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This is only a preview of the October 1997 issue of Silicon Chip. You can view 28 of the 96 pages in the full issue, including the advertisments. For full access, purchase the issue for $10.00 or subscribe for access to the latest issues. Items relevant to "Build A 5-Digit Tachometer":
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polarity and correctness of all components, and replaced ICs 1 & 2 but still
have 1V at these positions. (B. M.,
Wellington, NZ).
• The fact that points which should
measure +4.5V have only 1V present
suggests that the voltage divider consisting of two 220kΩ resistors includes
a resistor with the wrong value or the
associated 10µF capacitor is the wrong
way around. Our bet is that one of the
resistors is 27kΩ instead of 220kΩ. We
assume that your 9V battery is fresh
and is delivering 9V.
Query on
TV sound
Can you please tell me if the TV
stereo sound format is the same used
for FM broadcasting apart from the
different deviation and perhaps different pre-emphasis? I am wondering if
the same stereo decoder chips can be
used. (P. C., Glenhuntly, Vic).
• While both FM radio and TV sound
are frequency modulation systems,
their deviation and de-emphasis are
different. However the major difference is in how the stereo channels are
encoded. FM radio uses a multiplex
method with the modulation switched
between left and right channels at a
rate of 38kHz. By contrast, stereo TV
uses two separate subcarrier frequencies, at 5.5MHz and 5.742MHz above
the vision carrier.
Hence, the methods of decoding
stereo TV are quite differ
ent from
FM multiplex stereo and different IC
decoders are required.
TV pattern generator
programming
I have purchased your new Colour
Television Pattern Genera
t or and
wonder if you could explain the addressing of the EPROM (IC1) via the
four 74HC193 counters. It appears
unusual and I wonder if it makes programming easier. The least significant
bit of the counter is connected to A1
on the EPROM but the most significant
bit goes to A10 instead of A15. Is the
software available now from SILICON
CHIP? (C. M., Salisbury, SA).
• This project used unusual addressing to the EPROM to simplify the PC
board design. Since the EPROM would
be programmed and run with the same
addresses then it is not important
which order the address lines are ac-
Guitar/PA amplifier
wanted
I wish to obtain information on
the building of a guitar/PA amplifier. I have long been impressed with
the tonal reproduction of valve
amplifiers. My requirement is an
amplifier with a 100W capacity
and some (or all) of the features
of the Fender amps of the 60s era
(reverb, echo, etc). I would most
appreciate any advice you can offer
me with regard to circuit diagrams,
sources of supply of valves, power
transformers, audio transformers
and speaker enclosure dimensions.
(J. S., Isle of Capri, Qld).
• We have not published any
circuits for valve power amplifi
ers and nor do we think they are a
practical or economic alternative to
a well-designed solid state amplifier. For a high performance guitar/
PA amplifier, we strongly suggest
the 175W module presented in the
March 1997 issue of SILICON CHIP.
You might also consider building
the Digital Effects Unit published
in the February 1995 issue, to provide effects such as reverberation,
echo, vibrato, etc.
cessed as long as it is consistent. After
all, the address labelling on the EPROM
is only arbitrary anyway.
The software is available from SILICON CHIP for $10.00, including postage
and packing.
4Ω loudspeaker. The only way to get
the full power would be to use a DC-DC
inverter. We published a suitable project along these lines in the December
1990 issue.
When do you
change the filter?
Notes & Errata
I have a water purifier but I never
know when to change the filter. Can
you design a project to detect when the
filter needs changing? (D. S., Miller,
NSW).
• Unless the conductivity of the filtered water changes over time, as the
filter deteriorates, we cannot think of
any electronic method which could
tell you when to change the filter.
Perhaps one of our readers can suggest
a method.
Amplifier module
needs DC-DC inverter
I am enquiring about details of the
50W amplifier module described in
the March 1994 issue of SILICON CHIP.
The kit runs on 25VAC and I was wondering if the circuit can be changed so
that the amplifier can be run off a car
battery with the same power output.
Also, I was wondering how to change
a stereo signal to a mono signal. Do
you simply join the left and right
channels to make one channel? (N. B,
Gladesville, NSW).
• In order to deliver the full 50W,
this module needs a DC supply rail of
±35V or 70V in total. If it was to be
run directly from 12V DC, the power
output would be less than 4W into a
Colour TV Pattern Generator, June &
July 1997: the patterns produced by
the TV Pattern Generator are slightly
off-centre on the TV screen due to a
slight displacement in the line sync
signal. In most cases, the normal over
scanning of each line on the TV screen
will mask out this small shift.
It can be corrected by adding an RC
network to delay the line sync by the
requisite 1.5µs. This involves adding a
4.7kΩ resistor between the D7 output
of IC1 at pin 11 and the sync input
of IC10 at pin 16. The pin 16 input
of IC10 is bypassed to ground with a
270pF capacitor.
The resistor is best placed instead
of the link on the PC board above the
three 330Ω resistors near IC10. Note
that IC10 has an incorrect pin 1 labelling on the PC board. The position
shown for pin 1 is actually pin 16. The
capacitor can connect from pin 16 to
pin 1 of IC10 on the underside of the
PC board.
Flexible Interface Card For PCs; July,
1997: there are two errors in the Basic
listing shown on page 28. Line 90
should read: B$ = RIGHT$(TIME$,2):
WHILE RIGHT$(TIME$,2) = B$:
WEND ‘wait one second. Line 220
should read: LOCATE 24,20: PRINT
“Line”;LIN; ‘print it. Note: do not put
full stops at the ends of the lines. SC
October 1997 93
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