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| Stylophone |
I just repaired a nice little original STYLOPHONE DJT. The poor thing had taken a fall onto a corner (the same thing that shatters IPod screens) and had a shattered circuit board and was not working. The device is simple enough that some solder bridging the cracks and a jumper bypassing a missing piece of board brought it back to life.
This unit (original 60s/70s) uses a PUT - programmable unijunction transformer - with a resistor ladder to form the main oscillator. A NPN transistor and associated .1uF caps and 68k resistors form a phase shift oscillator creating the vibrato by wiggling the gate voltage on the PUT. Another transistor acts as the output amplifier.
New models of this device apparently use a 555 timer IC as the oscillator. I have not seen one so I have no comment. Tonal differences have been reported. I might scope the oscillator output to get a look at the waveform of this original one if I get around to it. A UJT/PJT Osc is most likely a sine or triangle whereas the 555 is most often used to generate square waves (or a near triangle wave if the output is tapped from the timing cap).
The line out on this device has a 154Hz filter before it, which seems odd. The tiny speaker can obviously not drive bass frequencies. Maybe this was to approximate the tone of the speaker when using the line out. R31 and C6 (see schematic below) form the CR filter.
Fc=1/2πRC
R = 22K, C = .047uF
A good cleaning of the output jack and keypads finished up the job and we have a usable STYLOPHONE again!
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| from https://andymurkin.wordpress.com/2011/12/ |