MODEL TA-4 TRANSCEIVE ADAPTOR

GENERAL DESCRIPTION
The TA-4 Transceive Adaptor allows the SPR-4 to transceive with the T-4 and T-4X series Drake transmitters.

This modification should be made by a competent technician. If you need help or want the TA-4 to be installed by one of our factory authorized service centers or by the factory service technicians, please call or write our Customer Service Department. 

Disconnect the speaker cable and the line cord and remove the SPR-4 cabinet. Mount the TA-4 circuit board with two number 4 sheet metal screws and two lockwashers (with the lockwashers between the screw-heads and the mounting feet) in the chassis holes as shown hereafter:

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Picture: JM CHERRY

Connect the five color coded TA-4 circuit board wires by pushing the connectors at the end of each wire onto the appropriate mating pins in the SPR-4 until they are fully seated. See hereafter the location of the pins . 

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Picture: JM CHERRY

Some SPR-4’s have a 12 K l/2 watt resistor between T6 and the pre-mixer board. Cut the resistor lead from T6 and remove the resistor. Route the coax cable along the wiring harness and install the phono fitting on the end of the cable in the large hole near the center of rear chassis apron as shown in Figure. The nut and the flat washer should be on the outside of the chassis. 

Remove the SPR-4 S-meter lamp bracket by squeezing the sides of the bracket. Remove the lamp bracket from the crystal selector frame by removing the mounting screw. Locate the audio transformer mounting feet over the two holes in the top of the chassis which are in front of the crystal selector. The black transformer lead should be facing the front panel. Mount the audio transformer by inserting number six screws through the chassis holes from the bottom and into the speed nuts on the transformer. Route the two transformer leads through the rectangular chassis hole. Push the pin on the black wire into the clip on the head-phone jack. Some early SPR-4 Receivers do not have this clip. In this case, solder the black wire to the headphone terminal closest to the RF and audio gain controls . Route the red wire along the wiring harness to the rear of the chassis and mount the phono fitting in the 3/8 inch hole directly below the speaker jack. Some early SPR-4 Receivers may have a 1/4 inch hole in this location. In this case, enlarge the 1/4 inch hole to 3/8 inch. Replace both lamp brackets. 

It will be necessary to adjust the injection trimmers after the TA-4 installation. See figure hereafter for the required accessory crystals for tuning the injection trimmers . Tune in the signals in the chart below from a signal generator or the crystal calibrator and peak the preselector for maximum S-meter reading. Detune T6 by grasping a metal screw-driver shaft and touching it to the rotor contact of S4R and tune the rear most injection trimmer for the band under alignment for maximum AVC deflection or S-meter reading. Detune Tl by touching the rotor contact of S8F and tune the front injection trimmer for maximum AVC deflection or S-meter reading.

The TA-4 is activated by circuits in the Drake T-4, T-4B, T-4C, T-4X, T-4XB, and T-4XC transmitters, therefore, operation of the TA-4 is performed at the transceive switch on these transmitters.

The transcieve switch on the T-4, and T-4X series transmitter, controls the TA-4 by applying a negative or positive D .C. potential to the injection cable. When the transceive switch on the transmitter is in SPOT position, no D .C . potential is applied to the injection cable and the injection cable is not utilized. In SEPARATE position, the injection line is connected to the mute jack through a 68 K resistor. The mute jack in the SPR-4 becomes positive under open circuit (receiver muted) condition which supplies base current through R6, CR2 and CR3 to Q2 and turns it on. Whenever, Q2 is on, Ql, Q3 and Q4 are also conducting. The collector current in Q4 drops the gate voltage on the SPR-4 premixer transistor Q5 and cuts it off which eliminates the possibility of the SPR-4 injection frequency from getting into the transmitter and causing a spurious signal.

In RCVR position, the cathode of the premixer tube, V8, in the T-4 and T-4X series transmitter is connected directly to the injection line which makes it positive, but not enough to cause CR2 to conduct preventing Q4 from disabling the SPR-4 premixer. 

T6 in the SPR-4, drives Q5, an emitter follower, through C5 and R12 in the TA-4. The output of Q5 drives the injection back through C7 which allows the T-4X series to transceive on the SPR-4 dial frequency.

In XMTR position, the injection line is made negative which turns on Q 1, Q3 and Q4. Collector current in Q4 drops the gate voltage on the SPR-4 premixer transistor Q5 and cuts it off. The injection frequency signal from the transmitter drives the center tap on the link of T6 through Q1 which is used as a switching diode.

T1 in the TA-4 is an audio transformer which matches the SPR-4 audio output impedance to the anti-vox circuit in the transmitter.

A voltage chart, schematic, and circuit board layout  are provided for servicing the TA-4.

MEASUREMENT CONDITIONS: Conditions same as the SPR-4 R.F. voltage chart and including inter-connection with a T-4X series transmitter. Measurements taken in SEPARATE mode were with T-4X series transmitting .11 megohm VTVM used.

The carrier oscillator in the T-4X series transmitter must be adjusted so that the SPR-4 and the transmitter will transceive. Both SPR-4 carrier oscillators must be on frequency. The SPR-4 oscillators are set at the factory and should not require any adjustment. If the SPR-4 alignment is under suspicion, perform the 50 kHz IF and 5645 kHz IF alignment described manual.

(c) 1998-2012 D. PRODUCTS SA/NV - DRAKE is a registered trade mark of R.L. DRAVE COMPANY
No copy of text or image without a written authorization : email: JM CHERRY
Last updated on: 24 Dec 2011