Local new radio operator asked me to fix his first sideband radio. A Uniden PC 244, which is similar to the Cobra 146 GTL which he bought from a flea market for $35. He was told that it worked. It did, in that it turned on, received, and could transmit. However poorly. In fact, all modes had a nasty oscillation heard when TXing.
So I throw it on the bench and open it up and then test it for vital signs. Driver/final bias were waaaay off scale and couldn't be adjusted to specs. The loop osc freq's varied greatly from ch 1-40. At first I suspected that the AN 512/balanced modulator IC had failed, so it was replaced. No dice. So I then suspected that the mic amp was oscillating due to a bad cap. SO, I started checking caps . . .
Hmmmm; this cap is showing half its rated value. And this one is one third its value. Wow; this next one shows up as a diode. The next shows up as two opposing diodes! So I replaced each one that showed up bad - which was all the ones I'd pulled and checked so far. Uh oh; how far is this going to go? What I found was unlike any radio I've ever recapped.
With the exception of just three caps out of thirty-four, they were ALL bad. Regardless of their rated voltages too. Most often I have seen that the 10v caps are the first, best suspects for failure. But not this time; regardless of voltage they were testing bad. Surprised that it worked at all, given what I'd found. Even more surprised that this condition didn't take out other parts too.
The nasty oscillation went away when I found and replaced a cap that was so dried out that it read 'open'. Two hours later it was finished. Then the entire radio was aligned to specs.
Have a similar experience?
