Turn a Vintage Transistor Hearing Aid Into an Amplified Microphone for Nostalgic Sound

Hearing aids.. Shunned, ignored, unwanted when old. You want to talk about an electronic device that changes a persons world when new and has little value when old? It's the lowly hearing aid.

In this instructable we take an old transistorized hearing aid and turn it into a powered microphone. This can be done for effects as they distort in a real gritty way or it can be used as a poor mans powered mic. I came up with this idea while wanting to do some field recordings under the title of "life through a 1950's hearing aid".

Supplies

In this instructable we take for granted that you can solder, work with small tools, and have some basic understanding of electronics. You'll need an early transistorized hearing aid, a soldering iron with a small tip, and some basic small hand tools.

Step 1: Finding a Suitable Hearing Aid to Mod

Hearing aids come in all shapes and sizes. Just like any old device, some can be valuable on the collector market. If you're looking for one to hack on, try going for a unit that's cosmetically challenged as you may be modding the case anyways.

What type to look for?

You're looking for what is a called a body pack or pocket hearing aid. These are devices about the size of a deck of cards that had a coiled up cord that went to an earpiece. In modern times it seems everybody is carrying a little box about the size of a deck of cards with something plugged in their ear. Back 50+ years ago, there was quite a stigma about being seen with such a device so manufacturers went out of their way to make the device elegant looking and the cable and earpiece hide as best they could.

Body pack hearing aids come in 3 types, transistorized, vacuum tube, and hybrid. The tube units were made up until the early 50's. Hybrid units using both transistors and tube were made for a short time in the early 50's. Transistorized units were made from about 1953 onward. Hearing aids were the first commercial use of transistors. Early transistorized units will brag about being "tubeless" or "transistorized".

There were some tube and hybrid tube/transistor units made that look like their all transistor counterparts. Those can be spotted by looking in the battery compartment. Tubes need higher voltages to work along with needing a low voltage for the heating filaments. Hearing aids that use tubes will usually have multiple batteries and they will be of at least two different voltages. Transistorized hearing aids usually have a single battery that is 9v or less.

Step 2: Open It Up and Test It

So you found a candidate to mod. You'll need to open it up and test it. Many of these units used a 1.5v alkaline or 1.4v mercury battery in an odd size called "N". On those, you'll need to rig up something with a battery holder or alligator clips to get battery voltage into the unit. Any AA or AAA battery will work fine in those situations. If you're unit uses some other type of battery, all that matters is you get the correct voltage and polarity fed into the unit for test purposes.

On my unit I decided to solder in a AA battery holder from RadioShack. It looks ungainly but then again this is a hack mod so functionality is more important to me right now.

In the pic you see a Zenith unit with the back taken off. On the left side center you see two gold contacts on the edge of the unit. That is where the earphone plugs in. If you don't have the earphone that came with your unit, wiring in any wired earbud there will be good enough for test purposes. The connectors on these hearing aid units are usually two or three small pins that click in place when the cord is inserted.

On a two pin it's simple, on a three pin there may need to be some trial and error to figure out which two pins need to be plugged in. Those sockets aren't like common audio jacks. The whole socket is one connection and you need two to drive the headphone. Tinning the wires that go to your test speaker and inserting them should be good enough for test purposes. Make sure your wires don't short against anything else or you risk blowing out the transistors in the unit.

Once you have power and some way to hear your audio, you can test. There may be one or two rotary controls on your unit. One is volume and power, the second if present could be a tone control. On some units there is a switch to put it into telephone mode. This activates a pickup coil in the unit for use with old style home telephones. For testing you want to be in MIC not TEL mode.

TEL mode can be cool once you have a working unit as it uses a pickup coil to inductively pickup a/c signals. Putting it up to random electrical items can yields cool sounds.

If all is well and you are confident you wired things up right, you should be able to hear audio through the test earphone when the unit is on and turned up. Be careful if using the factory earphone. These things can get real loud real fast. If it seems to be working, you're ready for the next step. Even if you have low audio, it's still working so move to the next step.

BOO IT DOESN'T WORK :-(

That can happen. We are after all talking about some very old transistors here and there's no telling what abuse the previous owner subjected the unit to. Recheck your wiring. If you have a 3 wire earphone jack try different combinations with your 2 wires. Still no dice? Check the power switch with a continuity meter. The mechanism to the power switch is easily seen in the bottom left of the picture. It's just a lever that sits on a cam and clicks against a contact when the unit is switched on. That contact can get corrosion on it rendering the unit inoperative. A little clean up with a tiny bit of sand paper on the contact points and it may come back to life.

If the unit is still dead, it may be time to find another. Your unit may have some blown transistors rendering it beyond the scope of this instructable. There is a strong possibility your unit has a shorted capacitor causing it to not pass any audio. If that's the case you might be able to fix it by going ahead to the next step. Keep in mind though, you'll be doing a bit of work on a unit that may still be dead in the water.

If you decide to give up and find another unit, don't toss it. Collectors may buy it via online auction even if it doesn't work as most just display them on a shelf.

Step 3: A Little Circuit Refreshing

In order to assure best performance and sound, a little refreshing of some electrical components is in order.

Pics 1-6 show the parts that need to be replaced in this particular Zenith. Not all units look the same inside. The parts you need to replace though are similar for all units from this era. You need to replace all the electrolytic capacitors. They dry out and go bad with age. This will cause no, low, or distorted audio.

Modern capacitors will be substantially smaller than the old ones you will be replacing. Chances are you will not find the exact same values that are in your unit. It's ok to go higher in voltage but never lower. As far as capacitance is concerned, going as high as twice the original value in an audio circuit is ok.

When replacing electrolytic capacitors it's VERY important you do NOT reverse the polarity. Old caps will be marked with either a + or a - symbol to indicate which end is positive or negative. On modern capacitors they only tell you which end is negative. Don't reverse them.

In pic7 you see all 4 old capacitors have been replaced with modern counterparts of higher voltage rating.

Step 4: All Done, Enjoy It!

In the pic you see my finished test unit. I soldered in wires to the old headphone jack so I have an RCA jack I can just plug into a line level device. I also have an external AA battery holder which is behind the unit. I had no intention of buying expensive special sized batteries. The battery holder can be epoxied in place or mounted using heavy duty double sided tape. The end result is basically an amplified mic with a unique early transistor sound to it.

What happens when things go wrong?

My mod unit ended up having a transistor that partially failed once I used the unit a bit. I'm not surprised as hearing aids were the first consumer use of transistors. I *think* this thing may have a push/pull audio section and one transistor quit, giving me only half the waveform. Unfortunately I don't have a schematic for this unit. It's making for some crazy distortion a guitar player might actually find to be cool. After listening to a few recordings it dawned upon me what I was hearing.. Daleks from Dr. Who! Listen for yourself at a recording I made.

http://kiwi6.com/file/m6i7siho99

The failed transistor renders the unit useless for quality audio but makes for some cool effects possibilities. I will now call this unit my "insta Dalek" and seek another unit to hopefully build a clean audio version.