required for IC8 and IC9(a) to self reload to 26. Any fault in this
resetting will cause adress zero to be accessed and this will produce the
illegal character da-dah-dah-dah.
With number generation IC9(a) remains held with pin 1 high and IC8 counts
down from 26 to 17. Here the never-normally-accessed address is 16. If, due
to a fault, it is accessed the illegal charcter produced is dah-dah-dit-dit-
dah.
In the "numbers" mode D7, D6 and D5/R12 ensure that IC5 is allways loaded
with binary "3". The character lenght then being 8-3 = 5.
In the "mixed" mode IC9(b) extends the adress counter by an extra bit and its
output determines whether the next character is to be a number or a letter.
4. Typical faults
The following faults have occurred at various times in the past.
(a) Accidental unsoldered feed through pins or IC top pins can cause illegal
letters to be produced.
(b) Some specimens of IC10 have resulted in random clock pulses which are
too narrow to clock IC8 correctly. This results in the illegal
addresses 0 and 16 being accessed. The pulse train at pin 11 of IC10(b)
should look symmetrical about 2.5 volts.
(c) If Q4 does not turn fully "off" Q3 can remain slightly "on". When VR2
is set to a high value this can cause a very long, even an infinite
delay to occur as charge leaks through Q3 faster than VR2 can supply it.
R20 was added later to avoid this.
5. Other points
(a) Reducing C1 allows faster sending speeds to be achieved.
(b) Adding a jack to the rear panel to allow an external supply to be used
can short out the regulator if an uninsulated jack is used. This is
because the back panel is at +5 volts. If this happens the TTL PROM
will probably be damaged.
(c) If an output for a tape recorder is required from the earphone jack a
resistor (say 100R) is required to maintain a DC path for collector
current in Q2.
