Board Issue No 2 2 3
ULA Type 5C112 6C001 6C001
Component
R47 220 a Ik Ik
R49 8 k2 10 k 10 k
R56 220 n 470 fi Ik
R63 220 n 470 n Ik
1.3 Modifications - Issue 1 Board
A number of modifications were introduced, and these should be checked
for and, if necessary, introduced retrospectively:
(a) 100 pF capacitor between RAS and ground - necessary only when
using ULA 5C102.
(b) 470 pF capacitor between IC2, pin 28 and ground - required only
when IC2 and RAM ICs are all of NEC manufacture.
(c) 1 k resistor between RAS and 23V and 1 k resistor between CAS and
12V, only when RAM ICs are of National manufacture. R57 (330n)
must be removed, and the 470 pF capacitor is not required.
(d) 47 k
resistor
between
pin 13 of
LMI889
and
ground
-
required
only
if the difference between white and yellow colours is inadequate.
(e) If disc capacitors are used for C41 and C49 (47 nF) they must be
replaced by axial components.
(f) Capacitor C46 (1 uF electrolytic) should be replaced by a high
temperature component.
(g) This modification is required for efficient operation of certain
machine code software and should be implemented on all units. It
has been implemented in manufacture for all ULAs 5C112-2 and
later.
D14 replaced by C67 (100 pF)
R24 changed from 3k3 to Ik
R27 changed from 680 n to 470 n
R73 (Ik) added between Id/32 and +5V
(h) A significant cause of problems has been found to be failure of
the
internal
power
supply
transistor
TR4.
The
circuit
may be
improved by changing the value of R60 from 100 n to 270 fl and
connecting a 4.7 uF electrolytic (perferably radial type) with
positive end to TR5 emitter (positive end of C34) and negative
end to TR5 base (to R58).
4.4
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