Okay, supposing that you, our loyal reader, were qualified to solve the problems we found, or knew a qualified person to do it for you. (Refer back to the disclaimer at the beginning of the article.) What would that look like and how well would the repaired units perform? The dust is still settling on the workbench, but let’s take a look at the older supply first, in all its renovated glory.
What is this, HGTV for ants?
Those are some pretty hefty interior updates. The exterior has changed as little as possible, preserving the vintage look as might sometimes be desired. There’s a new fuse holder, a three-wire power cord, and a screw through one of the transformer mounts in place of the original rivet. But inside, all of the wiring has been revised, re-routed, and heat-shrink-insulated to mostly isolate the high-voltage section. The three-wire power cord has a compression-type ring terminal for the green ground lead, and the chassis and transformer frame are secured to that ground terminal with a machine screw and acorn-style lock nut. The black hot lead is first fused and then switched to both the transformer and indicator light, before returning on the white neutral lead.
Meanwhile, on the low-voltage side, the wiring has been rerouted and secured so that it mostly stays out of the high voltage side while keeping a reasonable distance from that heat-generating resistor and the transistor mounting bracket. A load bank test was run. Just over 3A of current, or about 75-80% of the maximum rating was drawn for about 15 minutes with the cover in place. We then removed the cover and checked for hotspots with an infrared thermometer. The following diagram shows the approximate results:
So hot, you could fry a reg.
Now we see the limits of a linear regulator design. The ambient temperature at our test bench was around 75 F (24 C), but the back of the chassis was easily reaching 140 F (60 C). That’s the threshold of pain for typical human skin, and if this tool were operating in an open garage or workshop on a warm summer day, the final temperature rise could be proportionately higher. Inside, three hotspots were identified: the transformer (125 F/52 C), the transistor bracket (175 F/79 C), and for that crispy fresh-fried feeling, the power resistor was at a scorching 210 F (99 C). The unit is only suitable for temporary service, as that kind of heat is not practical to remove passively when driving a high continuous load.
Minor changes, major benefits.
Now, what about the updates to our new unit, with its similar electrical design but external heatsink? The exterior didn’t change a bit, although the power cord was replaced. Since we had to correct wiring errors on the high-voltage side, it only made sense to do a cord replacement and correct the color scheme, too. Once again, we ended up with the US-spec black, white, and green. The grounding lead is configured identically to the other repaired supply, and so is the power routing method: black “hot” lead through the fuse, then switch, then load, and return on the white “neutral” lead. The same load bank test was performed, and another diagram prepared:
It’s a sauna in here, too.
For this unit, with a similar (75 F/24 C) ambient, the rear heatsink and the top cover were running just slightly cooler (135 F/57 C) after 15 minutes. Inside, the transformer was running a bit warmer than its competitor (140 F/60 C), and the power resistor was an equally blistering 205 F (96 C). Similar design, similar results, although the external heatsink did seem to help. Even so, the same caveat applies—in a warmer ambient environment, the temperatures of these critical components would rise by a proportionate amount, and that could lead to deep fried fillet-o-fingers.
Online sources vary on their claims about thermal burn risk (link references Wikipedia). But for individuals with thin or sensitive skin layers, it is generally agreed that even 130 F can lead to burns in less than a minute if the user remains in contact with the heat source, and 140 F can begin to cause damage within a few seconds in some cases.