Power to the People
Today we will forget the switching aspects and party like it’s 1979, when power supplies were linear and the phrase “smart phone” meant a clunky rotary dial device wearing a bow tie and cummerbund, which probably never happened. Let’s take a look at our subject devices.
Two denizens of the gray market, both offering 12 VDC nominal output.
Our hapless victims willing volunteers are respectively older and younger than your humble author, but both do the same thing in nearly the same way. Each power supply plugs into a standard wall socket, and outputs 12 VDC nominal, 13.8 VDC maximum, at a maximum current of 4 Amps using a conventional transformer and linear regulator circuit (link references Wikipedia). It’s an old-school analog design that Just Works. The main downside of linear regulation is the higher energy loss compared to the best-designed switch mode devices, but the approach does offer a rugged and relatively simple layout.
The primary concern is whether gremlins might exist around the incoming mains wiring. Bad high-voltage connections often have none of the prolonged light and sound show portrayed in cinema. High voltage can and does do that sometimes, but mains electrocution from a faulty device can also look like this: at one moment the user is alive, then they touch something metal or wet that is also touching a bad wire, their body might stiffen slightly, and then they’re on the floor with a stopped heart. At that point aggressive CPR or an Automated External Defibrillator (AED) might revive the unfortunate victim, but otherwise the ambulance may not arrive in time. “Silent but deadly” is a real thing when good electricity turns bad.
Fortunately, modern electrical standards are quite robust on the safety side. But since electrocution can and does happen, it’s a good idea to look for dangerous layouts or shortcuts in vintage gear or uncertified direct-import equipment before putting it into regular use. Along with that, a secondary concern is heat. If these units are either usable as found, or else can be made safe and then deployed, can we load them up for a length of time without issues? Only one way to find out.
Introducing the Contestants
The larger and decidedly vintage-looking unit of our time-separated twins was located at a thrift store, proudly displays “Tempo PS-4A,” and was made by Henry Radio in California. The company apparently still exists (link references their website). The visual and tactile aura of this supply, especially the veeerry thin power cord with a non-polarized plug and no third ground prong, all point to a pre-1980s manufacturing date. Also, no regulatory standards are listed on the chassis even though it claims US manufacture, while the internals do include an IC-based regulator circuit. These clues reasonably place it early in the decade famous for codifying modern rock music, a lot of illegal drugs, and the dread economic term “stagflation.” (Pretty sure those three would make a neat Venn diagram, but I didn’t take time to unwind it.) Let’s pin a number and say 1972.
Tempo PS-4A, from 1972 or thereabouts.
The newer unit wears inscrutable Audiotek branding, which seems to have no connection to a US company of the same name most notable for audio engineering at the Grammy Awards. It designates itself as model “AT-PS4,” and offers no manufacturing date. The chassis is somewhat smaller than the first unit and moves its heatsink outdoors for better cooling. The internal electrical design is slightly simpler, and technically older, than the previous unit. However, it was recently purchased in new packaging from an eBay vendor and the unboxing smelled like an asphalt paving operation, both indicating recent origin and the presence of very cheap paints and resins. Let’s call it 2018 at the earliest, if not this year.
Fresh off the boat: Audiotek AT-PS4.
Electrically, both units do the same thing and even house similar components, including a model 2N3055 transistor to direct traffic at the DC output. A load-bank test confirmed both units operated reliably at 75-80% of full output. The newer supply gives a nod to modern safety standards by including a fuse on the high voltage inlet and a third (ground) prong on the power plug, although neither means much until we dig inside.
So much for introductions. One device was nearly free at a thrift store but is very old and visibly lacking all modern safety standards. The other unit is new and has all the looks while being as cheap as new equipment will ever get for this application, thanks to the agency of direct import. Okay, technically we did buy it from a US-based eBay seller, but since the PayPal receipt arrived with a long string of simplified Chinese characters, we doubt the seller’s supply chain went much deeper than an Alibaba account.
Never judge a power supply by its cover, though. We need to pop the hood on these guys and take a look. Polished aluminum velocity stacks and four-barrel carbs? Probably not, as that would just be silly in a power supply, but there’s other stuff in there that we should know about.