PTT Modification for Hamtronics Synthesized Transmitters

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Back to Home PTT Modification for
Hamtronics Synthesized
Transmitters
By Robert W. Meister WA1MIK

Background:

Hamtronics made several transmitters. The earlier ones were crystal-controlled; the later ones were synthesized. I bought a T304 UHF transmitter kit, the matching R304 UHF receiver kit, and a pair of TD-5 CTCSS encode/decode kits in 2002 to make a low-power UHF translator. This would receive a signal around 440 MHz and retransmit that signal around 449 MHz, so I could access a distant repeater while walking around my house with a portable.

I like active-low signals. They're easy to test and if implemented with open-collector transistors, when power fails you don't get any circuit activation. I know I configured the receiver and CTCSS decoder for an active-low COS signal when a properly encoded signal was received, but I don't remember the details. I needed to configure the transmitter to use this same active-low PTT signal.

Suggested PTT Circuits:

The T304 expects constant 13.6VDC to keep the synthesizer and transmitter going all the time, by applying voltage to points E1 and E4. They recommend separating these points, maintaining constant power to the synthesizer through E4, but this means supplying 13.6VDC at about 0.6 amps of current to activate the transmitter through E1. I could have used a PNP transistor to "pull" E1 up to 13.6VDC but I didn't have a transistor capable of this much current (one of the TIP-series of transistors). I also could have used a small relay but I didn't want to deal with any mechanical contacts nor the current the relay coil might require.

Another method is to replace R24 with a small-signal PNP transistor such that grounding the base lead (through a 1k resistor) supplies 8VDC to the VCO circuitry. The Hamtronics FSK data transmitter enables the RF output this way, but disabling the VCO may require the synthesizer to take some time to achieve frequency lock.

Improved PTT Circuit:

I decided to supply power to both the RF stages and synthesizer all the time, but find a way to activate the low-level RF stage when I wanted the unit to transmit. I determined that the synthesizer IC "LOCK" output (U2 pin 2) goes high when the synthesizer is locked. This goes through U5c (pin 10) to supply voltage to the first RF amplifier. If the synthesizer unlocks, this line goes low, disabling the transmitter. I added a transistor to ground this "LOCK" input to U5c when I did not want the unit to transmit. The PTT input disables this transistor, which then lets the high "LOCK" signal activate the RF amplifier.

Implementation:

In the schematic below, the added parts are in the lower left corner: two 1k resistors and one 2N2222 or equivalent NPN small-signal transistor. The resistors going to 13.6V turn the transistor on, which pulls the "LOCK" input to U5c low, disabling the RF amplifier. Grounding the new PTT input turns the transistor off, which allows U5c to supply power to the RF amplifier. Click on the diagram for a larger image.

I just hung the parts at the input terminals on the transmitter box (a Hamtronics enclosure that I ordered with the rest of the stuff) and ran a wire to U5c. It's been working fine since I installed the system. The only issue, which would occur regardless of what PTT circuit you use, is that you can hear the synthesizer's signal on a nearby receiver (within 100 feet or so) all the time, but no RF actually leaves the transmitter's output connector.

This same circuit should work in any other Hamtronics synthesized transmitter, although the part designations may be different.

Contact Information:

The author can be contacted at: his-callsign [ at ] comcast [ dot ] net.

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This page originally posted on 24-Sep-2018

This web page, this web site, the information presented in and on its pages and in these modifications and conversions is © Copyrighted 1995 and (date of last update) by Kevin Custer W3KKC and multiple originating authors. All Rights Reserved, including that of paper and web publication elsewhere.