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                     BIGBOARD MODIFICATIONS 

Released to the Public Domain 5/15/82 by
     Trevor Marshall,
     359 Wynn Ct., Thousand Oaks, CA 91360
     (SYSOP T.Oaks Tech RIOS & RBBS, phone (805) 496 9522 )

(C Copyrigh 198 b Trevo Marshall al right reserved thi 
wor i no t b communicate wit  profi motive.

Note:
The mods required for Double Sided Double Density disk operation,
 Extra banks of ram (using 64K RAM chips in the 4116 sockets),
 Ultrafast Disk Cache buffer BIOS (using one full 48K bank)
 an Infosof Operatin syste hav bee withhel unti  ca ge 
 someon capabl o adequatel supportin hobbyist implementin 
 them. Your suggestions will be welcomed.

ACHIEVING RELIABLE 5MHz OPERATION

YOU WILL NEED A HIGH SPEED EPROM FOR YOUR PFM MONITOR.
Th 2716-1 distribute b D wit man o thei kit wor fine 
 use th on the supplie fo th characte generator whic 
onl need an 'Garde variety EPROM Ensur yo progra you 
fas PF monito befor attemptin thes mods.

1.  Th RA arra i refreshe b 12 RA onl cycles Th dela 
needed to clock in the column data is provided by U76.
T operat a 5MH w mus cu th track t pin  an  o 
U76 and move the CAS line to pin 4 and the MUXC to pin 3.
The Z80 clock can then be changed from U96 pin 4 to pin 5.
Th cloc lin fo th Z8 chip ha t b dampe a i i 
resonant and the consequent ringing causes problems.
Tw 3 oh resistor ar needed on i serie wit th 
collecto lea o Q2 whic shoul b change t a MPS3640 
an on i serie wit pi  o U77 whic mus b change t 
 74S04  3K resisto mus b soldere betwee pin  an  
o U7 t yiel  mor symmettrica 20MH waveform.
Th trac t pi  o U7 i cu o th to o th boar 
between U77 and U79.
Th 3 oh resisto i soldere betwee U7 pi  an th 
plated thru hole adjacent to pin 16 of U80.

More formally,
    Cu trac betwee pi  o U9 an pi  o U7 o botto o 
board.
Link pin 5 of U96 to pin 9 of U78
Cu trac betwee pi  o U7 an plate throug hol 
adjacent to pin 3 of U65
Link this plated thru hole to pin 4 of U76
Cu trac betwee pi  o U7 an plate throug hol 
adjacent to pin 8 of U52
Link this hole to pin 3 of U76.

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LOWER CASE MODIFICATIONS

YOՠ WIL NEE  NE DISPLA EPROM se fil DISPPROM.B fo  
hex image for your prom programmer.(It is ORGed from 100-7FFH).
(The display EPROM was coded by Steve Hope, TNX Steve.)

  Remove link from pins 14 to 3 of U11
Cut trace to pin 4 of U11 below board
Lin pi  o U1 t plate throug hol adjacen t pi  o 
U21 (below board)
Link pin 4 of U11 to pin 11 of U9
Cut trace to pin 11 of U9 below board
Link pin 5 of U25 to pin 9 of U60
Link pin 8 of U22 to pin 19 of U73
Cu trac o botto o boar fro pi 1 o U73 i come 
through a plated through hole adjacent to pin 4 of U73
Cut trace to pin 11 of U9 between U9 and U10 on top of board
Cut trace to pin 12 of U10 between U10 and U11
Cu trac t pi  o U25 I emerge betwee pin  an  o 
U2 an disappear betwee pi  an  o U2 o to o 
board.
Lin pi  o U1 (earth t th plate throug hol o th 
trac whic use t g t pi  o U11 i i adjacen t pi 
13 of U22.

IMPROVING THE VIDEO DISPLAY CIRCUITRY

1. Improve composit vide waveform resul fro linkin TB 
pi  t pi 2  56 oh resisto shoul b betwee TB pin 
 an 4 R2 shoul b reduce t 56 ohms C14 shoul b 
omitted. TB1 pins 10 and 11 should be jumpered.
R2 should change to 1200 from 820 ohms.

ICs WHICH SHOULD BE PURCHASED WITH CARE

U75 should be 74151
U77 must be 74S04 SHOTTKY TTL
U76 should be 74164
U59 and U58 must be 74157
U112 should be 7414 to drive the floppy disk signals
U109 should be 7445 for the same reason
Al Z8 chip shoul b Z80  types althoug onl Z80 
CTC,SIϠ an PI ar currentl available The see t wor 
well, as does standard 250 nsec memory.

......Trevor Marshall, last revision 20 Sept 1981.
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