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Drake TR-7 A.M. Transmit Filter Modification
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| The Drake TR-7 is not your typical rig to be known for A.M. operation, since it is “solid-state” and not as famous as the old “tube” rigs that many of us folks have used through the years. I have had TR-7’s for many years in my shack and until recently, was wondering what could be done to improve the transmit audio; since the receiver is great using the optional 6-Khz wide filter installed. |
| Description: The TR-7 is an all “solid-state” transceiver including the final PA amplifier that will deliver about 120 watts output across 1.8 to 30 Mhz. Now running A.M. you have to turn down the output to about 25 watts with a little headroom for the peaks on A.M. The A.M. is derived from the balanced-modulator at the 5.645 Mhz lower I.F. by re-injecting the carrier with audio to develop the A.M. signal. This works great, except it is defaulted through the 2.3 Khz SSB filter now cutting the lower Side-Band off on transmit limiting the nice higher quality audio that is coming from the balanced modulator. The reason it is defaulted through the 2.3 Khz filter, the TR-7 has three positions for optional filters in the receive side independent of the “mode” selected. So, if the 6 Khz “wide” filter is selected and you operate SSB, then you have Double Side-Band and that would not be nice to your neighbors on the band and you might get a nice QSL from the FCC doing that! |
| Solution: I looked into what Drake did to default the 2.3 Khz filter in transmit all the time and found a simple way of making the 6 Khz wide filter work on transmit.  There is a diode that is used to steer
the voltage toward the 2.3 Khz filter and it is only on the
“transmit” 10-volt line. I also found a very seldom-used switch on
the front panel, called a “store” switch. The “store” switch was a
gimmick at the time to hold the digital-dial frequency (not a
memory) for what reason I could never understand, but it was there
and a handy one at that. There is also a seldom used “fixed” lamp
that comes on for a frequency that you might use for a net or in the
Marine Band when the TR-7’s were used in the Commercial Market. So
now there are two items that I needed to make this all happen. I
need a switch and a lamp to tell me that I am transmitting in the “WIDE” mode. |
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Pictured here is a front panel view of the modified and relabeled "Store" switch on the TR-7 |
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Pictured here is a front panel view of the relabeled "Fixed" lamp on the TR-7 Transceiver. |
