This modification enables the addition of an INRAD 8-pole 6.4 kHz AM
filter to the Drake TR-4 transceiver.
For installing the 9 MHz filter for AM, the new idea is to switch the
filter with relays driven by a PowerMOS and an optocoupler that
intercepts the small constant current (through the 3.3 Mohm resistor)
that unbalances the diode balanced modulator while transmitting AM (the
current is present also while receiving). In the Figure 1 you see the
Inrad AM filter and the two relays that are used to intercept the two
wires that go to the SSB crystal filter that is near the VFO connector
(non X filter).
Figure 1 -
Added Switching Circuitry
AM and non-X SSB filters are therefore interchanged by the relays. The
unused filter is connected to ground. One relay for input switching and
grounding, the other for output switching and grounding. The schematic
the switching circuitry is shown in Figure 2.
The 3.3 MOhm resistor that must be intercepted is on the board with the
red wires near the 6AQ5 socket. See Figure
Figure 3 -
Tap into 3.3 Meg Ohm Resistor
The additional custom board with optocoupler, PS rectifier and MOS is
near the VFO socket. In the brown insulating tube there is a 10 ohm
resistor in series with 12 VAC, it powers the board. The MOS has a 6.2
Mohm resistor between gate and source to convert the small current from
the optocoupler to gate voltage. The LED of the optocoupler is
connected to +150V, then its cathode is connected to the 3.3 Mohm
resistor that biases the balanced modulator and (also) to ground via a
4.7 Mohm resistor (or a combination of resistor and trimmer) which
gives more bias to the LED.
As before this arrangement is connected to the balanced modulator only
in AM mode by the mode switch. No modification to the front panel is
required, the mod is 100% reversible.
For better shielding I later used shielded cables for connecting the AM
filter to the relays and I also connected the coils of the two relays
to the nearest ground with 10nf capacitors (not shown in Figure 1).
To operate in standard AM mode with SSB selectivity filter, please
follow the instructions as reported in the TR4 manual.
To enable the 6.4 kHz AM filter for receiving, the SIDEBAND switch must
be rotated from the X position to the other position. Transmit
frequency will not change and the transmitter will emit the AM rated
AM filter can only be used by selecting the AM mode with the mode
Transceiver tuning to the correspondent station is best performed with
the SSB filter (SIDEBAND X position), then it is suggested to switch to
the AM filter for optimum reception.
IN3IEX - Giorgio Fontana, Trento, Italy (JN56NB).
is a PDF version, suitable for printing.