100 Watts on 2304 with a Thomcast MDS Transmitter
Ron Marosko, Sr. (K5LLL)


This is one of several articles Ron has done on 2304 MHz equipment conversions. He describes his initial intrepedation at attacking the problem of commercial surplus equipment and how he went about the process.


Introduction:

I was lucky enough to secure a couple of the 50-watt MDS transmitters that Greg Boren, N5SKE was offering before Hamcom. Following is what I know/learned about them.

Before you highly technical microwave engineers jump all over me, remember that I work in the oil industry and this is a hobby for me. I am still learning!


Internals

The MDS transmitters labeled 10-50 watt are actually a goldmine of parts for the microwave enthusiast (aren’t we all?) They have also been found labeled ‘Comcast’ with the same essential features. Generally, they are a complete Video transmitter, with synthesizer, mixer, filters, transmit amplifiers and ending with a pair of final amplifiers, each capable of about +47 dBm. These are combined and filtered to provide the linear output of 50 watts as listed. I have seen +50 dBm at slight compression. Some versions also included an Aural final at about 10 watts.

Power supplies are internal, including a 10.8V, 40+ A switcher for the final and pre final driver as well as other positive and negative supplies for the logic and other circuits.

One of the transmitters I secured had a 10 MHz oscillator internal to the unit, the other used external 10 MHz source. Look on the back panel, just above the power receptacle and switch, the F connector lists either ’10 MHz IN’ or ’10 MHz OUT’ that should give you a clue. I checked the oscillator against my 10 MHz GPS locked reference and it was well within 1 Hz.


Capabilities

The transmitters come in different frequency ranges. Check the labels on the final amplifiers; there should be a 2Gx in the part number. 2G6 indicates it is tuned for 2.6 GHz; 2G3 would be 2.3 GHz, etc. All are useful at 2304 but may require snowflake type tuning for maximum output.

The final amplifiers have 13 to 15 dB gain using six FLL-120 type GaAs Fets in the output stage, driven by two of them. A pre-final driver provides the drive and splits it into two signals for the two amplifiers. The mixer/amplifier box mixes in the video signal and amplifying to the required level. A tunable band pass filter is included.

The synthesizers in my units were labeled 2G6. They are fixed (dip switch) programmed units with 10 MHz input. Step size is 100 KHz and lock range is about 2350 to 2700 MHz. There are 4 outputs on the synthesizer, one of which is connected to a front panel SMA connector and one to the mixer. The bottom of the synthesizer (mounting bracket side) must be removed to access the dipswitches. A green LED on the top indicates lock. I have not pursued lowering the frequency, but it uses a standard MC145152 synthesizer IC that will go lower. The VCO may be the limiting factor.


Availability

These units are starting to show up on the surplus market. I saw one at the Eastern conference flea market in CT last year and even some in Australia. Some were labeled Comcast, others Thomcast, but they were all very similar. Just the amplifiers would make a nice power chain for a low power DEMI 2304 transverter, though some tuning may be necessary, depending on the original frequency range.

If one wanted to use them for fast scan TV on 2418 MHz, they should be very easy to convert. No comments on what that might do to 802.11b/g WIFI in your area, but since we are licensed users of the band, that is not our problem.


Front Panel


Internal Top View

Pictures courtesy of Greg Boren, N5SKE.

If anyone has comments or questions, I can be contacted by e-mail at K5LLL@hughes.net and will be happy to share information on this unit.


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