HIDARG - PHOTOS KF7MAX's "Hack a Spectra" Project

A few years ago, HiDARG accepted a donation of some radios from a public agency in Washington State. We currently have about 10 UHF units and 10 VHF units. Each unit is able to pump out 110 watts of power. These were headed for the re-cycle bin unless some kind of use could be found for them. I started to do some research on the radios and found out that the VHF units are able to support the upper VHF ham band. Some issues we had were getting the software and cables to program them. Plus once programmed the control heads would only allow about 9 channels or so to be accessed. Very limited to say the least. Then an idea occurred to me. What if I could modify one and turn it into a basic power amp for an HT, or other mobile rig. I began to do some research and track down just how I could accomplish this. I hope to have more details about what I found out. Suffice to say, with the information obtained, I proceeded to come up with a basic design for an interface to drive the 110watt power amp with my HT. In the weeks to come I will document some of my progress. The goal is to determine if this is a viable method of putting these surplus radios back to good use. The initial prototype interface is done, and testing will commence once the circuit boards are obtained and populated with the components. Ideally it will be nice to be able to have two or more power settings if possible say 50w and 110w. From the Motorola specs I believe the board was designed to do this. That remains to be seen. My initial testing showed a very clean signal. However once I have the prototype board setup and working, I will need to put it on a spectrum analyzer to see just how clean things really are. Given Motorola's tight specifications, I don't' see any issues here unless my HT is out of whack.

This is a nice 7lbs or so of an aluminum heat sink. Just after I gutted the whole unit. What is nice about these units is how modular everything is. It can easily be reversed if need be. I decided to remove the power amp portion in order to reseat the power transistors and put fresh heat sink goop on them. In this picture you can just notice the red wire with its connecting lug peeking through from underneath. This supplies the unit with the 13.5vdc in normal operation.

Sorry poor picture of the power amp component. Besides the 12 pin ribbon cable, the only other connections are the RX, TX, and antenna cables. On the ribbon cable, there are only a few pins that are primarily needed to actually make this thing work like I expect it to.

The power amp, reseated and back in position after it's cleaning and re-applying more heat sink goop.Notice the white coax coming from the top of the board. This is the TX input to the drive circuits on the board. Note the small connector on its end. In normal operation this plugs into one of the main circuit cards in the radio.

Part of my test setup with the unit. Referring to the schematic for the unit I was able to ID the appropriate pins that would be required. Using a cable from my junk box that had the correct connector, I was able to plug right into the board. I also removed the vhf connector on the board and put a SO239 in place. It connects inside the small enclosed area that is normally sheilded. This is where the final filters are and the circuit with the pin diodes to block any TX RF from being fed back into the units pre-amp. Which for my needs will not be utilized, though I can see maybe using that at some future point.

The basic test setup. I wanted to be sure that this amp in fact could be driven to 110w and actually be used. So I cobbled together a setup with a relay and DB pad, so that I could drive it with 1watt from my HT. Notice a switch to the left, this allowed me to toggle the relay so that I could feed the HT antenna into the amp during transmit and toggle the antenna connection back to the HT during receive.

Close up view of my manual switch setup. I did some testing with several values on my DB pad to insure that I did not overdrive the amp but still kick the amp to 110w. After some basic adjustments, I was able to determine that with 1watt in, I could drive the amp properly. In a review of the schematic it was easy to determine that things were not linear so to speak. 1/2watt in would not drive the amp to say 55watts, at least not with this test setup. It was not until I had taken everything down, that it occurred to me that I had never checked to see if it would do SSB. All my test to date have been with FM. I will wait until the prototype board is done and in place to conduct those test, research did not discover any SSB capability with the amp and due to many gaps in my knowledge about these types of circuits, I am not so sure it would support it.

This is the connector that the TX input on the power unit plugs into. I decided that if I was going to build an interface that this would come in very handy. So time to heat up the solder iron and remove it. Easier said than done, this was a chore and if your not experienced in doing this sort of thing, best to have some one who is tackle it.

The connector removed and put in place on a printout of one of the earlier prototype board layouts. I found a simple RF detection circuit, thanks to of some obscure site on the Internet. Plus I have added the required voltages etc that the board needs to operate with. I did make some changes to the basic RF detection design to better suit the needs of the project. This should be ideal for kicking the amp into high gear when the HT is in TX mode.

This is a printout of the current placement of components on the prototype board. This is checking basic fit and placement. The current design here is 1watt in = 110watts out. I would appreciate any comments, questions, suggestions or corrections in my thinking here. You should be able to note J3 on the left of the printout. This is where the connector mentioned above will go, and the coax to the left of it will plug in here.