Telephone Cassette Eavesdropper Mods

I am living in a temporary house at the moment whilst my normal place is undergoing extensive damp treatment and repairs, as a result of this I don’t have access to my workshop to continue with my current projects, so I have been doing some little hacks instead just like the old days. Here is a good’n:

I picked up a telephone conversation recorder from Caerleon Car Boot sale for £1, this is basically a compact and portable cassette recorder with an inbuilt mic, speaker, line input and headphones out. I added a couple of mods to the unit detailed below:

The unit including mods

The unit including mods

When I first tested it out I was impressed by the sound quality of the inbuilt microphone and mini speaker which can kick out a fair volume as well! The line input is also good, I was playing drum loops through it from my laptops headphones out and get some lovely tape distortion when the levels were cranked. The headphones output wasn’t so good however with a more-than-noticeable electronic ringing noise whenever the cassette was in operation. I wanted to open the unit up to see if I could find a speed regulator for the motor, and to make a better output from the speaker terminals. Before I opened the unit up though I made this:

Loop Tape

DIY Endless Loop Tape

I wanted to see if I could make the unit a self contained tape delay but after some research and a lot of poking I decided that you need more than one tape deck, or a unit that has separate heads for recording and playback (this one just has the one). I made this rather sketchy endless loop tape anyway to see what it could do. I glued a rubber band around one of the cassette spools and just made a loop out of the tape using sellotape to join. This loop lasts about 5 seconds at normal tape speed, I tried making it longer by wrapping the tape around some of the structural joins in the cassette, but this adds to much tension causing the player to stop constantly. Next tape I make will be a little tighter as this one is quite loose, though I like the feel I get with the results as you get little bits of random warble when recording. Be warned, if you try this yourself it is very fiddly, very damn fiddly!!

Before modding the unit I tried recording onto the loop tape but ran into problems, cassette recorders work using a combination of heads, sometimes three; playback, recording and erasing. This unit only has two, with one of the heads switching between recording and playback and one for erasing (which is why there was no chance of this unit working as a delay as the play/record head can only do one thing at a time). Anyway, the problem I had was the tape would erase a couple of seconds in normal tape time before recording anything, so in a five second loop you would have around 3 seconds of zilch. In an old repair book, I read that the erase head and record head work in conjunction through some kind of oscillation recording and cancellation process, so I hoped I would be able to get inside the unit and find some cables I could cut and add a switch in order to bypass when I wanted to.

The erase head was difficult to reach as I could only just see it poking out from the cassette compartment, and it was buried under electronics and mechanical parts on the inside, I was determined so I completely disassembled the player to get to it. I was surprised to find that the erase head had no circuitry and was just a magnet that was mechanically shunted into place when record was pressed, there was no simple way to bypass this. I used a pair of angled nose pliers to pinch a bit of plastic and remove the head completely:

The Eraserhead

The Eraserhead

With this removed, interesting things start to happen: when you record onto the loop tape (or any tape) you can actually start layering up sounds, instead of older sounds being recorded over, they become ghostly background elements instead – this is an amazing tool for creating drone loops!

Onward to the mods…

On the circuit board I quickly found what I was looking for, even from the underside I could see that there was a preset potentiometer, and as I suspected, turning this altered the tape speed. Tracing the circuit, I could see that a resistor was connected in series with the preset pot, which was probably there to act as a maximum speed cap to protect the motor from overload. I decided to replace this resistor as it was easy to cut away, and turn the preset pot to have a little resistance to stop the motor burning out – with my replacement pot I can control the unit to go anywhere from super shlow-mo to rewind/fastforward speed! Layering loop tape + variable record/playback speed = hours of fun! I didn’t have the best potentiometer to hand to do the job so I will replace it with one with a more dynamic range once I have access to the workshop again. An added bonus for me was that when recording with the inbuilt mic at high motor speeds, the unit picks up a lot of the mechanical noise of the unit, on tape this is a lovely whirring sound kinda like you would imagine coming from a vintage reel to reel film projector, I love it anyway!

The output was simple, because the speaker amp is volume controlled, I just wired a mono mini-jack socket in parallel with the speaker terminals, tested and works a treat! I want to add a switch later to toggle between the two outputs, though it isn’t really necessary. Here is a pic of the units guts with the mods added:

Tape Player Guts

The Guts

All in all, this was a great and fun project, I would recommend anyone give it a go, it is fairly straightforward and the units are really intriguing to take apart as they are real masterpieces of design, coming from an era of hybrid electro-mechanical construction! All in all, the whole project including the materials to do the make the loop tape cost under £5!

Sound clips:


I think some reel-to-reel experiments are in order next…

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