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Build an interface


The circuits depicted herein are available on the www in different variations. Note that each rig and computer are in a sense unique, so that there are different com ports, different mic/ptt connections or mods needed for your operation. You must have a 16 bit soundcard (SB-Pro) in your computer for this to work, and a CPU running at least 100 Mhz is recommended. This is made available to you at no charge, and any hints, mods etc. are from the author’s experience with the interface.


The first section is all that one requires to just receive only.... Enjoy

Reference Fig: 1
Parts needed for the receive part of the interface are:
1- 1:1 audio isolation transformer, or one may usea 1k to 8 ohm transformer in some cases.
Also a stereo audio cable to go to the soundcard.. use tip and ground.
And a (normally) mono audio cable from the receiver to the x-former

You will also probably want to have a small project box to put all the parts in. This part of the circuit will in itself, with the proper software, allow you to copy any of the above modes using your computer and rcvr/xcvr. Simply use the volume control on the receiver to adjust your audio level, or adjust the audio level via the soundcard settings. All modes use a very small amount of the available audio.


Reference Fig. 2
Parts needed for the transmit section of the interface:
1 ea. 1:1 Audio Isolation Transformer
1 ea. 3.3 uf Tantalum capacitor - Note polarity
1 ea. 50k ohm trimpot
You might also want:
a shielded STEREO audio cable, to go from the computer line out socket to the interface box,
a stereo socket for the "box".
Special note - on the box, you will only use the shield and tip sections of this socket. Do not use it as stereo! This is the easiest way to do this.
Also needed is a shielded audio cable,
and socket to suit, which will run from the "box" to your mic line on your rig, and it’s ground.

TIP: This application will be MUCH simpler on a normal 4 conductor mic connector, where there is usually one wire for ptt, one for ptt gnd, and mic and its gnd. Most rigs are different in this hookup. In short, you need stereo line from soundcard out to the box, and a mono line to your mic input …. Whatever plug or connection that you need to connect to your rig. When testing your unit in transmit, you will probably need to back the audio level down for the transmit audio. Do Not have your speech processor, or special effects turned on, and try to show next to no ALC when transmitting.

The Circuit used for PTT:

Figure 3

Reference Fig.
3 Parts needed for the PTT section of the interface:
1 ea. Reed Relay
1 ea. DB9 connector and hood
1 ea. 1N4148 diode

As shown, all you need are 2 lines from the com port. If you have an available DB9 connector on your computer, you will use RTS is Pin 7 and Ground is Pin 5, and two wires to your ptt line. On earlier interfaces, Optoisolators were used for keying, however on newer computers there is not enough voltage available on the serial lines to key the reed circuit. A reed relay which will operate between 5 and 12 volts will work nicely.

Addendum: AVOID COMMON GROUNDS… That is the main purpose to go through isolation transformers and reeds. Common grounds are a hazard to your equipment, and will breed "Ground Loops" which are horrible! There are other ways to switch ptt circuits, but reeds do the job well.Some modes tend to be touchy in tuning, until you get used to it, and in all cases, you need very little audio in, and also need much less audio to your mic, so as to not overdrive. Stay below ALC on HF, and avoid overdeviating on vhf. Software that I use is MMSSTV for SSTV, Hamscope for PSK31, RTTY, and CW, Hellschreiber by IZ8BLY. There are many programs out there, and easy to find one that suits your particular needs. There are many locations on the www where the mentioned files can be downloaded. If you need help finding them, drop me a note. Some locations change, so I am reluctant to add links to current download sites, as they could change shortly.

When tuning for a SSTV on HF… don’t tune to the louder sounds, as you will not copy. Start at 14.230 Mhz, and tune in voice signals till they sound normal. When they send SSTV, you will be on the proper freq. On 2 Meter FM, you will already be on the right freq (145.500). Using PSK on most software requires very little effort to tune the signals in, and is basically click on the line(s) on the waterfall, and you are there. The software does the tuning. The best softwares seem to be the FREE ones. Don’t add too much audio from the rig on any of these modes, and in all cases, Read the Friendly Manual for the software. These modes are a BLAST! As these modes are basically a "key down" operation when sending, back your drive down on HF rigs so you have 15 to no more than 25 watts output, which is more than ample, and on vhf rigs, run in a lower power (10 watts). If you push the rigs hard, you will run the finals out of it in a hurry. I have been working all corners of the world on about 20 watts into a longwire antenna on the HF bands on these modes. I hope this helps -

73, "Whitey" K8BE
Questions… not in a hurry?

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