Had some questions about modifying the Drake L7 amp for 10 meters. It seems that it is difficult to find credible, accurate information about the mod, let alone somebody that has actually done it. My questions are...
1. My understanding is that there are two bandswitch designs that were used in the L7. However, I cannot find information about which switch was used for which serial numbers. With one design, a nut is loosened and the stop removed. With the other design, I have no info at all about it. 2. One mod indicates the need to add two silver mica capacitors, labeled as C6 and C7 (both 68 pf), on some Drake schematics for mods related to the input coil assembly. However, I can't understand why both caps are needed. C6 is connected from the 15 meter input coil, input side, to ground. Since there is no 10 meter input coil for that band, C7 is connected from the 10 meter input wafer to ground. If one looks at S1A on the schematic, the input side or the one closest to the coax input (there are two sections for S1 that are both labeled A) is the section where that cap would be located. You can accomplish the same configuration just described by using only a single 68 pf capacitor. How. For the switch wafer closest to the coax input lead, merely short the 10 and 15 meter taps together, and put a 68 pf cap to ground as the mod describes. What does this do? On 15 meters, it puts that cap at the input end of the 15 meter coil. On 10 meters, it puts that cap to ground the same as if two caps are used. This approach leaves coil L4 (15 meter) connected to the input lead on 10 meters to BUT since the other end of the coil isn't connected into the circuit, it is just floating and a few pf of stray capacitance is present. Is there some compelling reason for using both caps? 3. This question is the most puzzling, and is related to question 2 above. As part of the 10 meter mod that I found described on the Internet, C6 is one of the 68 pf capacitors that is added to the circuit. However, this cap is added to the input side of the 15 meter input coil. Why is it necessary to add a capacitor to the 15 meter coil since the mod is for 10 meters? With the 15 meter input coil out of the circuit on 10 meters, that cap would seem to be unneeded for 10 meter operation. And why would the 15 meter coil need added capacity because of adding 10 meters? No rationale is given in the article I obtained. I've had two requests in the past month alone for making the 10 meter mod as part of rebuilding power supplies and/or other repairs. However, I have been unable to obtain documentation that seems believable and I have been unable to discuss the matter with anybody that has actually done the mod. Your thoughts? As a side note, I hope you can make it here for Hamvention and the Drake Forum. 73, Evan, K9SQG
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