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. http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320130-1.gif On the 2001, we see a 2n3904 flipping the polarity of the MOTOR signal, and the darlington setup of 2n4401 and TIP29 used to drive the MOTOR The 2001N and the B series uses the same circuit http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8032/8032029-03.gif The 8032 swaps out the 2n3904 for a 2n222, likewise for the 2n4401 http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/8296/324644-3of9.gif The 8296 uses a 7416 instead of the first stage transistor, and ditches the darlington arrangement for a single TIP110 http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/vic20/251027r2.gif 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. Interestingly, I can understand that in light of the lack of darlington (no need to figure B-E voltage drop through the first part of the darlington), but it seems odd that the 8296 didn't use a darlington and used the 7.5V zener as earlier circuits. To me, that implies the 8296 cassette motors run overvoltage. http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/250469-rev.A-left.gif The 64 keeps the 2sc1815, but uses a 2sd313 for the drive 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 http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c128/310378-3-left.gif As does the C128. http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/b/4256041-11of15.gif B goes back to the PET circuit. Questions: * Why was the darlington pair needed with the 2n3904/2n2222 setups but not with the 2sc1815/7406/7416 setups? Is the TIP29 that hard to toggle? * The 2n3904/2n2222 and TIP29 are commodity. Why did the VIC go with more specific transistors? * 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? Jim -- Jim Brain firstname.lastname@example.org www.jbrain.com Message was sent through the cbm-hackers mailing listReceived on 2012-02-17 07:00:03
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