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Priority |
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Priority |
The basic CHIPSWITCH
Microcomputer chip has the capability of allowing the user to program any frequency as a
"priority channel". But for the priority channel feature to
work properly, the radio's squelch and agc time-constant circuits must be
modified. The Priority Board installation accomplishes this.
Note that it is not necessary to read the following THEORY
OF OPERATION and CIRCUIT DESCRIPTION sections to install the priority board. They are
included for those who wish to understand why the board is necessary in the first place.
THEORY OF OPERATION
When the priority channel frequency (feature #44) is
programmed and the priority channel is activated, the CHIPSWITCH
Microcomputer chip will periodically perform the following steps:
1 Mute the audio (a positive-going signal from pin 63 of
the microcomputer chip)
2 Send the priority frequency to the Phase-Locked-Loop (PLL) chips (change to priority
frequency)
3 Wait for the PLL to "lock" at this new frequency
4 Sample the "squelch" status to see if squelch is broken at the priority
frequency
5 If the squelch is not broken, goto step 6. If the squelch is broken, goto step 10
6 Restore the original frequency to the PLL chips
7 Wait for the PLL to "lock" at this frequency
8 Un-mute the audio
9 END OF STEPS
10 Un-mute the audio to allow receive on the priority frequency
11 Wait until squelch is not-broken for 3 seconds
12 goto step 6
Since the microcomputer must perform the steps above is
less than 100 mili-seconds (1/10th second) to avoid a long "dead time" when
checking the priority channel for activity, the test performed at step 4 must assume that
the squelch status (broken or not-broken) is based upon the signal level at the priority
frequency, and not the original frequency. And that is what the priority board is for. It
uses the "mute" signal to temporarily "disconnect" a couple of
capacitors from the squelch circuit that are needed for normal (non-priority channel)
operation. The purpose of these capacitors is to "hold" the squelch open during
short signal drop-outs and at the end of a transmission as required for normal radio
operation. As soon as the mute signal is turned off in step #8, the priority board
"re-connects" the capacitors for normal squelch operation.
CIRCUIT DESCRIPTION
* The RED wire brings +8 volts
from the main RF circuit board to supply power to the Priority Board. Power supply Ground
is supplied through the mounting screw and solder lug on the Priority Board.
* The BLUE wire brings the MUTE
signal from the main RF circuit board to the Priority Board. The Microcomputer chip drives
the mute signal to +5 volts to "mute out" (silence) the audio
from the speaker, and to 0 volts to "un-mute" the audio.
* The YELLOW and GREEN
wire pairs are the F.E.T. switch outputs of U1 and U2
respectively.
When the mute signal is present on the
blue wire, current flows through R4 to the base of Q1,
biasing it on. R7 keeps transistor Q1 biased off when no
mute signal is present. When transistor Q1 is biased on, current flows
from the +8 volt supply, through R1, through R5,
and into the collector of Q1. This causes the voltage at Q1's
collector to drop to about .2 volt. Since the collector of Q1
is also connected to the base of Q2, this .2 volt level causes transistor
Q2 to be biased off. Resistor R6 provides an additional
current path to guarantee that Q2 stays completely off even in the presence of stray
Radio-Frequency (RF) that is likely to be present inside the radio during normal
operation. When Q2 is biased off, no current will flow in it's collector
circuit, thus the Light-Emitting-Diodes inside U1 and U2
will be turned off, thus the Field-Effect-Transistor (FET) outputs of U1
and U2 will also be biased off. When the FETs inside U1
and U2 are off, a very high resistance exists between the outputs (pins 4
& 6) of the photo-isolator, thus they act like a switch that is now OFF.
When the mute signal is not present on the
blue wire, resistor R7 assures that transistor Q1 is
biased off, even in the presence of any stray R.F. that may be inside the radio. Since Q1
is biased off, no current flows through it's collector, thus allowing current to flow from
the +8 volt supply through R1 and R5 to
the base of Q2, causing transistor Q2 to be biased on.
With transistor Q2 biased on, current will now flow from the +8
volt supply through resistors R2 and R3 and through the
light-emitting-diodes in U2 and U1 respectively. Since
current is now flowing through the LEDs inside U1 and U2,
the FET outputs are biased on, causing a low resistance (100 ohm approx.) to exist between
their pins 4 & 6. Thus they act like a switch that is now ON.
The connection of the Priority Board to the main R.F.
chassis allows the microcomputer chip to switch a couple of capacitor circuits on and off,
based on the mute signal. The charges developed across these capacitors
are needed during normal radio operation, but interfere with the priority function during
that critical time when the radio must go out and quickly determine the signal level at
the priority frequency. Since these capacitors were previously charged up to the signal
level at the main frequency, and would take about 2 seconds to assume the signal level of
the priority frequency (much too long), a cut is made on a circuit-board trace of each of
the two capacitor circuits and the outputs of the photo-isolator switches on the Priority
Board are connected across the cuts to switch the capacitors in and out of the circuit as
needed.
Since the 3 radios (HR-2510, HR-2600, LINCOLN) each have different main R.F. circuit boards, with minor circuit and layout differences, as well as different reference designators (resistor/capacitor/transistor numbers on the schematics), the generalized schematic on the next page is used to show the basic squelch/agc circuit area and the connection of the Priority Board.
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PRIORITY BOARD PARTS LIST
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The Sharp PC619 F.E.T. can be replaced with surface mounted HP Part# HSSR-8060 or HSSR-8400. R2 & R3 would then be changed from 330 to 511 ohms. |
At the end of this manual are three diagrams, one for each
of the radios HR2510, HR2600, and the LINCOLN. Before
referring to the correct diagram for your particular radio, the following notes apply to
all 3 radio types:
a. The board mounts to the chassis using the supplied
metric screw and spacer. Refer to the BOARD MOUNTING DETAILS figure.
b. Note that the original screw that is removed from the
main RF chassis is no longer used. The new screw (longer) and aluminum spacer must provide
the same ground to the RF chassis that the original screw did, so it's important that it
be tightened down firmly (but not so tight as to strip the threads). Also, the priority
board gets it's ground from the same screw, so make sure there is a good connection
between the head of the screw and the solder lug on the priority board.
c. Before soldering the 6 wires to the main RF circuit board, route them
together and cut them individually to the proper length so as to have enough wire to droop
gracefully across the board, but don't leave the wires excessively long.
d. Cut the two (2) indicated traces on the board before soldering the
wires. (otherwise the wires will get in the way). Use a sharp "X-ACTO" type
razor knife and be careful you don't slip and cut more traces than the ones shown.
Also, use an OHM METER to verify that the traces are opened-up.
e. After the wires are cut to the proper length for where they have to
go, strip about 1/16th inch of insulation off the end and pre-tin the wire before
attempting to solder it to the board.
f. Use the nylon tie-wraps to dress and bundle the wires together after
soldering is complete for a neat and professional appearance.
g. All three diagrams are pictured as if the radio is upside
down with the front of the radio facing you. The metal tray that
holds the smaller microcomputer/pll board is shown rotated up for access
to the mute line's connection.
h. It is possible that there may be some radios with different RF
circuit boards than the ones pictured in the diagrams. Thus, be sure that
you properly identify the 6 solder points and the 2 traces to cut before proceeding. In
the event the diagram for your radio is not correct, contact CHIPSWITCH
and ask for technical assistance.
i. After the board is installed, perform the following checks before
applying power to the radio;
- Use an OHM METER and make sure the solder-lug under
the head of the screw shows a good ground to the chassis
- Check one last time that all 6 wires are soldered to the correct places and that the 2 circuit traces that were cut were the correct ones.
- Make sure the wires are not going to interfere with the
speaker when the bottom cover of the radio is re-installed
- Be sure to re-install and tighten down the screws that
hold in the microcomputer/pll board's metal mounting tray.
Diagrams of each radio's PCB:
