The famous sign of death for many Kenwood transmitters. Switching frequencies with the tuning knob, results in a short display of desired frequency and then ---.--- in the display. This is a cry from deep within the radio that all is not well in the PLL circuit.
The radio had been sent in for repair to a well known German radio shop and came back with the same fault. They were unable to repair it and so it landed on my desk to see if I could find anything.
After some initial tests at test points around the board, I could confirm that the PLL was synching and then drifting off lock. So the first thing was to pull the PLL board and take a closer look.
The board itself looked fine from the top and there were no signs of anyone having worked on it. So far so good.
The board was brought over to my solder station to get a closer look and that's when I noticed that some of the caps were going bad and the board had dry solder joints all over it.
I decided to do a re-cap and reflow the solder joints on the bottom of the board. Considering this radio was produced from 1988-199x, it could be anywhere between 23 and 33 years old, so depending on how good quality the caps were back in the day, it is not hard to imagine that they need replacing.
I like to measure caps for leakage using a tester that I built some time ago.
It is a design thought up by "
Mr. Carlson" and the circuit diagrams are on his
Patreon channel. If you have never watched a few of his videos, stop now - head over there and really learn from a Pro. The device will test different kinds of caps and over an array of LEDs it indicates how healthy the cap is or how much it is leaking.
Once the board was back in the radio, things looked much better. The display was back and stayed on frequency.
However, this radio has a main set and a sub set. The sub set is on the top right in the picture. Separate volume knobs allow you to listen to both radios at the same time or to flip back and forth. In the case of this radio the sub set was not producing any sound. So the circuit diagrams were studied and with my signal tracer I could locate the area where we were losing signal.
The issue was with Q14. There is a little 4.7K transistor on the emitter leg that ties it to ground. This transistor was located on the underside of the main board.
As you can see if you look closely, it had become detached from the trace and was sitting up 45 degrees in the air. You can click on the picture to make it larger.
It was tacked back in and sound was once again available on the sub unit too. A closer inspection of this board, showed a lot of badly placed components and it must have come from the factory that way. I am amazed it passed quality control!
If looked after, these radios will last a very long time. Another happy old man.
