Tuesday, September 5, 2023

Converting a Kenwood TM-2550A into a 2 Meter Amplifier

How to turn an old radio into an RF power amp for less than $20


I received a partially working Kenwood TM-2550A mobile rig as part of an estate sale last year. The transceiver did not come with a mic, mount, power cable, or accessories. I was able to get the unit powered up and transmitting, but the receiver was blown on this unit and the microcontroller was damaged from over-voltage. The cost to repair and restore this radio far outweighed the cost to convert it into something more useful. So the best option was to turn this unit into an amplifier to be used with HT's and other low power devices for analog and digital modes such as FM, WinLink, APRS, Packet, DSTAR, DMR, and Fusion.

I already had a great starting point as the RF module was in good shape. The one I scavenged from the Kenwood is a 43 Watt module which appeared to output full power. I reused the heatsink and original mounting holes for the module and the SO-239 connector from the Kenwood. The next step was to either inject a low powered signal in the final stage of the PA amp circuit on the existing radio with a relay, or simply install a new PCB with the T/R circuitry built in. I chose to go with the PCB and use the rest of the radio for spare parts.


This PCB is compatible with RF power modules such as the Toshiba RA/SA series and Mitsubishi M series to provide an amplification interface for low power radios under 5 watts. There are two LEDs onboard which indicate TX and Power In. The low power TX signal is detected at the first relay (J1) through Q2? via a current sensing circuit which enables power to the RF module pin 2, then to the second relay (J2) with the amplified signal. The relays are switched off in RX mode to pass the receive signal back to the input (radio). There is a bias voltage adjustment pot for pin 2 of the power module at the bottom of the board to set voltage for gain of the final amplifier.


 The RF Power module I used from the Kenwood is a two transistor, class C design which has filtering onboard for 144-148 MHz to reduce harmonic distortion. The module is broad-banded to support a frequency range of 140-152 MHz with a typical input of 400 milliwatts to produce up to 45 watts at the output. As you can see in the photo below, the module is a Mitsubishi M57726.


Here are the results on a nearly discharged FT-70. I was able to pull over 30 watts into a dummy load. I noticed the bias pot did not have enough adjustment to supply 12.5 volts to pin 2, so I had to remove the series resistor R6 to increase voltage to get more power output. The next step is to reuse sheet metal and screws from the Kenwood to make a shield and print a 3D case for the front. I will also figure out cable management and upgrade the wiring for a more durable and permanent solution.






Saturday, September 2, 2023

HF Signal Identification and Where to Decode Them



Have you ever tuned around or performed a band scan on your HF receiver or transceiver and wondered what those noises where? Have you ever wanted to expand your listening experience by trying different modes other than AM or SSB?

A couple of great resources are available to you free of charge.

The Signal Identification Wiki is a great place to learn about different modes used by amateur radio operators, broadcasting stations, military, aviation, marine, and commercial or private entities. The wiki is maintained by the community and and provides detailed information to those who want see and hear what those signals sound like. It also describes what the signal is, where to find it, and software used to transmit (encode) or receive (decode) it.

Check it out at https://www.sigidwiki.com/wiki/Signal_Identification_Guide


Another great resource is BandPlans.com. This website contains an extensive database of recommended places to find analog and digital signals for the HF through 6 Meter Spectrum used by amateur radio operators. It lists frequencies, mode information, comments, and websites sorted by specific band.

Check it out at https://www.bandplans.com/







Wednesday, August 30, 2023

In The Shop - August 2023 Update

 The summer slowed down for about two months and is starting to ramp up again here in the shop. I've been spending time working on a FTM-10R, FT-90R, IC-751A, several DX-390s, DX-398, IC-706MK2G, and some other non-radio items.

I have a DX-390 in the shop right now as well a TS-130S and FT-101B. I will need to slow down for a bit as I am scheduling surgery in the last week of September and will be unable to perform any repairs, upgrades, or modifications. Be sure to get your radios in before the middle of September!



Tuesday, March 28, 2023

Yaesu FTM-10R 50 Watt Dual Band Radio

 Gone, but not forgotten. This very unique radio from Yaesu was ahead of its time with these features. I hope the new owner enjoys it as much as I did.

Sunday, March 19, 2023

What test equipment should you get as a new ham?

Here is a question sent to me recently and wanted to share some  ideas with others who are new to the hobby.

"Relatively new ham here and note sure what I might even need. Was thinking a meter at least but wondering what you think would be good for a new ham?"


My first observation is that these types of questions can get many responses from different people. My goal is to make an attempt at pointing to the north star and giving them some sort of logical direction. 

Here is my response.

It really depends on what you want to do in the hobby. Many types of test equipment can be used to diagnose issues and make adjustments in your shack for peak performance.

At minimum, a ham should have
  • external watt meter
  • multimeter
  • SWR bridge
  • reliable power supply
  • dummy load
The list of analytical equipment can go up from there including
  • audio analyzer
  • service monitor
  • VNA or antenna analyzer
  • oscilloscope
  • tone (audio) generator
  • waveform function generator
  • signal generator


All of these tools can be used in conjunction to either test, analyze, or repair ham shack equipment such as radios, antennas, coax, etc. 

One of the reasons we use this equipment is to verify our radios and antennas are working well even if the most advanced radios have some of these features built in. Those built in features can either fail or provide faulty measurements. And we all know how expensive these radios can get - We use this equipment to protect my investment among other things.


What would you recommend? Feel free to share them in the comments.






Sunday, March 12, 2023

Icom IC-2300H 2M High Power Transceiver

Another excellent sounding and easy to use 2M mobile rig. I hope the owner enjoys it as much as I did.


This radio is for sale. Please see my QRZ listing. Thank you.

Wednesday, December 14, 2022

Updated Station List & Timetable for Shortwave Broadcasters 2022 Version A and B

Here is the 2022 version of the HFCC broadcasters list for shortwave. It was last updated in late 2022 and lists the station details such as frequency, time, and transmitting location. This should help you find something on the dial to enjoy!




Happy DXing!