Service manual

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CIRCUIT THEORY
U6D
6522
+5V
PBo~1~0--~-----+-~2 V~
r Triger
Rewt >--+_"""1
4
rewt
r--_-+-----.,;6:..j TH
OUT 1-"3_..JV'''---I
022
CR28
R39
7
20 kn"loE----~DIS
uaa
555
cv GND
5 C8
.r.
0
'.'"
TIMING CHART
10 JLs
-tt-
R37 2.2 k
~U~6~D.~P~BO~----·U~-----------------
USB·OUT
Q22·COllECTOR
TP1
I. 500 JLs
.1
Print Wire Coil Protection Circuits
+5V
Reset circuit
.----~TPl
R23
3.9 k
When a print wire is fired, the output PBO on U6D goes "low". This triggers the 555 USB. The output
pin 3 goes "high" for 500 microseconds. This turns on 020,021, and 022. The potential developed
at the collector
of Q20 is used as + vee for the open collector Hex/Inverters U5B and U6B. R39 should
be adjusted
to obtain the 500 microsecond pulse.
14