Interesting Junk

Of an electronic variety, both home made and manufactured, Stain explores the workings and reverse engineers some of the internal circuits.

There's loads of junk out there although this page will concentrate on odd/rare stuff of interest. Loads of VHSs and DVD machines were built in the last 20 years or so but because they're so common and not particularly interesting, these will not be included.

This piece of junk is the Royal Call all-transistor tape recorder. You might think that, "This tape will self-distruct in five seconds. Good luck Jim!" The basic design is inspired by the Mission Imposible TV series of the 1970s and no doubt, helped the sale of many units. However, the actual machine featured in the programme was a much better precision instrument.

Watch the Youtube demonstration:

Upon first encounter of one of these, it was owned by a family with connections in Cyprus. Because phone-calls were quite expensive in the 70s people overseas recorded greetings to each other, sent them via the post on birthdays and similar. For this purpose, it and similar units performed quite well. Vinyl LPs and 7" singles were also costly and trying to use one of these for music purposes led to dissapointment. Because of a headphone out socket, The younger Stain tried connecting a similar unit a to valve amplifier and speaker system in a vain attempt at improving fidelity but it was a complete waste of time and effort.

These tape recorders can be internally divided up into two: the (1) mechanism and (2) electronic amplifier. The Royal Call motor is mounted on a rocking bracket set into place by the yaxley switch behind the control knob. In this example, the little ball bearing inside the switch keeps falling out and has frequently been lost/replaced. In one switch position, the rewind is activated, centrer position is off. Turned to the right, the motor rocks and makes contact with the spool table and the unit is set to playback. In a similar way that a intercomm works, the switch also reverses the input/output to the amplifier to either the record head, speaker and microphone. The push-pull amplifier is a simple paxoline panel containing a germanium four-transistor transformer coupled circuit. Close scrutiny of the circuit board (right) will reveal a black capacitor amongst all the other grey ones: this is the replacement and is a 10uF/16v. The original one failed, disabling the record function. This is a common fault on these units and the capacitors tend to fail because of the low polarisation voltage applied to them in this configuration. Electronic engineers will tell us that electrolytic capacitors need a polarisation voltage for correct operation. This capacitor isolates the relevant transistor stage from the tiny DC voltage driving bias current applied to the R/P head in record mode, and if it wasn't there, the DC conditions of the relevant amplifier stage would be upset. A non-electrolytic capacitor would be more suitable here, if it was possible to make one small enough of sufficient capacity.

 The Miny (pronounced my-nee) all transistor tape recorder.

Unlike the Royal Call, the Miny has two motors, one for each spool table. The record/play amplifier for both models is a four transistor transformer coupled germanium unit. Output: about 250mW.