When a vintage component network goes bad and you can’t exactly go down to the corner store and buy a Sprague TC-24 anymore, sometimes you just have to build a new replacement yourself.
Dial strings are, quite frankly, about the worst things in the world. They’re usually a complex and finicky mechanical system parked right in the middle of an otherwise straightforward electrical project, and if a string breaks good luck getting it back together again in all but the simplest of dial string arrangements. And they break easily. On older, tube gear the dial strings have often worn out and snapped with age and friction. On newer gear, the dial strings have often been snagged on the case at some point during a previous repair attempt, or even worse, they were accidentally nicked with the soldering iron and burnt or melted apart.
That’s a problem which has happened to me quite a few times, even with a steady hand and the best of intentions. After spending many hours re-stringing the dial on a 1970 Toshiba tabletop transistor radio after my soldering iron caused it to snap where the string passed very near the amplifier PCB, I was inspired to come up with a solution that’s a bit more reliable than “just be more careful”: copper shielding tape!
Copper shielding tape is an extremely important part of an electronics test bench. It’s very handy to shield a sensitive part of a circuit from electrical interference, but also, it can shield from thermal interference as well! In this case, because the copper foil is a thin piece of metal with a high melting point, the soldering iron brushing up against the foil won’t damage the string under it, and won’t heat it up nearly enough to cause damage to anything underneath for a short tap. This is the perfect solution to the problem of dial strings snapping when trying to solder too close to them. Copper shielding tape can be soldered, so it’s perfect to provide some protection against an errant soldering iron. A small 2″ section wrapped around itself with only a small section of the adhesive removed to form a cylinder was all it took.
Copper shielding tape is extremely useful to have around. It comes in a variety of styles, but I’d recommend one that’s about 2″ wide and has a conductive adhesive so it can act as a shield without soldering as long as it’s touching a metal chassis somewhere.
I stock a full roll of 2″ x 55 Yards ($56.95) as I use this while repairing Bose equalizers and stereo receivers, but it comes in other sizes. A five-foot section ($15.95) might be a better choice if you don’t see yourself using it often, or if you’ll only use it as a soldering iron shield. It’s available in smaller, narrower sizes also: 1″ x 5′ ($10.99), 0.75″ x 18′ ($8.28), and 0.25″ x 18′ ($5.05) but these smaller and narrower sizes are really more appropriate for actually shielding seams, etc. than trying to protect a dial string.
If you try this out yourself, let me know how it goes!
Belgian engineering student Gert-Jan built a great looking, and functional, linear regulated power supply for a test bench from an old radio transformer and a handful of supporting parts. This is a pretty nice looking adaptation of the common LM317-based power supply project. It only has a few parts and can make a nice, workable bench supply.
The supply uses an LM317 linear regulator. It can offer an adjustable voltage from 1.2-20V, since that’s the maximum output of the transformer, even though the LM317 itself is rated up to 37V. The case and meters came from eBay.
I do like what looks like a precision pot he’s chosen for the adjustment. I’d love to see a version of this build with a complimentary negative supply built around the LM337 as well. There’s plenty of room on that main piece of perfboard!
Making your own version of this supply would be pretty easy. Gert-Jan goes over the math of LM317’s adjustment range and includes schematics for each part of the system (rectifier and regulator). It’s up to you to string those two together, but that shouldn’t be very difficult. Transformers like that are pretty common, too. This would be a fun afternoon project. I might end up building one myself, I could use another adjustable supply. Maybe I’ll add a negative side to my version, too.
Starting in the ’50s, they’d figured out how to start miniaturizing components. “Couplets” and “printed electric circuits” started showing up in equipment around then. These components were combinations of several resistors and capacitors in a single package.
Like most components from that era, they do start to go bad over time – although you can’t exactly run down to the local Radio Shack and buy a Centralab PC-191 or a Sprague TC-24 anymore.
So, sometimes it’s necessary to roll your own.
Those two boards replace a set of tone control circuits in the EICO HF-81 amplifier. A little bit bigger when made with discrete components, but not too bad! These will be much more reliable, too.
Enterprising French amateur Claude Paillard , F2FO, creates beautiful modern vacuum tubes by hand in his shop. These would be effective as an 01A replacement or similar in an early radio.
It’s a real work of art. Check it out!
Fred Clift wrote over on his blog about why it’s worthwhile to pick a more expensive multimeter. He’s comparing a $200 Fluke to a $6 Harbor Freight multimeter, which really is a no-brainer. I’m not sure the Harbor Freight model would be safe to use on any of the higher range measurements with those whispy, poorly insulated test leads.
I use a few mid-grade meters for basic bench measurements and was interested to see how they compare with the Fluke 87V and the Cen-Tech from Harbor Freight.
I use the Mastech MS8268 meter which I recommended in my “Getting Started with Basic Tools” post. It’s durable, rugged, and costs only $25. Really, it’s about what I expected to find. As you go down in quality you lose accuracy, but the Mastech meter has all the same functions as the Fluke – just not quite as tight.
The Fluke does have some cool features that Fred picks out. It will tell you min/max/average over an interval, faster continuity beeps to quickly check a strip of connections, and a higher voltage continuity test that can actually light up an LED. Otherwise, though, the Mastech has the same basic capabilities including 200 kHz frequency measurements. I’ve been able to use that in a pinch as a signal tracer – set the meter’s range to measure frequency, and probe the stages of an audio amplifier. Meter shows the tone? Move on to the next stage.
A lot of applications don’t need 0.05% accuracy on a DC voltage reading, so I stand by my recommendation of the Mastech as a “good enough” meter for most hobby work and vintage equipment repairs. There’s definitely situations where it’s worth it to spend a bit more, though.
A few people have asked me variations of “What tools do I need to fix my first radio?”, “What do I need to put together this soldering kit?”, and such. It’s not as simple as you might think. There’s a lot of equipment out there, and if you’re starting out it’s not always easy to tell what’s good from what’s garbage. Picking the wrong tool for the job will give you a frustrating repair experience at best, or could even damage your project.
You do only need a few things to get started: a soldering iron, a multimeter, a wire clipper, a wire stripper, and some basic safety gear. If you’re a casual hobbyist who will do a few projects a year, you can get started for under $100. If you’ll be working on electronics a little more often than that, you’ll want to spend a little more for more durable tools, but they’re still pretty affordable.
Below are some tools I’ve selected which will help you get through your first project and then some:
Probably the most important item on this list, you’ll use the soldering iron to make connections between wires and components. You need to make sure you get an iron that will be durable enough, and produce enough heat, to quickly melt the solder and heat up joints without taking too long and causing “heat soak” which can damage other near-by components you’re not actively working on. While it might seem counter-intuitive, too small of a soldering iron is actually a bigger problem than too big! I recommend a soldering iron around 50W for most hobbyist applications. Stay away from any of the “cold heat” soldering tools which aren’t very effective for this kind of work, and stay away from any gas-powered soldering iron as those are more suited for plumbing or off-the-grid work.
For electronics repair, you can get started with a no-frills soldering iron for under $10 (left). I’d recommend spending a little more, though, to gain temperature control and easier replacement parts if you need a new tip or heating element. For about $20, you can upgrade to a Velleman 50W adjustable soldering station (center) which has a handy cleaning pad and soldering iron holder built in. If you’ve got deeper pockets, Hakko makes excellent professional-grade soldering products like the FX888D digital soldering station (right) which has a digital temperature control, and you can get dozens of different size and shape tips and replacement parts very easily. I use the Hakko FX888D in my shop, and it’s fantastic.
You’re also going to want the right kind of solder. Rosin Core Solder is the right kind for electronics and includes flux inside for a good connection. There’s new RoHS-compliant (“lead free”) solder out there, but personally I find it’s more difficult to work with. Wash your hands after soldering and you’ll be fine using traditional lead solder, like this basic 60/4o Electrical Repair Solder (left). Standard tin/lead solder comes in a several varieties; 60/40 and 63/37 are pretty common and there’s not a major difference. If you’re going to be doing work on sensitive audio or RF circuits, some kinds of solder can offer a little better performance. Silver-bearing solder forms joints with a lower resistance; tube radios won’t really benefit but some high-end vintage hi-fi gear can benefit. Tenma makes a very nice 96.5% tin, 3% silver, 0.5% copper solder (right). Be sure never to use acid-core solder as you’ll damage your device, acid-core solder is only suitable for plumbing.
You’ll use the multimeter to make a few circuit tests, including checking for the correct voltage at a few points in the circuit, and for measuring the value of resistors. A homeowner’s multimeter isn’t a good choice for these repairs, but even an inexpensive digital multimeter will be more accurate than most of the tools in a repair shop back in the day. At about $25, the pictured (left) MASTECH AC/DC Auto/Manual Range Digital Multimeter will measure AC and DC volts up to a higher range than you’ll ever likely need. It also can measure current, resistance, test diodes and continuity with a buzzer (useful for checking coils!), roughly measure the value of capacitors, and do some basic frequency measurements. Sure, it’s no Fluke (right) but it’ll get the job done just fine. A meter very similar to the MASTECH is my everyday go-to meter that I use during all but the most precise measurements, and it’s never let me down.
You’ll need to cut old component leads and cut wires down to size when doing repairs. A set of flush-cut wire clippers makes short work of this, and they’ll last longer and won’t get the cutting edges nicked up as if you tried to use a general cutting tool like a pair of household scissors. They’re very inexpensive – under $5 – and come in handy around the house as well as in the workshop.
Right along with the wire clippers, you’ll need to prepare old and new wiring for soldering by removing the insulation. An adjustable pair of wire strippers is critical so you remove the insulation without damaging the wires underneath. At the low end, around $5 will get you a set of TEKTON continuously variable wire strippers (left) with an adjustment nut, and they have a cutting edge as well. This is okay if you have only one wire size, but it’s a bit annoying to adjust every time. I prefer wire strippers with their own sized guide holes. Moving up to around $20, you can pick up a nice Klein Tools wire stripper (center), or a Greenlee Communications wire stripper (right). I own the Greenlee and it’s got a comfortable grip and is very sharp and easy to use.
Soldering does produce some toxic fumes, and you’ll want to make sure you’re in a well ventilated area. Soldering indoors isn’t a big deal occasionally, but you don’t want to breathe in the smoke too often. If you can’t open a window, you can use a variety of smoke absorbing filters for your workbench. They start at about $35 for a smaller desktop model, suitable for light work, up through many hundreds of dollars for a professional fume extraction system like the Hakko FA-430 which Rain City Audio uses.
Sometimes molten solder can splash, or a piece of wire can go flying, and you’ll want to protect yourself. Safety glasses are highly recommended, and can be had for as low as $3 if you’re buying some of the other items. That’s a low price to pay for peace of mind! Some people like to use a surgical filtration mask while soldering as well, to help with residual fumes. They’re available for about $1 each in boxes of 20. I don’t use a mask, personally, but if you don’t want to spring for a fancy ventilation system and don’t have good airflow in your location, you might consider it.
A tip cleaner and debris catcher is also very handy for keeping your iron clean but isn’t strictly necessary. At a minimum, keep a damp kitchen sponge on hand with a scrubby side and use that to clean your tip after every few joints.
That’s all you’ll need to get started. If you get further into the hobby, or run into functional issues once you’ve finished replacing the needed components, you’ll also want a few pieces of test equipment. Look for my recommendations on hobby test gear for radio and electronics repair in a future article!