Sunday, November 19, 2023

Yaesu FT-767GX Repair No Audio

I was contacted by a fellow ham who wanted me to look at a couple of broken transceivers. I offered to repair them for him, but he gave me a good deal so I bought them :)

The first radio is a Yaesu FT-767GX. If you've had experience with these radios, then you know how difficult they are to repair because of many undocumented changes between the service manual and the radio. Yaesu was known for that.

The FT-767GX was one of the flagship radios from Yaesu-Musen which debuted in 1986 as an all-mode, all-band transceiver. It was a first to feature a built-in antenna tuner and modules for 6 meters, 2 meters, and 70 centimeters. It was a shack in the box!

Here is a QST product review from September 1987 courtesy of N4ATS

This specimen came to me with the following symptoms. I will break these out into a series of posts.

  • No audio output from internal speaker, external speaker jack, or headphone jack
  • Low/No Receive Indication on signal generator

The block diagram for this radio show that there are two audio amplifier transistors and a transistor for the squelch circuit in the chain. These components are on the IF Unit Board.

Yaesu provided voltages for these transistors and we found that there was not enough voltage to drive the MB3713 at Q1037. Measurements were 0.68v, 1.31v, 1.31v, 0v, 0v, 0v, 0.6, and 0.6v. 

I checked the capacitors for shorts to ground and bad ESR at the input voltage and output stage of the transistor (Pin 2 and Pin 1). The capacitors in the green boxes were checked and passed. The capacitor C290 (red box) was replaced due to a high ESR reading on the capacitor checker (see image below). 

I also noticed that Yaesu omitted L16 and replaced the component with a NPN transistor. Voltage at emitter was 1.31v and 13.3v at the collector. I removed the transistor and found that it was testing as a diode. This transistor was replaced and we now have proper bias voltage at pins 1 and 2 as well as great, loud audio!

Here is a redraw of the L16 circuit substitute on the IF unit board. Notice the positive lead of C290 was moved to the base of the 2SD667C.


Wednesday, October 4, 2023

Power Supply Repair

Yes, I repair power supplies too. This is a Tripp Lite 10 amp 13.8 Volt Supply. The design is similar to an Astron using the infamous LM723 Regulator. Feel free to contact me if you need your equipment repaired.



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


Another great resource is 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







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.



Saturday, April 29, 2023

Bird RF Load and Wattmeter 25KW - RCA Collectable

here is a fun little watt meter that I used to own. 

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.