Reconditioning a touch tone dialer.

28 December 2020

One of my holiday break hobby projects, a palate cleanser if you will, was reconditioning a classic Radio Shack touch tone dialer I'd picked up on eBay somewhen around Thanksgiving. They're retrotech to be sure, dating back to the days when the touch-tone dialing that we take for granted these days (so much so that we don't even hear them anymore because we use mobile phones) was actually pretty rare.

Note: A lot of the following history of telephony has been edited to reflect only the salient points for this article. Telephony experts out there will probably rankle a bit because I skipped over stuff. Sorry, not sorry. Needs must.

Way back when, once upon a time (the 80's for me, because I was a kid back then) the telephone system as we knew it was largely based upon electromechanical switches. Racks and racks of not-computerized rotary switches, covered with electrical contacts and hooked to cleverly designed circuitry that figured out the signals sent from people's phones and other switches and (eventually) connected your call to whatever number you dialed. If you've ever heard someone with a really nice gaming keyboard with mechanical switches rocking and rolling you've probably got a good idea of what they sounded like back in the day. Starting in the mid- to late 80's, systems that responded to touch tones (the precursors to today's voice mail and IVR systems) started appearing. However, they only responded to touch tones and not the pulse dialing phones most folks had around then.

This was back in the day when you weren't actually allowed to go out, buy your own phone and plug it into the wall jack. You had to rent your phone (a big Bakelite covered dealie that was built like a tank) from the phone company (back before [Ma Bell was broken up into separate competing companies). For quite a while they'd only let you rent pulse dialing phones, which essentially tapped the hangup switch really fast to dial numbers. Even when the telcos relaxed their restrictions and you could freely buy telephones at the corner store, most of them still only did pulse dialing. IVRs couldn't read those pulses so unless you had a touch-tone phone you were pretty much out of luck. Enter the hand-held touch tone dialer from Radio Shack, a device a little smaller that a deck of cards with a speaker on the back. The idea was that you held the speaker to the mouthpiece of the phone, hit the keys on the front, and it generated touch tones that went through to the system on the other end and let you press 1 for banking, 2 for investment, or whatever.

Tone dialers started vanishing as a common carry thing in the late 80's because computerized telephone switches were being rolled out by the Baby Bells across the country, and one of the big features they implemented was native touch tone service by default, without needing a lot of expensive signal processing circuitry and wiring changes. You could just turn it on in the switch's configuration and go about your business. This was also around the time when touch tone phones got to be as cheap as the old pulse dialing phones. Enter the phone phreaks.

For a time phreaks used tone dialers for dialing under circumstances where either you didn't have a full phone handy, or for whatever reason the keypad on the phone (usually a payphone) was diabled. It wasn't until somebody figured out how to build something later referred to as a red box out of them, a device that produced the same tones that payphones made when you dropped coins into them. The idea was that you could swap out the timing crystal in a dialer, which would change the frequencies of the tones it generated. Coincidentally, this modification made the * key on the dialer emit the same tone generated by a payphone when you dropped a coin into the slot. It was typical for phreaks of the early to mid 90's to program five asterisks into one of the memory slots of their red box to emulate the same signal as a quarter. Red boxing pretty much died out in most areas by 1998 because the new generation of switches were smart enough to tell the difference. Every once in a while an operator on station would break into illicitly placed calls to yell at the people on the line.

So, all of this is for historical purposes. It doesn't work anymore.

Anyway, I got it into my head to buy a Radio Shack touch tone dialer off of eBay to restore and add to my collection of retrotech. Finding one was a bit tricky but not too difficult, and I didn't care if it worked or not because I was just going to put it in my trophy case on display. When it arrived a couple of days later and opened the package, the smell nearly knocked me over. The seller apparently kept his stuff someplace where animals usually hung out, so the smell of old dog pee filled the kitchen in short order. This particular problem was taken care of by folding a paper towel in half, pouring a generous layer of baking soda in between and wrapping the tone dialer up in it. The bundle was then placed in a zip-lock baggie and allowed to sit for a couple of weeks (the holidays being a busy time). When I unwrapped the bundle the smell was gone, modulo for a little left over in the battery compartment because I'd left it closed. I also gave the dialer a quick test with fresh power cells - nothing.

The reconditioning process was significantly shorter than my C64 earlier this year. The first step involved popping the case open to expose the guts. Using an off-brand guitar pick as a prying tool makes the job much easier than back in the day. Six screws held the circuit board in place and the battery terminals were press-fit (necessitating a pair of hemostats to pry them loose). The power switch just rested inside the case, and unfortunately fell out during the disassembly process. It took some rooting around in the leaf litter below the back porch to rescue it. That done, the circuit board lifted out, followed by the conductive rubber keypad and the individual number keys.

The speaker on the back, however, was epoxied into a ring molded into the case. It took about half an hour with a craft knife and a prying tool to pop it loose. I then took some cotton swabs dipped in nail polish remover to the case and rim of the speaker to scrub off the now-gummy epoxy.

The PCB was cleaned with an old toothbrush and some 95% pure ethyl alcohol, which is pretty much the gold standard for this sort of thing. The thin coating of... I really hope it wasn't dog pee.. came right off. The keys seemed pretty clean but I threw them into a pill bottle half full of hot water and a few drops of dishwashing detergent, shook it up a bit, and let them sit. The two halves of the case and the battery compartment door got a good scrubbing in the kitchen sink with a toothbrush, hot water, and some dishwashing detergent. This brightened up the case considerably, and didn't require any more work, other than letting everything dry overnight (due to the sheet of mesh glued to the inside of the speaker grille).

Reassembly was just as easy as the original tear-down. I keep little parts sorted into separate pill bottles so they don't get lost, so it didn't take very long to reverse the process. The speaker was glued back into place with a few dabs of hot glue (every cosplayer's best friend). Once everything was snapped back together I thought it couldn't hurt to put new batteries in and see if it would work. While the power LED did light up the dialer didn't respond to the keypad. It could be any number of things, from a bad solder joint someplace to a bad UM9559E tone generator chip. I could troubleshoot it, if only to learn how to use my oscilloscope, but at the moment it's not a high priority for me. I have a new museum piece, so I'm happy with it.

If you'd like to see the pictures I took here's the gallery.