Watkins Copicat IC400 Belt Drive
aka WEM Vari Speed
More eighties than sixties, but very interesting anyway
Drawings are provisional and not checked
This information comes from a strip down of serial number 85542. It's too complicated to show in one readable schematic and so it's presented in three main parts;
the audio control path
the bias oscillator and PSU
the motor control section
The bias oscillator and motor control share the same circuit board, the audio circuit is on another and parts of the PSU appear on the chassis and both boards.
Judging by the marks on the back of the pcb, what was probably a bridge rectifier has been replaced by four 1N4002 discretes. The only other changes seem to be a replacement send / return jack socket and a T2A fuse in the mains circuit. The serial number label specifies a T160mA for a mains input power of 30VA.
The main problem with this one was a loud mechanical hum while the tape was running. It was cured by stripping out the motor and lubricating both end bearings with sewing machine oil. Oddly enough, it turned out to be the top one causing the trouble.
RV103 is best left alone, but it can be set to give a sensible range of adjustment on the front panel vari-speed control pot. If it's set too fast the speed will max out before the pot is fully clockwise.
Although it's not a problem with an amplifier, the 'Send-Return' facility is a bit annoying if you want to use it in a mixer effects loop. Like the IC300, it doesn't disconnect the straight through signal. Input sensitivity in this mode is set by the Channel 2 level pot (not marked but, logically, next to the socket).
Component reference numbers are shown for convenience only. They're not WEM factory numbers since I couldn't find any at the time.
Copicat IC400 Audio Path
Copicat IC400 Audio Board Layout
Copicat IC400 Motor Control
Copicat IC400 Bias Oscillator and Motor Control
Copicat IC400 Bias Oscillator and Motor Control Board Layout