                     MOTOR SPEED CONTROL UNIT
                     ФФФФФФФФФФФФФФФФФФФФФФФФ

       +12...15VФФФТФФФФФФФФФФФФФФФФП
                   Г                Г
              47R ллл               Г C
              1/2Wллл              /       BD649 or equal
                   Г         B  Г/      NPN Darlington transistor
              1K  ллл<ФФФФФФФФФФГ
       speed      ллл           Г\\
       adjust      Г                \\ E
     potentiometer Г two diodes      Г            to tape recorder
                   Г 1N4007          Г            motor + terminal
                   РФ>УФФ>УФФФФФФФФФФСФФФФФФФФФФФФФФФ>
                                                 (remove internal speed
                                                  control unit from motor
                                                  case). Connect motor - 
                                                  terminal to ground)
If RFI problems exist, add 1nF ceramic capasitors
between base and emitter and also between collector and emitter.

You should get typically about 70 RPMs (or more) at the beginning of the
tape on the take-up (smaller) C-cassette reel, when recording. This makes
1000 to 1200 LPM speed decodable with the tape recorder.
When playing back, the speed shoud be able to go down to around 5...10 
seconds / round on the take-up reel.
   This is just my quick and easy proposal to make the speed control work. 
There are numerous other, less easy ways to do it, just don't believe 
the pulse control gives any better results without a feedback from the motor, 
than any other type of DC control without feedback.