Oct 14, 2019

FT-7900R ZUM Repeater Build

Here is a project involving a ZUM Radio duplex MMDVM and a Raspberry Pi 3 B+ combined with my trusty old Yaesu FT-7900 and FT-7800 radios, and a couple of mini-DIN cables. Together, a trial version of small digital all-mode repeater capable of full duplex D*Star, DMR, NXDN, P25, POCSAG, and Yaesu System Fusion should be the result. If this works out as expected, I will likely be seeking out an old GE Master II, or any other robust analog repeater, to handle whatever mode is selected.

Simplex Mode

So it's been a little while, and I finally figured it all out. Part of the problem I found was the Yaesu CT-39A mini-DIN wire color was incorrect for the pin location. So lets forget about colors and talk straight up pin number to pin number. Conduct a simple ohms test on your mini-DIN cable pins to determine what color goes to what pin (my Electronics-101 oversight...)

As stated on page 10 of the Yaesu FT-7900R user manual, the pins are identified by both number and function. Using this, combined with the pin information from the ZUM Radio, I found the following connections to work properly using one FT-7900R as a simplex digital mode repeater.

Note: Two wires on the CT-39A mini-DIN are not used: Yellow (RX1200), and second Black (shield ground).

The pins - looking at the back of the FT-7900R - are identified in the following locations. The locator tab will be at the bottom center.

Pin 1: top left - PKD (DATA IN)
Pin 2: top right - GND
Pin 3: middle left - PTT
Pin 4: middle right - RX9600
Pin 5: bottom left - RX1200
Pin 6: bottom right - PKS (SQL)

Keep in mind, this will transmit when anyone keys up on the WIRES-X, YSF, or FSC rooms you are connected to. Be sure to keep the power as low as possible for your needs, and share the air.

Now to get a repeater frequency allocation from the Wisconsin Association of Repeaters and get this working as a duplex repeater!

Cool RasPi

The Raspberry Pi ZUM repeater board gets a new case and a custom fan install to keep things nice and cool. This is a HiFiBerry box, which is tall enough to have a GPIO hat and a fan inside. I cut a 1.125" in the top and mounted a Pi fan inside, blowing directly down on the ZUM Radio hat and Raspberry Pi. Temps stay well under 40 degrees F.

Repeater Case

There it was, down in the basement. A lonely old Windows computer that has seen it's better days. Perfect! I gutted all the hardware out, saved everything that was still good, and prepared the case to be converted into my new ZUM repeater box.

I was able to rewire the existing power supply to reduce the number of wires clumped together for the previous occupant of this case. Now I have 17 amps of 12 volts, 18 amps of 5 volts, and 11 amps of 3.3 volts available, all regulated and fan-cooled. This works out perfect for all my needs.

Getting everything positioned in the case is pretty easy, so long as the fans have coordinated air movement over the radios and through the case.

I'll need to fabricate a new front face plate to incorporate the two radio faces, a Nextion touch screen, the voltage and temp monitor, and the power switches. The Raspberry can be controlled via SSH, but a re-program of the Nextion screen can provide the shutdown capabilities I am looking for. It should work well and look pretty nice when it's all done.

Propagation Estimation

I found a tool online to help estimate propagation from the repeater's site at Radio Mobile Online. The parameters I used for the estimate are a 6 db vertical antenna at 55 feet on 146 MHz. This, of course, only gives a general idea of coverage. The actual specs will be used as soon as I get the frequency allocation from the Wisconsin Association of Repeaters.

This map is at 5 watts output. Since the goal is to serve the surrounding communities, I would like to keep the repeater at 5 watts to prolong the life of the FT-7900R driving the repeater, but a real-world test today around the area had the 5 watts barely making 5 miles; at 6 miles there was almost no squelch break on the Yaesu FTM-400XDR in the car. I was always told to expect one mile per watt on the ground, so I'm not really surprised. My Diamond X300 antenna, at 55 feet, is basically on the ground, here in the woods.

For comparison, I can hit the WE9COM repeater in Plymouth from my QTH using my FT-70D and FT-2D on 5 watts. The RFinder app lists this repeater as 14.1 miles from my QTH. This is 2.82 miles per watt. Here is an example of antenna height making all the difference.

Below is a map I made with a 5 mile, 10 mile, and 15 mile radius. If one watt per mile on the ground holds true, this map should be better aligned with real-world expectations and performance.

Dressing Up

I was able to acquire a nice piece of 14ga steel for the front panel of the repeater case from a friend at local metal shop. I mapped out where the various parts will be located and look forward to getting into the shop to get it all cut and finished. Hey, if the repeater looks good and works good, I will me a happy ham. Is this metal floating over my desktop? Well anyway, we're on the road to success here, I think... Maybe I should call this project the KMS-7900ZUM repeater(?).

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