A Tale of Two Power Supplies

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.

Both power supplies

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.

Old power supply

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.

New power supply

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.

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Aaron Vienot

Engineer by day, hobbyist by night, occasional contributor, and full-time wise guy.

35 Comments
    • cegras
    • 3 weeks ago

    Great article!

    Reply
    • LiquidSquid
    • 3 weeks ago

    Thanks for the trip down memory lane. I have a few of these 13.8V supplies around (Car battery voltage for running HAM equipment and other car related crap)
    What is cool is a tiny SPS wall-wort for $10 can have more output power than one of these behemoths, and is much safer and far more power efficient. However, the SPS will also be far more noisy as far as electrical noise.

    Reply
      • ludi
      • 3 weeks ago

      Indeed, although the attractiveness of a bench supply is that the power switch and terminals are Right There for attaching whatever. The coworker I referenced in the video segment is squarely in the millennial generation and had somehow turned up one for hobby use, so that’s a pretty good sign that these things are as popular as ever among the DIY crowd.

      Reply
    • dragondaddybear
    • 3 weeks ago

    I love the dual post style. Also, I really love the humor.

    Reply
      • juzz86
      • 3 weeks ago

      Me too! Good job ludi, a great read mate!

      Reply
    • continuum
    • 3 weeks ago

    Slightly off-topic question, what’s with the grey text in parenthesis after the links? Is it some attempt at WCAG 2.0 compliance without proper tags in the CMS? Or some attempt at footnotes without actually doing footnotes? Just curious since I’ve never seen a site do that before…

    Reply
      • Ben Funk
      • 3 weeks ago

      It seems to be a way to differentiate a real link from an automatic one.

      Reply
        • ludi
        • 3 weeks ago

        Bingo. It also gives mobile users an easier way to determine if they want to visit the link, since mouse-hovering isn’t implemented as conveniently on a touchscreen.

        It may or may not be used [ever again / by any other authors / cheese ], but I thought I would try it out as an experiment.

        Reply
    • willmore
    • 3 weeks ago

    Before anyone says it again, at the voltages you’re going to find in low voltage sides of these boxes, the caps aren’t going to give you any shock at all. Now, in a modern switching mode power supply, the front end caps can have more than 2x peak AC voltage on them which is around 340V. That will gladly kill you.

    Reply
      • ludi
      • 3 weeks ago

      Yep. And as it happens, both designs in the article (and all three designs shown in the video feature) are constructed such that they bleed down pretty quick when shut down. Since neither I nor the TechReport could invest the proper resources to discuss all possible hazards illustrated in this review, we opted to cover it with multiple, plainly-worded, “don’t try this at home” disclaimers.

      Reply
    • willmore
    • 3 weeks ago

    Can you get some close up shots of the control board and pass transistor for the older power supply? We can probably pull some date codes off of there. Can you see what value the large resistor on the output is? It’s likely a shunt for overcurrent protection. I’d be the control board has an LM723 on it.

    Nice teardown–now on to the new design…

    Reply
    • DPete27
    • 3 weeks ago

    “See how I just nonchalantly fingered the solder terminals on the bottom of those larger capacitors without verifying that they’re discharged (or stating this step had been done prior)…don’t do that”

    I applaud the video content and i think it’s a strong start in the right direction. If I can offer any constructive criticism (no offense intended) it would be to spice up the delivery of the [admittedly dry] review content from the speaker. The effects were a welcome addition to keep things lively, but [IMO] the idea is to make this [boring] power converter sound like the coolest/funnest thing on the planet.
    Obviously most of us Millennial+ viewers are turned off by the LTT-style of delivery, but remember that Gen-Z is responsible for probably an order of magnitude more views and likes comparatively. I’d say that LTT generally doesn’t have the most in-depth or insightful content, but their STYLE of delivery is what attracts views in droves.
    I don’t have the answer whether to go full psycho on the video content in order to cater to the younger crowd while allowing the written content to continue drawing readership from the older generations, or if there’s a happy medium that can be struck.

    Reply
      • ludi
      • 3 weeks ago

      If that’s what you think people want, I’ll wait for you to make that video 🙂 It’s harder than it looks.

      Reply
        • DPete27
        • 3 weeks ago

        I absolutely agree.

        Reply
    • chuckula
    • 3 weeks ago

    Bonus safety tip since I see a nice big blue capacitor in there: Capacitors often hold substantial charge if you turn off the power supply without discharging them.

    So you need to watch out for a scenario where you run the power supply, fully disconnect it from the wall, and still have to worry about the potential for getting a nasty shock if you open it up to do work without making 100% sure the Capacitors are discharged

    Reply
      • dragondaddybear
      • 3 weeks ago

      When I was younger that was an evil trick I would do. I’d hand a charged cap (using a transformer and a battery to charge it) to someone and shock them. It was a take on the sock box I did in school. Very appropriate/fun/evil way to learn electron flow, basic circuits, and transformers.

      Reply
        • crabjokeman
        • 3 weeks ago

        We used to play “pass the capacitor” in lab, where we’d soft toss around an electrolytic capacitor and try to catch it by the cover to avoid getting shocked.

        Reply
    • chuckula
    • 3 weeks ago

    Sweet article and finally some subject matter on TR that can get you killed if you don’t do it right!

    Reply
      • Krogoth
      • 3 weeks ago

      “Our shilling can’t repel potential of this magnitude!”

      Reply
    • Krogoth
    • 3 weeks ago

    Unlimited Power!!!!!!

    Reply
    • redocbew
    • 3 weeks ago

    OMG WHO IS THIS WTF SO SAD

    Reply
    • usacomp2k3 (AJ)
    • 3 weeks ago

    Enjoying the read so far. Does Aaron have a forum handle that we recognize?

    Reply
    • In the video he clarifies that he’s ludi—as well as in the comments yesterday where he accidentally doxed himself, lol.

      Reply
        • ludi
        • 3 weeks ago

        Aaron? Never heard of him. *flips trenchcoat and scurries off*

        Reply
        • Can one of the mods ban this joker for running multiple accounts? 😉
          Seriously though, thanks for your work on this, it could be life-saving information and will directly impact my future purchasing decisions (in a good way).

          Reply
          • usacomp2k3 (AJ)
          • 3 weeks ago

          Great job sir! I really enjoyed the rest of the read. My dad’s dad was a big electrical hobbyist (worked for Ma Bell back in the day) and while the curiosity is genetic, the skill is not. It’s still somewhat black magic to see someone understand what’s going on.

          If you’re even in the Orlando area, you’d love Skycraft (https://www.skycraftsurplus.com/). They have all sorts of vintage electronics like old Agilent Oscilloscopes.

          Reply
            • ludi
            • 3 weeks ago

            Thanks!

            My dad is a lifetime electronics technician and also a hobbyist. I had the early advantage of learning a lot from him early in life, including proper soldering technique, and then went deep into DIY audio around 15 years ago. There are also a lot of good YouTubers that enjoy delving into electrical equipment, which is a great teaching shortcut that didn’t really exist 10+ years ago.

          • Mr Bill
          • 3 weeks ago

          Well written, useful, interesting, and humorous… [quote]We thought about doing so as a side-feature for this review but didn’t want to risk being stoned to death with Mountain Dew bottles.[/quote]
          You should write reviews more often.

          Reply
            • ludi
            • 3 weeks ago

            Sadly, I don’t tax my computer hardware as much as I used to, and that gives me less relevant material in which to geek out. The video editing and encoding for this review was the hardest I’ve worked my main desktop system in over a year, and my last from-scratch system build was a low-end Skylake HTPC nearly three years ago.

            If TR sticks around for a while and I find something interesting, I’d be open to doing it again.

          • Aether
          • 3 weeks ago

          Great lunchtime read; thanks!

          Reply
    • Captain Ned
    • 3 weeks ago

    The traditional “brand name” for inferior Chinese-sourced components is “Yum-Cha”.

    Reply
      • ludi
      • 3 weeks ago

      Interesting. I’ve been heavily into DIY electronics for a while and hadn’t heart that slang yet.

      Reply
        • ludi
        • 3 weeks ago

        *heard

        Reply
        • GhostofNappa
        • 3 weeks ago

        I’m not sure what this Yumcha is, but it sounds like a Raditz to me.

        Reply

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