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!
1972 Marantz 2270 AM Radio Module – Rebuilt with Nichicon Fine Gold audiophile grade capacitors.
I recently got to work on a little tabletop transistor radio from the late ’60s, maybe 1970 at the latest, from Toshiba: the 11H-540F. Not the catchiest name, but it was near the top of its model line-up featuring 11 transistors, AM and FM, and a line input. Audio power output about 1.4W into a 4 Ohm speaker.
It’s the transistor-age equivalent of a personal radio from the tube era. Solid middle of the road performance and a decently stylish package. Its owner was reporting that it wasn’t sounding that great, and she got an electric shock when hooking up an iPod to the back. It also had a lot of noise when first turned on, and the band switch was dirty and wouldn’t stay in one position. Basically it just wasn’t working well at this point.
I pulled the chassis out to get started. Definitely time for a rebuild. This 500 uF 6V capacitor had cracked and started leaking out the top.
The power supply board, with one pass transistor and a set of rectifier diodes. Replaced the 500 uF 15V capacitors with new 470 uF 16V models.
To work on the rest of the boards, the dial face has to come off:
In progress replacing components. I’d power up every few capacitors just to check, since these very early PCBs can develop cracked traces very easily. By checking regularly, I’d know if a trace broke with the last component I installed. Fortunately, none did!
Ultimately, everything ended up getting replaced.
A shot of control cleaner into the band switch and into the volume control cleaned up the scratchiness and intermittent connection. With all new caps, the static and noise on turn-on was completely gone, too.
It’s back to like new condition. No more shocks, no noise and crackling, no randomly cutting out and needing to fiddle with the switch. Just a warm vintage sound. This radio is rated for 1.4W max into its internal 4 Ohm speaker, and it managed this at around 2.5% THD. That’s after adjustment, but the nature of the distortion, the frequency response, and the speaker setup meant that it actually was still a pleasant sounding and non-fatiguing audio source. Perfect for background music in an office, for instance. It looks pretty sharp, too, with the atomic design above the dial and a nice wood cabinet and reddish grill cloth.
Here’s to many more years of happy listening!
I had the distinct pleasure of working on one of the earliest Bose® Model 901 Active Equalizers from Series I’s first production run. Serial #230, in fact! It’s made entirely by hand and shows traces of hand re-work as the design was being tweaked – really a piece of history!
This particular equalizer was a gift from Dr. Amir Bose himself to its current and only owner as an undergrad on a factory tour in the early ’70s, and after a lifetime of enjoyment it was time for a rebuild.
Incredibly, this one still has the “Acoustic Suspension Loudspeaker” decal in the center of the treble contour knob.
The owner reported it was having several issues, and opening it up one problem was obvious: the bass contour inductors had broken free of their mounts and were rattling around inside.
It was fairly similar to the later Series 1 Early boards I’d serviced, but had different style inductors and a few extra jumper wires.
It did have discrete Zener diodes installed instead of the reverse-connected transistors. And as for the transistors – the early production models used the 2N3393, and on this one they were so worn out they were only delivering about 20% of their normal gain tested out of circuit. That’s definitely no good!
It cleaned up very nicely, though:
This equalizer is restored back to full performance for its owner to enjoy for another few decades. With all new Nichicon Fine Gold electrolytic capacitors, 1% resistors, new miniature wirewound inductors, rectifier diodes, transistors, Zeners, and a thorough switch cleaning this one is good as new and will sound great for a long time to come.
I’m continuing to work on getting my HP 143A ocsilloscope mainframe fixed up as a dual-trace X-Y display.
A lot of the time has been spent on reading and researching, but I’ve picked up an important tool that will help get me there: a Heathkit Oscilloscope calibrator.
This is a simple but important little device which will help me calibrate several of my oscilloscope projects. It produces a precise DC output level from 1 mV – 100V in multiples of 10, along with precise crystal-controlled frequencies for calibrating a timebase, etc This one was a kit, but it was put together well and is in great cosmetic shape for its age.
The heart of the calibrator is a 4.000 MHz crystal oscillator. This reference is passed through a series of frequency dividers and multipliers to obtain the calibrated reference frequencies.
I hooked it up to my scope to test. The frequencies were spot on with my scope’s internal counter (4-digit, so 1.000 MHz). I didn’t check against my more precise counter but this will certainly be good enough – I’m not an NIST cal lab (and don’t want to be!) so a “bench cal” will be just fine for these old scope repair projects.
All the functions check out. I’ll probably replace the 4 electrolytic capacitors as a preemptive measure just to ensure it doesn’t develop problems while I’m using it just to be sure. Otherwise, though, it’s going to be very useful for the 143A, and also my HP 130C oscilloscope project as well.
I’m still looking for another HP 1402A dual-trace amplifier, since this mainframe needs two identical amplifier modules to be used as an X-Y display. If you have one, please let me know!
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.
Over at RTL-SDR.com, I found out about a great new product that’s just come out for more precise SDR applications: a customized R820T-based stick with a high stability temperature compensated crystal oscillator (TCXO) in place of the stock crystal, giving an incredible 1ppm frequency accuracy.
A precision TCXO like this nearly eliminates frequency drift. The stock crystal on the dongles isn’t that accurate, which can cause your tuned frequency to migrate around a bit. That’s not good for reception, especially if you’re trying to receive digital data or precisely time align multiple dongles like in a trunking or diversity application. RTL-SDR.com made these great recordings of the stability to illustrate the point:
On the left with a stock crystal, the dongle’s tuning wobbles around the center frequency. On the right with the TCXO, it’s dead stable.
You can pick your own up on eBay for $65 with free shipping. It’s a little more than a basic dongle, but given the cost of the custom part and the very precise surface mount re-work needed to make the swap, it’s quite reasonable. It’d work well with an upconverter, too.