The Awkward Engineer AWK-105 Analog Meter Clock

A few years back, I did a roundup of the various meter clock projects which were floating around the Internet those days. Some were really intricate:

I’m a bit late to the party, as this came out a few years ago, but there’s a neat manufactured product that looks a bit like a piece of audio or lab equipment, doing the same thing.


That can be yours for $139 plus shipping. Sounds about right, given putting your own together would take at least a handful of hours, not to mention tracking down the parts.

I think I’m getting one for the shop. Or maybe for my bookshelf. I wonder if they have discounts for multi-packs!

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Neat Little 60FX5 Stereo Single-Ended Tube Amplifier

While digging around, I came across a really simple circuit in the RCA Receiving Tube Manual describing a 1WPC stereo amplifier using only two tubes, the 60FX5 power pentodes, and a single silicon diode. The 60FX5s have high enough gain they don’t need a driver stage, and were designed to use a crystal or ceramic phonograph pickup which is compatible with the output voltages of most modern electronics, too.

60FX5 Power Pentode Stereo Amplifier

Looks pretty easy. This is something like you might find in a portable suitcase record player or similar, on the lower end of the cost spectrum.

If I were going to build this – and I might if I ever get a spare minute not working on repair projects – I’d probably start with an isolation transformer like the Triad N68X for safety, and eliminate the 0.22M resistor and 0.1 uF capacitor in the chassis network. The diode would be a 1N4007, naturally. The output transformers might be a tougher, though. I’m not sure of the specs on the Triad S-16X they specified, but with a low-power, economy amp like this one would probably not have met the day’s hi-fi spec, 40 Hz – 15 kHz. Small output transformers just don’t have enough iron to really couple bass well, among other things. There’s the Edcor XSE10-8-3K, offering 70 Hz – 18 kHz +/- 1 dB at $19 a piece. A transformer that’s flat 20 Hz – 20 kHz would be massively oversized and cost considerably more, like the Edcor CXSE25-8-3K coming in at over $90 each unit.

The 60FX5 tubes themselves are about $8 a piece on eBay. Ceramic 7-pin sockets are only around $2 each, too. The controls are probably about $10 – I’d just use a dual 1 Meg audio pot, and find a 2 Meg trimmer for the balance control and pre-set it during construction. The rest of passives would cost about another $10, a power cord, a cake pan from the grocery store for a chassis and you’re at $100 in parts to build the amp from the ground up. You’d need to use very efficient speakers, though!

If anyone builds one of these, and you send it to me, I’ll measure it’s specs with the Audio Precision analyzer!

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Friday Link Roundup

What’s good this week? Let’s see:

Internet Radio in a Classic Cathedral Case – rather than destroying an old radio, nick.r.brewer built an entirely new case in the same style to house an Internet-streaming radio platform for the Pi Hack-a-Day contest. No vintage radios were really harmed in the making, either! [Hack-a-Day]


Dyna SCA-35 Integrated Amplifier – Seventies Stereo takes the clothes off a 1971 kit amplifier, featuring 12AX7s and 7199s feeding stereo push-pull EL84 output tubes good for 17.5W per channel. It’s pretty! I’d change out those caps, though. [Seventies Stereo]


Restoration of JVC MX-J170V Component System – Repair shop and electronics repair training center owner Jestine Yong in Kuala Lumpur, Malaysia shows off a fairly complex repair of a modern CD boombox system. Interesting! [Jestine Yong]


The Swamp Cooler – The Daily WTF reminds me why I left a career in IT administration, with a tale of ad-hoc server rooms and swamp coolers that brings back a flood of memories. Electronics, heat, and water don’t mix. [The Daily WTF]


Discogs vinyl-tracking app goes wide – If you’re a record collector, chances are you spend some time digging through bins at music stores and anywhere else, hoping to get lucky and find that one rare album. This app for your phone tries to help with that goal, aggregating market pricing, searching for record info, and managing your collection and wishlist. Looks interesting, shame I’m not much for vinyl myself! [Engadget]


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The Speaker Spotter – March 3rd, 2016

Some interesting stuff on Craigslist in this week’s The Speaker Spotter, a curated selection of Seattle’s local speakers for sale online. As always, Retrovoltage is not affiliated with any of these sellers and you should click through the link to the original post if you’re interested. If a link is dead, most likely those speakers are gone!

Vintage JBL L55 Speakers
$250 in Burien, WA

Just south of Seattle, this pair of vintage JBLs have some cosmetic damage to the cabinets but look to have intact drivers, and will for sure have that vintage JBL sound you can’t find anywhere else. As far as the condition, well…speakers are meant for listening, not watching, anyway!


Bose 901 Speakers
$325 in North Kenmore

This looks to be a solid deal for a set of Bose 901 speakers, which come with their original equalizer and paperwork. They seem to be in decent cosmetic shape, although with an equalizer that old it probably could use a once-over for the best sound. Series I and II have cloth surrounds, so there are almost never driver issues with these older pairs, too.


Tannoy DC-200 Dual Concentric Speakers
$425 in Lake Stevens, WA

It’s a bit of a drive, but if you’re a fan of Tannoy it might be worth it to check out these DC-200 Dual Concentric Speakers. Their current owner reports he’s their second and they’re in good condition. I’m not very familiar with Tannoy speakers, other than having heard the name a bit, but they seem to use concentric/coaxial drivers like KEF and some older EV speakers. Looks worth checking out!


KEF Q65 Tower Speakers
$250 in Olympia

While we’re talking about concentric drivers, here’s a KEF Q65 with a 6×9″ bass driver and concentric mid/tweeter, UK built and ready for bi-amping if desired. They look to have a decent frequency response, 38 Hz – 20 kHz at 91 dB sensitivity. I’m always a fan of strangely-shaped drivers, too.


Phase Research Transmission Line Prototype Speakers
$140 in Everett, WA

These look interesting, and worth including, since I’m a sucker for anything that’s a one-off, prototype, or just a generally-unloved speaker from the ’70s. These are from 1978 and feature an interesting driver array and transmission line cabinet with rear opening. I wonder how they sound!


Totem Acoustic Hawk Speakers
$1600 in Olympia

Speaking of transmission lines, if you’re in the market for something a bit higher-budget, these might be a good choice. They look like they’d be a great speaker if you’re looking for something that delivers tight sound, while still having a nice “furniture” look and likely wife-acceptance-factor. Spendy, though!


ESS Tempest B2 Speakers
$350 in Everett

ESS was and is famous for their “air motion transformer” (AMT) tweeters which are a very interesting electrostatic hybrid design. Plenty about them written elsewhere, but if you’re looking for that kind of speaker in a large bookshelf form factor, this one’s for you.


Dali Helicon 300 Home Stereo Speakers
$1995 in Tacoma, WA

These boutique-brand speakers feature a 6.5″ woofer, and both silk dome and ribbon tweeters. I bet they’re fantastically accurate while still being smooth in the high end. Great looking finish on these, too. I’d love to give them a listen!


Carver Loudspeakers
$2000 in Redmond, WA

Haven’t seen enough ribbons and planar tweeters yet? Or maybe your speakers have a small surface area and you’re craving wide open spaces? These gigantic Carver electrostatic planar speakers might be what you need. They look to have two active elements, upgraded ribbons, and each appears to be about the size of your average door. I’ve never heard these myself, but Carver has a great reputation for amps, of which this seller also has many.


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New Bench Storage

Some pegboards, and wall mounting some parts bins, really cleaned up the benchtop!


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555 timer teardown (and a Retrovoltage drum trigger project)

Ken Shirriff’s blog has a fantastic and detailed teardown of a 555 timer, the ubiquitous decades-old timer circuit that turns up in so many places:

If you’ve played around with electronic circuits, you probably know the 555 timer integrated circuit, said to be the world’s best-selling integrated circuit with billions sold. Designed by analog IC wizard Hans Camenzind in 1970, the 555 has been called one of the greatest chips of all time with whole books devoted to 555 timer circuits.

Given the popularity of the 555 timer, I thought it would be interesting to find out what’s inside the 555 timer and how it works. While the 555 timer is usually sold as a black plastic IC, it is also available in a metal can, which can be cut open with a hacksaw revealing the tiny die inside.


via 555 timer teardown: inside the world’s most popular IC.

Ken takes us through the history, use, die, and individual transistor circuits implemented on the die on his blog. It’s a fascinating read well worth spending an hour exploring.

I don’t get into 555s that often myself, but last summer Retrovoltage built an LED flasher circuit centered around a 555 for local Seattle band Breakaway Derringer‘s drummer. (They’re a great band that’s worth checking out if you enjoy cowboy punk rock!)




The 555 operates in single-shot mode, driven by a drum trigger sensor to switch on a TIP31C power transistor supplying power to LED strips.

The drum project was based on an Instructable that he found. You can find that Instructable here…but if you do, pay careful attention to the pin numbering on the schematic, which borders on nonsensical and definitely does not match the physical layout of the chip.


He’s only using one, but you can do some pretty neat effects with this idea.

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Antique Radio Shop near Kunming, Yunan, China

My friends over at Crimson Lotus Tea sent me this photo of an antique radio shop near Kunming, Yunan in China they ran into during a recent trip.


They’re stacked to the ceiling in the shadows. Looks like a fun place to explore!

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Building the SSTRAN Part 15 Low Power AM Transmitter Kit

Have a bunch of old tube radios, but nothing good on the air in your area? That’s a common problem, and SSTRAN has the solution! I just built one of these to give as a gift, and thought I’d write up the experience. It’s a somewhat complex kit to build with quite a few parts, but if you’re decent at soldering and have some patience, you shouldn’t have any trouble.



Everything came neatly packaged in a box with a detailed set of instructions. Inside, the parts were kitted out based on their type and which step of the build process they’d be useful.



The instructions are very detailed, including which color codes you can expect to find on the coded parts, and the assembly steps follow a logical path building up the bare PCB. There are even tips about how to get the best solder joints and soldering techniques on the plated through-hole board.


One chip, a surface mount IC, came pre-soldered; everything else was for the recipient.


I followed the instructions, documenting each step along the way. Resistors first:




Small chokes next:



Rectifiers and small-signal diodes:



Next up was the resistor network, a set of 9 x 10K resistors in a SIPP arrangement with a common pin.



The board is starting to fill up! Next up were the IC sockets. This is always a nice touch – it’s easy enough to put ICs directly on the board if you’ve perfected your technique but can be tricky, and it’s easy to burn up an IC by accident. Sockets make it easy to fix a mistake.


Jumpers and switches next. Later these are used to set the frequency range according to tables in the back of the manual.


Next up were the small fixed capacitors:



Getting there!


Just a few more parts: jacks, the ceramic trimmer for the output circuit, front panel controls, and some other bits.





Transistors were one of the last items to finish on the board:


Followed by big power supply chokes:


Last was the voltage regulator’s heat sink, and the crystal.


Time to fire it up!



The transmitter accepts L+R audio input, downmixed to mono internally, and a power supply; the antenna and counterpoise are also connected via an RCA jack. There are adjustments for audio gain, audio compression, and modulation. These controls interact somewhat, and vary a bit depending on what you’re using to receive, so tend to need to be tweaked for best sound quality once you’ve got the system on the air.

The next step was to tune the output. The construction manual lists an easy procedure to measure a voltage across a set of points while adjusting the trimmer. Here I did diverge a bit to use my spectrum analyzer with a small antenna and measure the output that way, since I had already been using the analyzer earlier.


Finally, it was time to snap it together into its case:



All done!


This was a very straightforward project to assemble, and I expect it should be able to be completed by anyone. It took me about 4 hours to complete this project (stopping to take photos along the way); if you’re on a mission I think it could be done in as low as 2 hours. If you’re pressed for time or are new to the hobby and want to go slowly, it’s easily divided up into steps which you can work on one at a time, a few minutes a day, until you’re finished.

As far as performance, it sounds great playing through a selection of vintage tube radios – just like it’s supposed to! I’d highly recommend this kit if you need a low powered AM transmitter solution for your own collection.


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Retro 1956 Cyclometer VTVM – Hycon Model 615

A pretty cool, very rare piece of test equipment turned up on eBay recently: a 1956 Hycon model 615 AC/DC VTVM. This was a precursor of today’s digital, using analog servo circuitry to drive the numeric digit display.




These were a big-ticket item back in the day:$374.50 in 1956 would have been around $3,200 in today’s dollars, adjusted for inflation!

Hycon 615 ad

Cyclometer drive displays started turning up in the 1930s, and disappeared by the 1960s as direct digital read-outs (numitrons and decatrons, nixie tubes, and later VFDs and LED/LCDs) became available. They were initially used in clocks, with cyclometer meters appearing in the 1950s and lasting for about 10 years. Most turned up in even higher-end lab equipment, with cyclometer displays being quite rare on benchtop equipment.

This would be a great collector’s item, especially if you could get it running again! If the servo motor is good, it should be possible; there’s a schematic over at Hycon Model 615: Radiomuseum.

[Vintage 1956 Hycon Model 615 VTVM Vacuum Tube Voltmeter *Non Working*]


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Building a Better Voltage Regulator

Glancing through my feeds, I stumbled across a note on The Paleotechnologist describing a new replacement for the venerable LM7805 linear regulator IC. It turns up in a ton of devices, pretty much anything with a medium-current 5V rail including some stereos, computers, power supplies…mostly anything you can think of. And the old version isn’t that efficient:

Take the LM7805, for example. It does a good job of regulating voltage — from a minimum of about 7V or so, it will provide a steady 5VDC output. The only real problem is that it does this by basically adding a dynamic resistance to simply burn off the excess voltage at whatever current you’re using. If you were to power a 5V, 1A load through a LM7805 connected to 12V, it would need to dissipate 7W of power, since it would basically be acting as a resistor; that 7V voltage drop, combined with the 1A of current, means it would be putting out 7W of heat. Without a BIG heatsink, it would quickly get too hot to work. Also, you’d be wasting over half of the power for the device, even if the rest of your circuit was 100% efficient.

via A Better Voltage Regulator | The Paleotechnologist.

Looks like CUI came out with a new, drop-in 7805 replacement which implements a DC-DC switching converter for voltage regulation, rather than a linear regulator. Way less heat and lost power with this module! I’ll probably spec it in future projects if I end up needing to replace a 7805 in some old gear, looks very interesting!


The folks over at Hack-a-Day have already found a hobbyist who put this module through it’s paces. Daniel over at Daniel’s Electronics Blog does some bench testing the switching drop-in replacement for the 7805. to test it’s efficiency.


The graph of efficiency versus load is shown below, the peak efficiency is around 92%. Not bad for a 12V input.


Not bad for a 12V input indeed, the linear chip would be wasting a ton of power as scorching heat with the same conditions.

The Paleotechnologist: A Better Voltage Regulator


Daniel’s Electronics Blog: Testing a DC-DC Converter Module

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