Click On Any Section of the Schematic
Below for Information on That Part of the Circuit:
Negative Keying Inverter:
The circuits in the CW T/R System are all designed to be positive keyed so that
any key, bug, or electronic keyer will work with them. However, many vintage
transmitters or transceivers use negative or grid block keying. This circuit
uses a high voltage PNP transistor to key a negative keyed vintage transmitter,
while presenting a positive voltage at the key input.
Negative Keying
Inverter
Click On Any Section of the Schematic
Below for Information on That Part of the Circuit:
Operation:
When the key is up, the collector-base junction of the transistor is reverse
biased and no current flows through the transistor. Since the transistor is
turned off, no current flows through the 390 ohm resistor or the 200 ohm
resistor and the voltage drop across both of them is zero. This means that
there is no voltage across the base-emitter junction of the transistor, and the
transistor is turned off.
When the key is pressed (grounded), current flows through the 390 ohm resistor
from the positive supply
to ground, and there is now a voltage drop across it. Some of this voltage drop
gets through the 200 ohm resistor to the base-emitter junction, so there is now
a voltage across the base-emitter junction (with negative on the base and
positive on the emitter). The base-emitter junction is thus forward biased,
turning on the transistor, causing current to flow from the XMTR- terminal
through the transistor and 200 ohm resistor. This removes the negative voltage
from the transmitter key jack, keying the transmitter.
Key (+) Positive Keying Input:
The key is connected here. The circuit is designed to present a positive keying
voltage at the input, so any key, bug, or keyer should work. In the CW T/R
system, this runs to the Key Line.
MPSA92 Transistor:
The MPSA92 is a high voltage PNP transistor capable of withstanding 300V and
500mA. This is more than enough to handle any vintage transmitter. Virtually
any PNP transistor can be used here provided that the following are met:
a. It must be able to handle the key up voltage of the transmitter plus 12V,
since the voltage across the transistor is the sum of the transmitter keying
voltage and the positive supply voltage.
b. It must be able to handle the key down current of the transmitter. This is
usually very small, only a few mA.
200 Ohm Current Limiting Resistor:
The 200 ohm resistor limits the current through the transistor to a safe value
under key down conditions.
390 Ohm Bias Resistor:
When the key is up, the collector-base junction of the transistor is reverse
biased and no current flows through the transistor. Since the transistor is
turned off, no current flows through the 390 ohm resistor or the 200 ohm
resistor and the voltage drop across both of them is zero. This means that
there is no voltage across the base-emitter junction of the transistor, and the
transistor is turned off.
When the key is pressed (grounded), current flows through the 390 ohm resistor
from the positive supply to ground, and there is now a voltage drop across it.
Some of this voltage drop gets through the 200 ohm resistor, so there is now a
voltage across the base-emitter junction (with negative on the base and
positive on the emitter). The base emitter junction is thus forward biased,
turning on the transistor, causing current to flow from the XMTR- terminal
through the transistor and 200 ohm resistor. This removes the negative voltage
from the transmitter key jack, keying the transmitter.
1N4002 Protective Diode:
The diodes that are used here can be any of the 1N400X type, such as the
1N4002, 1N4007 etc. They are not strictly necessary for the proper operation of
the circuit, but protect the transistor or warn the operator in case the T/R
system is connected incorrectly. For example, if the output is accidentally
connected to a positive keyed transmitter, the output diode will conduct,
keying the transmitter, warning the operator that the wrong jack has been used.
V+ Positive Supply Voltage:
The inverter requires a positive voltage with respect to ground for proper
operation. This is obtained from the
system power supply. If
the inverter is used by itself, this could be a 9V battery.
XMTR- Negative Keyed Transmitter:
For this inverter to operate properly, the keyed transmitter must present a
negative voltage at the key jack. If a positive keyed transmitter is
accidentally connected, the protective diode will key the transmitter, warning
the operator of their error.
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comments, you can send E-Mail to Dr. Greg Latta at
glatta@frostburg.edu