17 November 2017

Linear PSU failing with overvoltage

According to timestamps on components, I have owned this linear power supply for 30 years (rms K135). It powered my first CB station (Midland Alan 48) and who remembers how many other devices. Lately it has been powering a 12V LED strip in the shack-lab.

One of these days I wanted to take a picture of a multimeter (will be feature on the blog, don't worry) and I needed both something to measure and less light in the shack. So I unplugged the LED strip and wired the PSU to the multimeter.

To my great surprise I read 24V. What?! That would have burned the LED strip and everything else I had connected in the meantime. But, with the LED strip powered, it behaved as expected and outputted 12V or so. Of course I cross-checked the reading with several DVMs and they all agreeed.

A bad capacitor? I opened up the PSU and I was greeted by a large amount of dust. And two 78S12 in parallel, as I remembered. Hard to remove the dust, but once components were in the clear, I still had 24V out without a proper load.

Capacitors looked OK. No leaks, no deformation. Unwillingly I reached for the solder side of the PCB and disconnected one 78S12 at a time (just the output pin, lazy me). One regulated at 12.5V, the second at 24V. Well, it didn't regulate at all then!

I temporarily replaced the bad one with a 7812 and the output is still 12.8V.

So, watch out for this simple technology too! Until today I would test the Amps rating of a PSU. Now I will check the open circuit voltage!

07 November 2017

My first HF RTX, in 2017

Got home a bit early tonight from work and didn't want to stare at a screen, so I finally dug out my first HF transceiver for a quick check-up.

It is an Icom IC-728, 100W on HF bands, AM/CW/SSB, triple conversion receiver. I think I bought it in 1993 after the high-school graduation. The price was a bit less than 1'000'000 lire (that's about 820€ in 2017's value).

First I did a visual inspection of the circuits for leaking capacitors: no signs. Then I grabbed the DC power cord from the IC706 and powered it up. It was exactly as I remembered it when it was last used .... 20 years ago or so. Whew!

I reached for the coax that enters my little shack and inserted the hot pole into the antenna socket. At last HF noise filled my room!! Why just the hot end? Because there's no antenna on the other end! It's a coax left over from my balcony experiments 9-10 years ago (seek blog archives if you want). But it does provide few metres of pseudo-antenna. Just don't press the PTT.

I quickly tuned around the bands. I found CW stations on 40m (and I could even make sense out of it!), some AM broadcastings too.

I caught myself few times looking for a "MENU" button, but this radio has no menu: each key has one function (some have two). That's it. All functions are one press away.

Then I wanted to reduce the incoming noise from the open line antenna and ... no DSP, of course! All you get is a Noise Blanker and Pass-Band Tuning: neither of them is effective against modern QRM. Well, I "retuned" my ears and it was fine.

I love the smooth effect of the large tuning knob. I could keep tuning for hours.

I will do my best to put it on the air even if propagation doesn't seem to be helping.

(I didn't take a picture. I will next time I take it out of the box ... hopefully soon!)