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VD-1221 Replacement Substitute Diodes

November 25, 2014 2 comments

We’re lucky that these days you don’t need to stock very many diodes in the parts drawers for refurbishing most vintage gear – pretty much everything except for the main power supply rectifiers can usually be replaced with some combination of 1N4007 and 1N4148.

Some special diodes do still turn up, though, and they’re a bit harder to replace. Varactor diodes frequently turned up in the bias circuits of ’70s amplifiers where they were used to keep a very stable reference voltage for bias under changing temperature and current conditions. One commonly encountered varactor is the VD-1221, which turns up in the Sony TA-4650 VFET amplifier (3 diodes), the Sony TA-5650 VFET amplifier (4 diodes), and the rest of that VFET line. They also happen to not be the most reliable, and a failed VD-1221 will destroy all the VFETS in that channel. Those are unobtanium these days, so if you’re servicing any VFET amp it’s practically mandatory to replace those diodes as well.

Fortunately, in this application, they can be replaced with a pair of 1N4148s in series. Observe the correct polarity, but it’s as simple as that! It’s not the prettiest fix, but it’s cheap and effective.

VD-1221 1N4148

2 x 1N4148 in series will also work to replace the common VD-1212, VD-1220, VD-1222, and likely several other similar series diodes.

Destroying Hard Drives with a Drill Press

November 5, 2014 2 comments

I’ve had this stack of old hard drives lying around for a few years as drives become unreliable or I upgrade. There’s a 160GB, 320GB, two 400GB and a 1TB that failed recently. Most of them have never been even in my personal systems, just in servers, but a handful were my daily-use desktops over the years and have old business records, bank statements and the like on them. Secure wipes take a long time, and there’s no point really with them destined for the trash anyway.

Fortunately, that new drill press of mine can make short work of them.

Now, off to the trash. That’s probably the most fun I’ve had using the drill press yet, too.

Sometimes you just have to build it yourself.

October 28, 2014 Leave a comment

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.

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Use Copper Shielding Tape to Protect Dial Strings from Hot Tools [Tips and Tricks]

October 24, 2014 1 comment

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!

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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.

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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!

Project Epsilon: Linear Regulated Power Supply from Spare Parts

October 14, 2014 Leave a comment

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.

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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.

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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.

Project Epsilon

DIY Replacements for Vintage Couplets and Printed Electric Circuits

October 9, 2014 Leave a comment

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.

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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.

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So, sometimes it’s necessary to roll your own.

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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.

Modern Hand Made Vacuum Tubes

October 7, 2014 2 comments

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!

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