being gun-shy by now, I was careful to follow the PK-232 installation

  instructions. Lo and behold, the instructions said to wire the DCD input

  and output to JP5 of the PK-232. Unfortunately, JP5 is the Transmit

  Data Lead, not the DCD lead. Moving the connections to JP6, the DCD

  lead, produced satisfactory results. The moral of this story is to 

  take all instructions with a grain of salt. 

        All four units are installed and do work "as advertized". Do

  take care when installing them to watch for these errors in the 

  instructions. If another type TNC is involved, be aware that there

  may be difficulties in the instructions for installing the mod in it.

        Give these units an "A" for performance and a "C-" for 

  documentation. Good luck.
                       RELAYED  BY...N0KGX...GENE