[Monday run: 3.4 miles]
In the bin I brought home from the Jackson hamfest was this small Ten-Tec model 210 power supply. It appears in the same style as the Argonaut 509 transceiver.
I plugged it in and measured the voltage, very high at 17+ volts. That’s not right. It is supposed to be regulated at 13 volts +/- 0.5 volts.
I found the schematic online and took the cover off. The first issue was a strange fuse setup. The original soldered-in fuse was piggy-backed with another fuse; the original was burnt out. So I sorted that out with a new soldered-in fuse. This thing really shouldn’t be blowing a fuse, the manual says it is “short circuit proof”. I wonder how that fuse got zapped?
Then I made resistance and voltage measurements in the circuit and tried to figure out where the voltage regulation was going wrong. The resistors all looked good.
That little zener diode (D6) is the brains of the regulation circuit. It is supposed to conduct from right to left when the voltage on the right side is 13 volts higher than on the left side. Anything above 13 on the output will start to apply voltage to the base of transistor Q3 turning it on just a bit which then nudges Q2 and Q1 more toward “off” to squeeze the line and not let as much current through which brings the voltage down.
I finally convinced myself that +9 on the base of Q3 should have nudged Q2 all the way to off long before it got that high. D6 may be broken but Q3 definitely isn’t doing the job.
I dug through my pile of parts and found a similar transistor, a Tip-31. I took out Q3 and measured it with the little all-device-tester thingy I bought off Ebay awhile ago. While the tester successfully figured out that the Tip-31 was a transistor, it couldn’t do that with Q3. The best it could do is say there was a diode from the base to the collector. Bad. I put in the Tip-31.
Now I have 13.1 volts on the output! Yay! I guess the next thing to do is measure the ripple and the regulation at different loads. I may do that.
My guess is that some surge took out both the power line fuse and the emitter of Q3 which is attached directly to ground, maybe a nearby lightning strike.