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.

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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.

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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.

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

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

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Small chokes next:

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Rectifiers and small-signal diodes:

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

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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.

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

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Next up were the small fixed capacitors:

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Getting there!

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

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

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Followed by big power supply chokes:

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

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Time to fire it up!

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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.

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

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All done!

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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.

[SSTRAN]

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!

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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.

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

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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

Hack-a-Day: BENCH TESTING A SWITCH MODE DROP IN REPLACEMENT FOR THE LM7805

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

Idea Poll: A tutorial on buying parts online?

Once again, I’ve been asked “Where do you buy your parts?” and that’s a question that comes up very regularly. (My answer? Mouser.com, of course.)

I’ve heard from quite a few people over the years that Mouser.com can be intimidating for new hobbyists to navigate, though, and I can understand it. They sell everything under the sun for electronics and it’s designed primarily for professionals. The ecosystem of smaller vendors selling curated selections of common capacitors, resistors, etc. with simple, friendly websites (and a somewhat higher price) serves as further support for this position.

Really, though, for many if not most types of parts you’ll buy for common radio and stereo repairs there’s not much to it and once you’ve seen it once or twice it’s second nature.

If I went through and wrote a tutorial on how to navigate Mouser.com and find common parts, is that something anyone would find useful? I imagine that of the people reading this blog, there’s a mix of advanced hobbyists and professionals who already know the drill alongside casual hobbyists and beginners who might not have taken the time to explore it just yet.

What do you think?

How to Read Capacitor Codes

I ended up with a massive stash of mil-spec CDE Mica caps recently, and was searching for a refresher on decoding their military part numbers like “CM05FD221GP3”, since the caps came marked for their values but not their voltage ratings.

I stumbled on the Xtronics Wiki page about reading capacitor codes, which has all the info I needed, and more. It has instructions on reading EIA and Miiltary codes, and tables of the tolerance, delectric materials, temperature coefficients and ranges, and working voltage codes.

Turns out, those are 500V CDE Mica caps, 2%, and with a +/- 0.05%+0.1pF capacitance drift over temperature. Not bad!

If you need a quick reference for all these different codes, definitely check out the Xtronics wiki.

It’s good to have adapters!

I’m working on a little amplifier from the late ’60s, maybe very very early ’70s, which uses RCA jacks for the speaker output connections. It was most commonly found on very economical and low-powered systems. (The one in question is 10 W per channel, definitely low power.)

Fortunately, a quick dig into the parts drawer turned up some RCA plug shells and a few minutes later, ready to connect.

Easy enough! I won’t be using this one very often, but it joins my collection of speaker cables for whenever I do need it.

What do we have here?

I was reading about how to build a capacitor checker from the December 1959 issue of Popular Electronics and stumbled across something that looked very familiar.

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I know that shape.

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Nearly identical to a Bose® 901 Series I/II cabinet, and even specifically references expanding the sound field with a reflected wave. The 901 series uses what Bose calls “Direct/Reflecting” technology.

Anyone know about these “Cosmos Industries” speakers? Does anyone know any of their names, who might have gone on to work for the early Bose® corporation?

Mailbag: What’s the best multitester for vintage radios?

I recently got this question in the mailbag, and it seemed like a good one to answer:

What’s the best multitester for vintage radio repair?

That’s actually a tougher question than it seems. The short answer? Well…it depends a bit, but most multitesters (not to be confused with multimeters, which are very useful) aren’t that useful for vintage radio work. They’re not a bad thing to have, but most of their functionality is lost on a vintage radio.

To re-cap, a multitester is a neat little microcontroller instrument which can do quick analysis of 2- and 3-terminal devices. It’s useful for checking capacitor value and ESR, quickly checking transistors and FETs, checking diode voltage drop and capacitance, measuring DCR and inductance of coils, and low-ohms resistance measurements. Powered by a 9V battery and a microcontroller, these devices take a lot of the guesswork out of quick go/no-go checks for a variety of types of electrical components. The most common model is the MK-168, available from a ton of different vendors primarily on eBay for $20-50.

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The trouble with these devices for vintage radio repair is that you won’t end up using them all that often. Resistance measurements for vintage radio can easily be handled by most multimeters, like those in my Basic Tools round-up. Transistors started turning up at the very, very end of the tube radio era and you’re unlikely to encounter one of those hybrids and multi-testers can’t do anything for a tube beyond what a regular multimeter can. While DCR and inductance of coils is occasionally something to consider, more often than not you’ll have a good coil or an open coil and not a lot of in-between. And with vintage radios (and even most stereo gear through the ’80s) it’s just not worth it to test individual components before replacing; you’re better off just replacing all the parts subject to failure at once regardless of what they might measure. Not to mention, with a 9V battery supplying the power, you’re going to be far below the hundreds of volts found in most vintage devices.

That said, if you want the vintage equivalent of a multi-tester for a vintage radio, you do have a few options.

First would be a signal tracer. Signal tracers are fairly straightforward devices with a probe, detector, amplifier, and speaker. By injecting a tone at the antenna terminals of your radio under test and moving the probe through the signal path, you can find where it disappears. The probe can detect an AM RF or IF envelope and turn it into audio or amplify a small audio signal to find out which stage of the radio fails to pass a signal. These fell out of fashion after about the 1960s, but you can still find them on eBay. They were typically made by bench service companies like Conar, EICO, Heathkit, Knight, PACO, Superior Instruments, and others. They’re all pretty much the same – although being old gear, if you buy one you’ll want to make sure it’s in good working order (or you refurbish it) and the probes, if any, are provided. You should pay less than $100 for a fully working model. You’ll need a signal generator to go along with the tracer, of course.

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Secondly, if you have a bit bigger of a budget or like rare test equipment, you might consider an RCA-Rider Chanalyst.

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These are pretty rare and hard to find, but they’re unprecedented if you need a full functioned signal tracer and generator. These offer RF and IF generators, an oscillator injector, a power meter, and audio output (and you can even hook the various stages together and use one as it’s own radio if you wanted!) Sadly, though, these are rare and expensive and aren’t any better than a signal generator+signal tracer combo.

In general, though, I’m not sure I can really recommend a signal tracer if you’ll be doing more repair work than just vintage AM tube radios. They have little use in an FM radio, stereo receiver, or even most other electronics projects beyond a radio with the AM broadcast band and a 455-ish KHz IF. I used to have one, and used it once or twice very early on, but quickly moved up to other test gear and it sat taking up bench space until I finally got rid of it. About the only place they have any real use, in my opinion, would be tracking down issues with a radio’s front end – between, say, the antenna coil and a first RF amplifier ahead of the mixer – where signal levels might be too low for most oscilloscopes to display. Signal injection with a generator would likely be able to overcome this limitation, though.

So, in conclusion: in my opinion, a multi-tester isn’t a great tool if you’re just doing vintage radio repair. You’ll be better off with a plain old signal generator and tracer (along with your multimeter and standard bench tools) for your first set of vintage radio repair tools, if you’re not sure about taking the plunge for bigger and more expensive test equipment. Multitesters are fantastic little devices, but have limited applications in vintage radio repair, and so you’d be better off saving your money if that’s all you’re planning on working on. As far as signal tracers, while they are useful for vintage radio repair, they do have a limited usefulness beyond AM radios so keep that in mind when deciding if one is right for you or not.