Electronic Vivisection: Elder Abuse Edition
The ancient Tempo PS-4A is first to go under the screwdriver. Interestingly, only the cover is secured with screws, and the manufacturer was kind enough to use six machine-thread inserts, ensuring a high-quality attachment that can be opened and closed repeatedly without damage. Unfortunately, they did not brace the front or rear panels, and those flex a bit much. All internal equipment is riveted. For the vendor, rivets were probably cost-effective, and neatly prevent the user from accidentally loosening equipment inside by turning the wrong thing on the outside. It also means we can be fairly certain this supply hasn’t been repaired or modified by a prior hobbyist, as they would have drilled out some of the rivets and replaced them with screw hardware. Whatever turns up inside should be factory fresh.
Inside the Tempo PS-4A. Need a little marinara with that?
Oh, dear. We might not find this much old spaghetti in the breakroom of a New Jersey waste-management company. The lack of a safety ground or fuse was obvious from the start and reflects the age of the unit. But the complete inattention to clean layout is an eyebrow raiser. Granted, a lot of electrical equipment was made this way in the bad old days, but the unnecessary tangle of high-voltage and low-voltage wiring with sharp pointy leads isn’t good news. Again, the original rivets are all present. It was made this way.
We also have a 125 V-rated lamp assembly mounted directly over the low-voltage regulator board. The rating does makes sense. It’s probably a type-NE2 neon lamp assembly with a dropping resistor, common for the time as LEDs were not yet common or cheap or bright, and a neon lamp is preferentially run on the AC input voltage, not the DC output, for maximum brightness. That noted, we’re pretty sure the front panel aesthetic was the priority here, and not electrical safety.
The regulator circuit: An oasis of order.
The regulator circuit itself is pretty decent. The chip part is a very old type of multi-feature linear regulator device. We won’t dig through all the details but suffice to say that bracket heatsink includes the bit output transistor which passes current to the load, so we should expect a lot of heat in that area. Likewise, that large rectangular power resistor is unlikely to keep its cool, as it must eat a bit of power to create a voltage reference for the regulator’s overload and short-circuit protection. The resistor could use a better mounting arrangement, but otherwise it would be a very passable layout were it not for the jungle of copper occupying the other half of the chassis.
A thermal overload protector, as it would appear.
The one nod to equipment safety is a glass tube glued firmly to the transformer body. The innards appear composed of a simple contact assembly and bimetallic strip, suggesting a thermal overload device. Purportedly, if the transformer reaches an excessive temperature, the strip will bend and break the circuit, then reset again after a cooling delay. It does not qualify as a primary protection device since it is apparently capable of self-reset. Modern transformers sometimes include a thermal fuse, but it is normally embedded in the windings and is a one-shot device that only trips when a truly damaging overheat has occurred, permanently disabling the transformer.
Verdict: this design may have been considered good enough for the early 1970s, but so were disco and Richard Nixon. It isn’t safe to use in present form and, as experienced hobbyists who accept the risk of working on mains-powered equipment, we would first do all the following:
- Replace the power cord and apply proper grounding technique.
- Add a chassis-mounted fuse holder and properly sized fuse.
- Insulate exposed high-voltage terminals as much as possible.
- Wiring cleanup. A few zip-ties can do amazing things.
The looming question: is all of that, worth doing? We will come back to that discussion, but first, let’s see if we can do better without breaking the bank by just purchasing an affordable new device.