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 [email protected] http://mailman.zerobeat.net/mailman/listinfo/drakelist
