I had been mulling over applying Jeff Covelli's (WA8SAJ) AM-wide
transmit mod to my TR7:
http://www.wb4hfn.com/DRAKE/DrakeArticles/TR7_AM_Mod/TR7_AM-P1.htm
I had actually thought about trying something like this some time ago,
but Jeff beat me to it. Nice work Jeff! This is the kind of info that
makes tech-ing the TR7 so much fun!
In thinking about what I have learned about the way the TR7 manages the
filter passbands, something just didn't seem quite right. The question
that stuck in my mind was, "Is the AM transmit signal properly centered
in the wide filter passband?" A trip to the service manual found the
relevant information. Referring to page 2-50, the oscillator
frequencies on the PBT/Ref board for AM are:
13.695 MHz RX
13.6936 MHz TX (also the USB frequency)
In RX, the AM signal is placed in the center of the filter passband for
proper demodulation. In TX, the signal is shifted down 1.4 KHz to place
the carrier on the skirt of the default 2.3 KHz transmit passband. This
provides the proper audio response for the USB transmit audio - the
carrier is reinserted later in the chain.
What this means is that the AM transmit signal, when passed through the
wide filter, is not symmetrical - being off-centered by 1.4 KHz. The
filter is 6 KHz wide, +/- 3 KHz from center. 1.4 KHz is almost 1/4 of
the total passband, and seems signifigant. This may not make a lot of
difference in the real world, I don't know; but the purist in me wants
that AM signal to be smack in the middle of the passband.
This can be accomplished easily enough, although it complicates the
switching a little if it is desired to switch AM-Wide transmit in and
out. A small relay can be used or a solid-state switching arrangement
can be fabricated on a small circuit board. I chose to make the AM-Wide
transmit automatic and full-time, and used a small DPDT 5-volt surface
mount relay with a suitable dropping resistor for the coil, hot glued
to the bottom side of the parent board. The relay handles the filter
switching , and is operated by the +10V TX AM control voltage. This
switches in the wide filter on transmit in the same manner that the
STORE switch does in Jeff's article. Care was taken to place the relay
in a location where there is nothing but ground plane on the other side
of the parent board, and it is not next to or on top of any signal
carrying traces. Also, wires were routed to they do not lay directly on
top of any signal traces. A 1N4148 diode was used across the relay coil
for spike surpression.
In this scenario, all that is needed to center the AM signal is to
unsolder a wire at the back of the parent board and add a jumper. The
two control singals of concern are the "USB POT." and "C.L. XMIT.",
both routed to the PBT/Ref board. "USB POT." is exactly what it sounds
like - the control voltage from the USB fixed passband pot on the power
supply board, fed directly to the PBT/Ref board. This voltage is used
ONLY IN AM TRANSMIT to shift the oscillator 1.4 KHz down in order to
generate a proper USB signal for the transmit default 2.3 KHz filter.
The "C.L. XMIT" voltage is used in all other modes to set the transmit
filter passband at the proper point on the DSB signal from the balanced
modulator, using the control voltage from the appropriate fixed
passband pot. Switching between TX/RX is handled by U1001 on the
PBT/Ref board. All that needs to be done is unsolder the wire that
feeds "USB POT." to the PBT/Ref board, and jumper that point to "C.L.
XMIT" - in other words, connect PBT/Ref board pins 10/26 and 10/28
together. That's it! The DSB signal will be centered properly in the
wide filter passband in AM transmit. USB operates normally, since this
control voltage comes in on "C.L. XMIT" when the mode switch is set to
USB. If it is desired to switch AM-Wide TX on and off as Jeff did, one
half of the DPDT relay can be used to switch these connections at the
same time as the filter control connections, and the relay can be
operated by the front panel STORE switch - or from some other switch if
preferred.
I don't have any on-air signal reports yet, but it looks good on the
scope and all voltages are appearing where they should be, when they
should be. This article is in no way meant to criticise or diminish
Jeff Covelli's great work on his AM-Wide TX article. I would have
missed this little tidbit myself the first time around. I wish all mods
were as well documented as Jeff's. Thanks, Jeff!
73
-Jim
NU0C
___
Drakelist mailing list
Drakelist@zerobeat.net
http://mailman.zerobeat.net/mailman/listinfo/drakelist
