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Background: There is a cassette recorder (RadioShack CTR-80A) and its owner would like to have the ability to speed up and slow down the motor to record and play the sound in a distorted way. We've done a similar thing already to a Sanyo TRC-3460 voice recorder, but its diagram was easier to understand.

Question: Where in the following diagram one would put a potentiometer to achieve a distorted effect (slow down or speed up a bit) when recording or playing audio from a cassette? What value should that potentiometer have?

enter image description here

Also, could you please point me to the resource describing the symbols which are used for switches in this diagram (the ones with white and black triangles). I've search high and low and do not understand what they mean. Well, not well enough, anyway.

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  • \$\begingroup\$ The black triangle indicates a normally-closed switch contact, while a white triangle is a normally-open contact. The earphone jackJ3 has a normally closed contact which will open, disconnecting the speaker, when a plug is inserted. \$\endgroup\$ – Peter Bennett Feb 18 '20 at 5:11
  • \$\begingroup\$ @PeterBennett Thanks! Could you please tell me what the dashed line from S3 to the AC socket (?) stands for as well? \$\endgroup\$ – A Small Gruffalo Feb 18 '20 at 10:05
  • \$\begingroup\$ The dashed line indicates that S3 is operated when the AC power cord is inserted. Note that the NC contact of S3 is connected to the battery, and the NO contact is connected to the diodes which provide power from the AC source. A dashed line like that is often used to indicate a mechanical connection between parts of a multi-pole switch, or other mechanical associations. \$\endgroup\$ – Peter Bennett Feb 18 '20 at 16:53
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In many of these old-school devices the motor speed was controlled by a centrifugal governor on the rotor of the motor, so faster was not an option only slower.

I built this: it worked well enough to be a fun toy:

schematic

simulate this circuit – Schematic created using CircuitLab

it just plugged into the 'Cue' socket (J4 REM on your schematic) placing it in series with the motor. It did not give stable or repeatable speed control

Any reasonably big power transistor should work eg:TIP31 the 2N3055 was just what I had on hand.

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This motor in the lower right corner is connected directly to the 6V supply on top right, so there is no speed control.

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The old school recorder is designed to run on four 1.5 Volt dry cells or a simple unregulated mains transformer .The nominal 6VDC motor is designed to give small speed variations for large voltage variations .This means that reducing motor volts using a 100 ohm wirewound pot or any other means will not give good results .Consider changing the motor pully or getting inside the motor and modding the internal speed governor .

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