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KPC 3 Plus and HTX-252   -   2002/12/09Viewed 174 times this month, last update: 2004/11/27

I've been asked how I wired up my Radio Shack HTX-252 2 meter FM HAM radio to my Kantronics KPC 3+ TNC.

Actually, I couldn't find out how to do it either, so I kind of had to figure it out on my own. The hard part is that the HTX-252 doesn't detect PTT as most radios do, by shorting PTT to ground. The HTX-252 detects PTT by shorting one of the mic pins to an isolated ground. The specifics of this mechanism are out on the 'net somewhere... I couldn't find it again. Sorry.
The basics are this:
  1. Connect the center pin of the HTX-252's audio out jack to pin 5 of the KPC 3+ DB9 connector
  2. Connect the ground of the HTX-252's audio out jack to pin 6 of the KPC 3+ DB9 connector
  3. Connect pin 1 of the HTX-252's DIN-8 microphone jack through a 5% 2200 k ohm resistor to pin 3 of the KPC 3+ DB9 connector
  4. Also connect pin 1 of the HTX-252's DIN-8 microphone jack through a 104k capacitor to pin 1 of the KPC 3+ DB9 connector.
  5. Open up the KPC 3+ and jumper the J9 center pin to the 'HT' pin.
  6. Optionally, connect pin 2 of the KPC 3+ to pin 2 of a DB9 connector, and pin 6 of the KPC 3+ to pin 5 of the DB9 connector to connect up to a GPS for position data, to be read from the GPSPORT of the KPC 3+
Here's my hand-made diagram:

Click to enlarge

Note: If I were to setup another APRS station, I would not use the Radio Shack HTX-252. It's signal is very noisy, and it's recive quality could be much better. Also, it's display uses quite a bit of juice, so, if I were to do it again, I would use one of the 'data radios' available, which operate at a set frequency and mode, and have no display. (Unless of course, this other radio may be used manually in an emergency.)

Update: 2004/09/01
Last week, I noticed that every time my radio transmitted a packet, my FM sterio would fuzz. Worse, when I was listening to my CD player via one of the fake-tape doohickeys, it would Fuzz too. Then I noticed that while working in my garage, the FM sterio on my workshelf woulf fuzz. It seems the HTX-252's noise gets worse with age. I've had to turn off my transmitter, since I'm certainly not licensed to transmit into the FM broadcast band!

Update: 2004/09/27
I need to take back the above. The HTX-252 did not die. It was another casualty of the Death Valley washboard roads. The antenna wire nut got vibrated right off! Yesterday I screwed it back on, and all is well.

Update: 2004/11/17
William Avery e-mailed me with lots of informational links about the HTX-252:
The HTX-252 Service Manual which you can order from Radio Shack is:
Cat# 12276358, MANUAL SERVICE 19-1127 ; NP Part # MS1901127

This was listed on a partial parts list at:
from the official HTX-252 web-support site:
"Product: Amateur Radios
Question: How do I get a schematic and/or pinouts for my radio?
Answer: For schematics or pinout information, you would need to reference the Service Manual for the radio. Service manuals can be ordered by contacting either your local RadioShack store or by calling RadioShack.com at 800-241-8742"

The second quote is from a more up-to-date-site at:

Additional Support is listed on:

The HTX-252 Microphone Pinout is at:

The commercial unit called "RASCAL GLX" with a HTX-252 cable, which is for TNC/sound card use, is available at:

There is a site which sells Ham Manuals, including the HTX-252 (#19, for $16.00), at:
A free source of manuals is at:
http://www.hamanuals.com/miscmans.html , where the Owners Manual is at:
And the 50 page Service Manual (for Albrecht AE-540, which equals R.S. HTX-252):

Finally, the Microphone Manual is available separately from the Service Manual at:

There is a site which has a few related comments on HTX-252 modifications:

There is a neat description of one man's efforts to make an interface, described in a newsletter called Smoke Signals [ http://www.qsl.net/k3smd/smok1001.pdf ], and the last article by Don Munson, W4GFQ, called "HOMEBREWING, A LOST ART?", has a link to another effort by Stephen A. Smith, WA8MLF, at: http://members.aol.com/wa8lmf/ham/tonekeyer.htm with 3 designs documented there.

KD7PLU / trooper (2003-01-23): Yes- KPC3+ and HTX-252 was a great idea on paper. But the HTX-252 radios are not worth the effort. I was dumb enough to buy six of them on close-out for 99 bucks each. Two of them worked, three didn't. Alinco just came out with an inexpensive TNC-integrated radio, and the Vertex VX-150 with a 4-pin to standard Spkr/Mic adapter makes an ideal APRS radio if you're usually close to some kind of Wide node.

Rich/N2ROW (2004-01-14): Can you email me the mic wiring for the htx-252. Thanks velezrd@fdny.nyc.gov

k1lwi (2004-10-12): looking for a schematic diagram htx 252 can you help 73 wen k1lwi e mail k1lwi@aol.com

Erik (2004-10-13): Sorry, I don't know where you could get the full diagram for the htx 252.

anonymous (2005-09-12): My HTX-252 transmitts OK, but the receiving side seems to have a problem with the squelch opening. I get NO audio on regular signals, but a strong repeater in the local area open the squelch OK. Is there a control someplace inside the unit to rectify this condition?

Erik (2005-09-13): W6ORS, you're saying your squelch is not opening enough? Maybe it's just the potentiometer, which can be replaced, if you're careful. Are you using this radio for APRS? If so, you can probably just short the squelch open, and let the TNC pick out the signals itself.

N0YXV (2007-10-17): APRS no longer uses 'WIDE' for digi's. To reduce QRM APRS now uses WIDEn-N in place of WIDE. To use WIDEn-N effectively you would replace the N's with a number between 1 and 3. (i.e. WIDE3-3). In place of RELAY APRS now uses WIDE1-1. Mobile paths are usually set with two settings WIDE1-1, WIDE2-1. I live in a sparcly populated APRS area and even a path as small as WIDE2-2 easily covers 200-300 miles.

Brian KJ4AGA (2008-11-21): Do you know where I could find another microphone for the htx252?

Brian South

KB1GVR (2008-12-21): I've had my HTX-252 on APRS/PKT wired to an MFJ-1274 since 2001 and got the schematics from my local RS dealer.

BJ452 (2009-02-25): IS anyone still interested in the 252 - I bought one for $40, soldered a spare wire into the microphone plug to allow both Mic and TNC use. The Mic line has 5v so I let the PTT line form TNC ground it through a 2.2K resistor wiht a .02uf cap to decouple - works a treat for may packet stuff.

Don Keyes (2009-02-25): Will some please help!!! I have the PK-232 and HTX-252 radio to use for packet. Is there a mod or wiring for this? Don N7OTO email: siriusconnection@q.com

Joe N4SP (2010-04-12): Approximatly what levl should the audio out be set to to run into the APRS TNC?

Erik (2010-04-13): Joe,
I've found that the KPC isn't too picky. Some experimentation may be worthwhile, but I found "somewhere in the middle" works well. If you live in a high traffic area, just watch the "Rx" light on the KPC as you manipulate the volume knob on your radio.

Bill (2011-01-23): i have a problem with my htx232 when i add offsets to my radio
then i transmit an e shows after i try to save in memory

Paul (2012-11-15): I have an HTX-252 and a KPC-9612+. I'm trying connect the two, but I don't have any resistors or capacitors on hand. Will it brick the radio if I wire it up like you have it and don't use any resistors or capacitors?

Thanks, 73

Erik (2012-11-15): Paul, I don't know if building this circuit without the capacitor and resistor would fry your radio or TNC, but I'm pretty sure the circuit would not work, so it's probably not worth trying. I suggest going down to your local Radio Shack to get the proper components.

See also: APRS


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