On 02/17/2012 07:32 PM, Jim Brain wrote: >>> 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... > Hmmm.... In the TIP29 C16, what did they use as a bias resistor? 470 Ohm, same as with the 2SD880. >> 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. > I was using this schematic: > http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/250469-rev.A-left.gif That's the schematic for the last rev. of the C64 board, the one with the 64pin Super-PLA. There they had optimised about everything that could be, down to eliminating the 556 for reset timing, using a gate from a 74LS14 instead and integrating the 2114 color RAM into the 64pin IC (250469 Rev. B). > Interesting that other versions used the darlington arrangement: > http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/251138-2of2.gif That's a 250407 or earlier, still has the discrete PLL, not the 8701 used in later revision. > > While others used the TIP29 without a darlington arrangement: > http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/c64/251469-2of2.gif That's the schematics for a 250425, using the 8701 clock generator. And looking at a photograph of said board, yes, it doesn't have the darlington circuit anymore. > The use of TIP29Bs in some of the designs without a darlington T to help > baffles me, based on Bil's response. TIP29B has better breakdown > voltages, but still the same small Hfe of 15. Looks like the tape drive didn't need that much power and the 470 Ohm resistor was good enough to get the TIP29 going. Gerrit Message was sent through the cbm-hackers mailing listReceived on 2012-02-17 20:00:14
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