11
allows free running oscillator IC10 to feed divide by 2 flip-flop, IC8B. This
divide by 2 feeds both the dot counter, IC4, and solenoid timer IC6. This
solenoid timer is used to set the "ON" time of the printer's solenoids. If set
too long, the solenoids will overheat, if set too short, the printed lines will
be too light. An "ON" time duration of 400 microseconds is the best compromise
setting. Decade counter IC4 has its B and C outputs tied back to its zero reset
inputs forcing it to become a modulo 6 counter. As the counter progresses from 0
to 4 inclusive, the selected character lines are decoded by the ROM, IC2 using
the ASCII data fed to it by FIFO memory, IC1, and fed onto the solenoids thru
buffers IC3 and IC7 and Darlington drivers 01 and Q7. Counter state 5 is decoded
by the ROM as a blank for the one dot space necessary to separate the printed
characters. In the latter phase of the five count the FIFO is shifted, the
counter resets, and the sequence repeats. When the FIFO finally empties, the ROM
is disabled and blanks the output for the rest of the print cycle. When the still
moving print head reaches its normal "rest" position, the roller arm microswitch
again changes states. This time RS latch NAND gate IC11C flips low setting
control flip-flop IC8A back to normally high state.
Timer IC5 is a power up clear circuit which empties the FIFO memory and
helps prevent random firing of the print solenoids during power up.
Note
When turning the printer off it is best to use the toggle switch on the
front of the printer chassis. This toggle switch not only interrupts the 120
VAC being delivered to the power transformer on the printer but also
disables the 40 VDC line feeding the solenoids. It is necessary to break this
line while the DC voltages in the system are bleeding down in order to prevent
the print solenoids from randomly firing and printing a vertical line on the
paper.
In Case of Problems
In case of problems, the best procedure is to remove power and recheck all
assembly steps. Since you are probably feeding the printer with a computer
system, don't rule out a possibility of a problem either in the interconnection
between the two or in the program driving the printer.
As far as the printer itself is concerned, check the voltages on the PR-40B
board to make sure the DC voltages are within 10% of their specified ratings as
shown on the PR-40B schematic. Other than this the only thing you can do is go
thru the PR-40A board components one by one until you can pin down the source of
the problem.
If you still cannot find the problem or do not wish to service the unit
yourself, repair services are available from us at a cost of $20.00 labor plus
parts.