I’ve taken my HF radio, a Heathkit SB-104A, offline for now. I had noticed some 60Hz line hum through the speaker when transmitting, however I didn’t really think much of it. I just assumed the audio pre-amp was picking up some hum which was getting through. It wasn’t until I got an audio report from a QSO who said he could barely understand me over the 60Hz hum that I got concerned. If the audio circuit was picking up a little hum, I could live with that (at least until I got around to sorting it out). But to be told I could be barely understood over it, that was more serious.
The SB-104 uses several voltages internally. It is supplied with 13.8V from the power supply, which is then regulated down to 11V for most things, and 5V for some of the digital stuff. However, I remembered that the final amplifier used the 13.8V supply voltage straight – and if, under load, the power supply was letting some bad ripple in, then this would essentially modulate the output with that 60Hz ripple. Sure enough, putting my scope on the power supply output when it was under load by the radio showed a huge AC component. Ouch!!
The power supply I’m using is the actual Heathkit HP-1144. The original version without the overvoltage crowbar circuit. I tested it when I bought it and it produced a nice ripple free 13.something volts that the adjustment trimpot inside easily tweaked to 13.8 exactly. I didn’t however, test it under load. There’s a newby mistake right there. I don’t actually have the resistors to test power supplies under load, which is an oversight I am now rectifying. I’ve now ordered a set of five 3.3Ω 50W resistors (from AliExpress) that I can wire in parallel for load testing at various draws. Since most amateur gear runs at the 12-13.8 volt range, this will work well in the future. In the meantime, I’ve been having a hard time determining where the problem actually lies.
It’s almost a sure bet the problem is in the filter capacitors. The HP-1144 power supply uses 2 x 10,000µF capacitors. After desoldering them from the circuit, I performed the very precise “snap” test. Which is, simply, charge it up then short it with a test lead and see if it sparks. Ok, so not the most scientific of tests, but right off I see that they both spark consistently. Wanting to do further tests, I take one out entirely.
Charging it up and discharging through a resistor to measure the time constant suggested it still has about 13,500µF of capacitance. There are no markings that tell me what the tolerance is, but I do know that +80/-20% wasn’t uncommon for large capacitors back then, so this is high but plausible. The same capacitor measures 12,500µF on my trusty LC200A, which ties in closely enough that it should mean the capacitor is good. So now I’m puzzled. It’s passing every test except my gut test, which says that since there’s more AC than DC on my radio’s power rail but only under load that it’s probably the capacitors. Physical examination is inconclusive. There is a slight bulge on the top, which could mean something, but I’ve seen older capacitors look worse and still work fine. Heck, that could be the way these came from the factory.
So… two tests tell me that it’s good. Both of these tests are relying on an RC circuit – the high capacitance mode of the LC200A is almost assuredly an LC circuit. And my “measure the time constant” method certainly is. What if dried out old electrolyte inside the capacitor is causing its internal resistance to be quite large? That would cause it to charge and discharge more slowly, throwing off the measurements. It could have much less capacitance, but still seem like it has more because of the added resistance. Measuring the current during a charging cycle should tell me if that’s the case. A little variable wall wart power supply I have gives 17.2V at full output. I also happen to have some 4300Ω resistors, which a my calculator tells me should give an initial charging current of exactly 4mA. How convenient. And when I measure it, I get 3.9mA.
I may have to give up on the idea that it’s my electrolytics that are bad. I’ve ordered two new ones anyway, a pair of modern 22,000µF capacitors that are half the physical size. They won’t be bad to have around, even if I don’t need them for this. But something is not adding up. More investigation is needed. Meanwhile, VA1DER is off the air.