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 https://www.n4ats.com/PDF/QST_FL7000_Review.pdf
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.