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[IRCA] TECSUN PL-300WT AM Sensitivity Variations
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,
ptdx@xxxxxxxxxxx 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
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