Re: CASSETTE MOTOR DRIVE Questions

On 02/17/2012 07:11 AM, Jim Brain wrote:
> Looking over the Cassette motor circuit on the various Commodore units,
> I noticed Commodore used a few different drive circuits through the
> years. My transistor theory is very rusty, and I'm hoping someone else
> can shed some light on the differences:
>
> All of them implement a similar circuit.
>
> An initial circuit flips the logic level of the MOTOR signal.
> THe second circuit bleeds 9-12VDC via a resistor into the base of a
> power NPN, while tying the Collector to the voltage, and the Emitter to
> the cassette motor. A Zener on the drive's base holds the voltage from
> getting too high.

That circuit is a voltage regulator (power transistor, resistor and 
zener diode) with a switch to turn it off. The circuit will supply you 
with about 6V unless you use the 7406 stage or transistor to short the 
zener diode to ground.


> The VIC uses a 2sc1815 for the first stage and a 2sd880 for the driver,
> no darlington, and the zener goes from 7.5V to 6.8.

Yes, since you only have one base-emitter stage now, you need a 6.8V 
zener to get the 6V output.


> To me, that implies the 8296 cassette motors run
> overvoltage.

 From what I remember, those motors were also meant to be run in battery 
powered equipment and don't care too much as long as the voltage doesn't 
get too high.


> http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/plus4/c116-251239-1.gif
>
> The +4 keeps the 2sd880 drive, but uses a 7406 for the first stage

I have a C16 with a TIP29 instead of a 2SD880 and a +4 with a 2SD313. So 
it looks they again used what was handy...


> * The 2n3904/2n2222 circuits have a 10K to ground on the first stage
> transistor base. I assume this weakly holds base at GND until a
> signal appears. This does not show on the VIC,64. Why not?

You sure? My schematics for a 250407-board show R3, a 10K resistor to 
GND on the first stage. And a photograph of such a board also shows that 
resistor being present.

  Gerrit

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