Hi Pete, Thanks for your generous comments. Regarding the AM sensitivity variations in the new Tecsun PL-300WT, an investigation of the issue has been made by myself and Steve Ratzlaff, and we have a pretty clear idea of how this happened, and the solution for the problem. The details of the investigation have been shared with the Ultralightdx Yahoo group, but those without access to the Ultralightdx Yahoo messages probably don't know the full details. Silicon Labs (the designers of the PL-300WT's internal DSP chip) intended for the DSP chip to provide good AM and FM sensitivity without the need for any alignment system. This was an innovative concept, and they certainly succeeded in accomplishing this in the FM section of the PL-300WT. In the AM section, however, the chip designers' original "no alignment" intention was greatly compromised by Tecsun's PL-300WT manufacturing process-- specifically, by Tecsun's habit of placing the loopstick coil at the far left side of the ferrite bar, without making any effort to match the inductance needed by the DSP chip's internal varactor to provide optimal AM sensitivity. The varactor in the DSP chip requires a certain fixed inductance to provide maximum AM sensitivity in the PL-300WT, and this inductance can be anywhere from 180-450 microhenries. The only way to ensure maximum AM sensitivity in the PL-300WT is to align the loopstick coil to provide the fixed inductance necessary to optimize the match with the DSP chip's varactor component. Unfortunately, this alignment process is not as routine as most AM alignments, due to the construction of the PL-300WT loopstick. To my knowledge, three such AM alignments have been performed so far on the PL-300WT-- two by me, and one by Steve. In Steve's case, he was able to remove the stock loopstick by disassembly, and was able to peak AM sensitivity by obtaining a final coil position providing a 205 uh inductance, to match the value needed by the DSP chip's internal varactor. Unfortunately, that coil position was partially off the ferrite bar, so Steve needed to cut away some of the left plastic mounting bracket to reinstall the loopstick. In my two alignments, being reluctant to remove the loopsticks because of an apparent overload of glue holding down the ferrite bars on both models, I attempted to shift the single coil by scraping away all wax along the coil borders, and patiently probing the coil sides until the coils finally started to shift along the bar. Both of them finally did, but the processes were like marathons, and the risk of breaking the Litz wire seemed high. For maximum AM sensitivity, my first model's coil ended up not too far away from the original position, at 1/8" to the right of the left mounting bracket (compared to an original position pushed completely up against the left bracket, which is apparently Tecsun's standard PL-300WT loopstick coil position). This was my original PL-300WT model, which had good AM sensitivity out-of-the-box (as reported to the group in the original post). The inductance at this position was 282 uh. Since that model's stock loopstick was removed in favor of a 7.5" loopstick transplant, I did have the chance to test whether the glue holding down the stock ferrite bar would indeed prevent removal of the bar. In fact, the glue was so excessive that it made the stock ferrite bar break into four pieces, when the bar was removed. My second PL-300WT model was received a week ago, and had AM sensitivity noticeably worse than the first unit. I reluctantly did the loopstick alignment procedure again, which was another marathon of wax scraping, probing and pushing. The coil finally did move (without any Litz wire breaking), and ended up peaking AM sensitivity at a position a half inch to the right of the original position-- at an inductance of 343 uh. For those DXers with a new PL-300WT providing less than optimum AM sensitivity, the alignment option (despite its tricky nature) is the only way to ensure correction of the issue. Because of the variable nature of the varactor components in the DSP chips, some units are going out with fairly good AM sensitivity, and others are going out with less than optimal sensitivity. Tecsun apparently thought that the "no alignment" feature of the DSP chip meant that the loopstick coil could always be pinned against the left bracket, to cut manufacturing costs. This is not the case, and we are now dealing with the consequences of their decision. I hope to upload a pictorial for a Tecsun PL-300WT AM alignment procedure soon, for those hobbyists willing to align their models for peak AM sensitivity. A fully aligned PL-300WT should have AM sensitivity fairly close to that of an aligned DT-400W, C.Crane SWP or SRF-T615. If your model does not, it could probably use an AM alignment, which unfortunately is a trickier procedure than the standard AM alignment. Hobbyists should proceed with due caution. 73, Gary DeBock In a message dated 6/14/2009 4:41:51 P.M. Pacific Daylight Time, p...@comcast.net writes:
Gary, thanks for your exhaustive and continuing research in the world of mini-radios. We all, and the hobby, have benefited immensely. Having said that, it looks to me like we might have some quality control issues; perhaps it is between factories or vendors. George Sherman has just reported results similar to mine. If you are getting hot ones, it would be of value to me to meet with you and compare mine with one of yours. I think we all agree that the selectivity is very good but we do seem to have variances in sensitivity not only between different radios but also, at least in my case, between adjacent frequencies. There are some quirky things going on... I finally figured out how to get it on LW (it doesn't work every time) but the preset 9kHz tuning doesn't always coincide with local outlets. The FM is quite good and I haven't even bothered trying SW. Onward! In the meantime, thanks again for all your work on this. Pete Taylor Tacoma, WA 12225w 4719n HQ180 + Kiwa air core loop ICF2010 + " " " " DX398; Palomar loop SRF-59 & -M37V Eton E100 _______________________________________________ IRCA mailing list IRCA@hard-core-dx.com http://montreal.kotalampi.com/mailman/listinfo/irca Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers For more information: http://www.ircaonline.org To Post a message: irca@hard-core-dx.com **************Choose the home loan that saves you the most $$$. Agents available at ditech.com (http://pr.atwola.com/promoclk/100126575x1221374924x1201371434/aol?redir=http:%2F%2Fclk.atdmt.com%2FDEG%2Fgo%2F153724534%2Fdirect%2F0 1%2F) _______________________________________________ IRCA mailing list IRCA@hard-core-dx.com http://montreal.kotalampi.com/mailman/listinfo/irca Opinions expressed in messages on this mailing list are those of the original contributors and do not necessarily reflect the opinion of the IRCA, its editors, publishing staff, or officers For more information: http://www.ircaonline.org To Post a message: irca@hard-core-dx.com