Hello Gregory (and Chris),
Thanks for reporting the results of the 7.5" loopstick transplant in your PL-310 model, and congratulations on the new stations you have logged with it.
Since this 7.5" loopstick transplant procedure was developed for the PL-380 model (not the PL-310), the 530-1700 kHz performance testing was only done here on the PL-380 model. Using the exact same material and procedure as described in the transplant article, the PL-380 model here had a very great AM sensitivity boost on all frequencies from 530-1700 kHz (averaging 20 dBu), with no issues on the high band like you describe.
Concerning whether this PL-380 transplant procedure developed for the PL-380 will work with equal effectiveness in the PL-310, as far as I know, you are the first hobbyist to try it in the PL-310 model. I have a PL-310 model here hooked up to the same Amidon loopstick with an 81-turn Slider coil, and no matter how I tune the Slider coil, the PL-310's AM sensitivity performance does not equal that of the finished PL-380 7.5" transplant model. It comes fairly close on most frequencies when the Slider coil is peaked for optimum inductance match with the DSP chip, but it is still about 5 dBu less than that of the PL-380 transplant model on most AM frequencies. The reason for this is a mystery-- it may possibly be due to improved circuitry in the PL-380 model installed by Tecsun to compensate for the smaller stock loopstick. In any case, the testing here indicates that the PL-310's performance with the 7.5" Amidon loopstick transplant seems slightly inferior to that of the equivalent PL-380 model, although there were no high-band issues on the transplant-model PL-310 here, like you described.
The symptoms you describe DID occur here on the PL-310 experimental loopstick composed of 113 turns of 40/44 Litz wire on the stock ferrite bar, which was tested here as a possible replacement for the stock PL-310 loopstick. This experimental loopstick (documented in a photo album on Ultralightdx) had a coil inductance of 554 uh, and did boost up AM sensitivity significantly on all AM frequencies up to about 1300 kHz. Above 1300 kHz, however, it was inferior to the stock loopstick in performance, with symptoms exactly like you describe. Presumably the coil "Q" of this experimental loopstick (practically covered with Litz wire) was unacceptable at the high AM frequencies.
Recent experimentation here (and by Nick Hall-Patch, and others) indicates that any change in an experimental loopstick parameter (ferrite bar size or length, ferrite mix, Litz wire, coil "Q" or Si4734 Radio model) can change the optimum loopstick coil inductance required to match the DSP chip varactor for peak AM sensitivity. The loopstick parameters in the PL-380 7.5" loopstick transplant article were developed and tested on the PL-380, but may not be directly applicable to the PL-310 model, Gregory. In any case, if you wish to optimize the AM sensitivity in your PL-310 transplant model on all frequencies, you may need to ensure that the 554 uh coil inductance is optimal for your particular experimental loopstick. The easy way to try this would be to add or subtract a few turns of Litz wire from the coil, to see if the high band issue would be corrected or not. You are also welcome to borrow the 81-turn Slider coil here, if you wish to perfectly fine-tune your new loopstick for optimal AM sensitivity.
73, Gary DeBock
In a message dated 1/1/2010 9:25:01 A.M. Pacific Standard Time, gmosherat@... writes: