So, as I wrote in my last post, I had taken my Heathkit SB-104A radio offline due to suspected power supply problems. Under load it appeared to be sending AC directly through to the radio. It turns out this wasn’t the case, that a couple of issues are at at work here:
- The SB-104(A) is prone to sending RF and the modulated audio signal back up the 13.8V rail. This “talkback”, as it’s known, is a well known SB-104A issue and is addressed in several QST articles.
- My “shack” has terrible, terrible, terrible AC noise and it’s obviously leaking into my radio. Which was then getting onto the 13.8V rail through the same mechanism the talkback audio was.
So, yes – it’s not my power supply after all. What’s particularly embarrassing is how long it took me to figure this out. I didn’t have anything handy on my bench with which to test the power supply under load (except for the radio itself which I wasn’t going to risk), so I pulled the filter capacitors out to test figuring they had to be the culprits. When it looked like they were good, puzzled, I began preparations to test every part of my power supply if need be to determine where the trouble lay. I was even writing a blog post about it – I was going to title it “combat troubleshooting”. My workbench, while increasingly well kitted, is woefully short of those odds and ends parts most old-time hams have which let them whip together pretty much any kind of test circuit they need at a moment’s notice. So I was going to do a bit of “combat troubleshooting” and (as Matt Damon said in a great movie recently) “science the shit” out of my power supply to work out where the problem was with a minimum of equipment.
That’s when it occurred to me, it should actually be quite easy to test my power supply under load – I don’t need load resistors, a few automobile headlight bulbs in parallel should be just the trick. And wouldn’t you know it, when I hooked them up to be the load, what did I see but a beautiful, stable, exactly 13.8 volt DC line without even a whisper of ripple.
Face, meet egg.