By Steve Rawlings G4ALG


Under transmitter test into a dummy load a small amount of RF pickup was found on the receive audio output wiring. As a result, the following decoupling was provided:

A 0.001uF 200v capacitor was wired across the key jack.

A 0.1uF 100v capacitor was wired across the 'AUX DC' jack in the power supply, PS7.

When transmitting full power Single sideband (SSB) it was found that supply ripple was occurring on the 13.6vDC to the TR7 due to a voltage drop across the two core supply leads on speech peaks. This showed up as a fluctuation in panel light brightness and foldback on the receive audio line. At G4ALG the strap between the two +ve connectors in the CD plus of the PS7 was removed and a seperate -ve wire run to the PS7 for the low current 13.6v rail. The existing +ve conductor thus being used to supply the PA only. Another -ve wire was provided between the plug and an earth tag in the PS7. Both extra conductors were fed through a length of plactic tubing before being laced to the existing DC supply leads.

The AM position was found to be useless on receive with only the standard 2.3kHz filter. To aviod the expense of the optional AM filter, a 'dummy' filter was provided in filter position "B". See Figure 1 - Download Link at bottom of page. Good results are obtained on the BC band and NBFM is resolved well using slope detection.

The PTT line was found to be very intolerant of contact bounce at the PTT switch and poor TX/RX switching resulted.Extra components were provided around the mic jack, J7, to effect the modification shown in Figure 2. The 1k resistor increases the PTT switch current to 10mA and the 15uF capacitor sets the transmit/receive changeover timing to about 100mS.

Under certain conditions associated with the VOX delay circuitry the TR7 will momentarily switch to TRANSMIT at the instant of POWER ON. This can be eliminated by generatinga 'TX inhibit' signal at power 'ON.' The delay circuit of Figure 3 pruducesa 3 second inhibit signal at the 'out of lock terminal (2/12) of the Digital Control Board each time the TR7 is swicthed on.

It is not essential to purchase the Aux 7 board in order to provide 0-1.5MHz receive coverage on the TR7. Without the AUX 7 board eleven signal diodes only are required. The diodes can be mounted on a small piece of 'veroboard' and the whole assembly mounted on the underside of the Parent Board.See Fig. 4. When using the LF provisions, rotate the bandswitch to the 1.5-2.0 position to extinguish the 'set band' lamp. Connect VLF antenna to the ACC socket at the rear of the TR7.

  • For 0.0-0.5MHz select AUX Program 1
  • For 0.5-1.0MHz select AUX Program 2
  • For 1.0-1.5MHz select AUX Program 3

The keying waveform on CW transmit was improved at G4ALG after permanently wiring a series connected C and R across the Key Jack, J15. See Fig 5. Also see note (19).

'PTO Braid Modification' to cure frequency jumping problems as directed by R> L> Drake, Ohio:

  • Remove all connecting cables from the TR7.
  • Remove the cabinet wraparound by removing the eight screws on the bottom and sliding the wraparound to the rear. NOTE: Perform steps (c) - (f) if the DR7 is installed
  • Unplug 5 cable connectors connecting DR7 to the TR7. Carefully position these cables to the side.
  • Unplug antenna coax and blue/shite bandswitch stepping wire from the high pass filter module and remove the grommet.
  • Remove the DR7 hold-down screw and lockwasher.
  • Carefully remove the DR7 by hooking the board puller under the read edge and lifting upward. Once unplugged, the DR7 can be removed toward the rear of the TR7.
  • Clean chrome rider bar with wood alcohol or similar solvent. See Fig. 6. Leave entire rider bar absolutely clean and dry. DO NOT OIL.
  • Solder braid to VFO fork and frame as per Fig. 6. Be careful not to allow solder to flow into the braid as the braid must be flexible - hold the braid 1/8 inch from the end with needle nose pliers.
  • Reinstall the DR7 by locating LED readout block in the proper slot in front panel and lowering the pins on the bottom of the DR7 into their respective sockets. Be sure all of the pins are aligned with the proper sockets, and the antenna coax is routed through the correct hole on the DR7.
  • Reinstall the DR7 hold-down screw and lock washer.
  • Reinstall the rubber grommet on the antenna coax and bandswitch stepping wire. Connect these wires to the appropriate connectors and dress the wires and grommet into the slot provided in the chassis.
  • Reconnect the 5 cables to the appropriate pins on the DR7. Be sure to install the connectors so that the black stripe is uppermost (facing you).
  • Check for broken or pinched wires, board misalignment, etc., and correct any problems. Dress all leads down into the chassis.
  • Reinstall the cabinet wraparound.

Check the primary fuse in the PS7. Many chaps with 220/240v main voltage are happier with a 5 amp fuse in place of the 8 amp fuse fitted as standard.

Those with TR&'s with serial numbers below 1400 may care to make the following mods to update their TX Exciter Boards. See Fig. 2-6 in the Service Manual.

  • R327 from 33k ohm to 150k ohm
  • R333 from 330 ohm to 180 ohm
  • C308 from 1000pF to 68pF
  • C311 from 220pF to 470pF
  • Q301 from 2N3904 to 2N6521

     The reason for the above changes is as yet unknown. Additionally, install 470k ohm across pins 1 and 4 of the microphone jack, J7. This is to provide an extra mic. Input for high optput microphones.

For a review of the TR7 by K1ZZ see 'QST' May 1979

The Predriver Board has been redesigned and GW3GHC has kindly forwarded the circuit diagram of the new board. See Fig. 2-37A.

To enable the TR7 to transmit on all frequencies 1.5-30MHz (excludint 2.5 and 5.0 MHZ bands) simply unsolder the collector of Q9001 on the DR7 board. See Figures 4-1 and 4-2 in the Service Manual.

PS7 Adjustments
The current limit control should be set to provide 30A and shut down at 31 to 32 amperes. If the power supply board is wired-in, adjust the white PC control clockwise to increase current. If the power supply board is a plug-in, adjust the white PC control CCW to increase current. Set the output voltage to 13.6v by rotating the yellow PC control

PS7 Troubleshooting Guide

     Unit does not turn on (0v at output):

  • Fuse blown
  • Fuse holder not soldered to primary switchboard
  • Primary wiring incorrect
  • Defective transformer
  • Apparent short circuit at output:
    • Large SCR (C228F3) is ON (shorted or triggered)
    • Check that wiring on board connector is correct
    • Primary wiring incorrect
    • Primary switchboard has short to ground
    • Large electrolytics reversed
    • Defective high current bridge (MDA3500)
  • High current DC line shorted to ground Unit will not handle peak loads (trips out):
    • (a) Defective pass resistor (2N5301)
    • Open 1.8 ohm resistor in emitter of 2N5301
    • Current pot.(white) not adjusted properly
    • TIP31 transistor defective
    • MC1723 regulator IC defective
  • Unit will not shut down at any current level:
    • Defective MC1723 (7v reference gone)
    • Defective 2N4402 at IC regulator
    • Defective 2N5060
    • Shorted 2N3566
  • Unit will not regulate (13.6v wanders with load):
    • Defective regulator IC MC1723
    • Defective pre-regulator transistor TIP31
    • Defective 2N3566 on PCB
    • Defective 1n4005 in rectifier bridge on PCB
    • Defective 470uF filter capacitor on PCB
  • Defective transformer
  • Large electrolytics reversed (15000uF)
  • Defective high current bridge (MDA 3500)

ALC Improvements for the TR7
By George Cutsogeorge, W2VJN

     On the air tests indicated that my new TR7 did not cut through the QRM as well as my 20 year old Collins S Line. This was true for barefoot operations as well as when driving a linear. In all cases the key down power output was adjusted to be the same and the VSWR was low. Both rigs were operating with ALC and the same microphone was used without any speech processing. The difference was traces to the time constants used in the ALC circuits of the two units. The 32S-3 has a fast attack and a dual delay time constant. The attack is less than 1ms and the delay is 66ms and 1.6 sec proportioned 17% and 83% respectively. The fast attack is necessary to prevent flat topping on peaks. The longer delay time constant adjusts the average gain for the particular microphone and operator. The shorter delay time follows the syllabic rate and brings up the gain between peaks which increases the average power.

     Examination of the TR7 ALC indicated an attack time of 10ms and the delay time constant was very long. Only leakage currents and the bias current of the op amp U1601 would discharge the ALC capacitor C1611. This not only makes the delay time constants long, but also quite unstable with temperature and life and variable from unit to unit/

     An 82k ohm resistor was added from the cathode of CR1603 to ground. This resistor may be added by removing the bottom cover of the TR7 and soldering it between the CR1603 end of R1617 and the ground end of R1615 on the top of the ALC board. It is not necessary to remove the board to make this addition which leaves the attack time at 10ms but gives a delay time constant of 92ms.

     On the air tests now show approximately equal average power outputs from the TR7 and the 32S-3. DX stations now report that there is little or no difference between the two units. Some small additional improvements may be obtained by utilizing a dual time constant similar to the 32S-3.

     Concerning Note(8) above, W2VJN writes: I installed the mod using the 330 ohm resistor and 1uF capacitor across the key jack. I noticed that on high speed CW the duty cycle of my keying was too heavy. The trailing edge of the wave was extended by 15ms. Also the leading edge (not affected by your mod) was too soft. The net result is keying that is not easy to read. I found that the trailing edge could be softened without extending it by increasing the size of the by-pass capacitor on the CW oscillator. In my unit a 1.2uF across C1143 on the 2ndIF/Audio Board did the trick. This left the leading edge too soft. The problem was traced to the crystal being very slow to start. It was determined that a resistor across C1136 would cut the crystal Q down somewhat and speed up the oscillator start-up. I believe that the required resistor value will vary from unit to unit, but in my case a 3000 ohm resistor was chosen. The rise and fall times are now 2.5ms when the ALC is adjusted to threshold. This mod reduces the oscillator output slightly and the carrier control may be advanced about 1/2 division to compensate.
((At G4ALG, mod (9) has been replaced by mod(19).))

    For those people wishing to make up their own RTM7 module GM3WIL has forwarded details of the required diode arrays for the proposed new amateur bands. See Fig. 7. For 'receive only' moduled, omit diodes to pin 10.

Sherwood Engineering of Colorado is marketing a range of high quality IF filters to suit the TR7 and has now added an RF speech processor, model 7-SP to their proce list. Address follows:

        Sherwood Engineering Inc.
        1268 South Ogden Street
        Colorade 80210
                                                        ISSUE: UK101/2
                                                        DATE : MARCH 81

     If you have any ideas for improving the Drake TR7 please send your input to:

                Steve Rawlings, G4ALG
                3 Hunt Road
                High Wycombe
                HP13 7RE

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