From: chuck@eng.umd.edu (Chuck Harris - WA3UQV)
Subject: Re: Hamtronics R-137 Info needed
Date: Wed, 09 Sep 92 18:40:03 GMT
Organization: University of Maryland, Department of Electrical Engineering
Distribution: usa

In article <1992Sep9.143930.19477@mercury.cair.du.edu> awinterb@diana.cair.du.edu (Art Winterbauer) writes:
>Has anyone built/purchased a Hamtronics R137 Weather Satellite
>Receiver?  I'm interested in:

I recently purchased and built a similar Hamtronics receiver, a R76-144.  It
had some plusses and a lot of minuses.

>
>1. Its bandwidth (> 30 khz? < 30 khz?)

The R76 BW is about 20Khz using a "homemade" crystal ladder filter at 10.7, and
a 20KHz BW ceramic filter at 455KHz.

>2. Ease of construction:

The construction instructions consist of a print of the PC board with the
part numbers on it, a parts list, and instructions that essentially said, "put
all of the resistors in the board, then put all of the capacitors..."
It is quite tedious to try and find the actual locations of the parts on the
non-silk screened, non-solder masked board.

>   a. Instruments required for alignment

Instructions tell you to connect your voltmeter to a test point, and peak
the oscillator coil.  Then put a 10.7MHz signal into the IF, and adjust the
discriminator coil for 2.75vdc.  Then to put an ac voltmeter on the
discriminator, and a whopping signal into the antenna jack, and adjust all of
the rest of the coils for min voltage on the voltmeter, bringing the signal
down as you go.  It works, but is kind of trial and error.  (but then so is
most RF work.)

>   b. Condition of kit (all parts actually came, quality of
>      of components, any discrepancies between the schematic
>      and the parts placement diagram, quality of pc board, etc.)
>3. General performance of unit compared to the more expensive receivers
>   (e.g., Vangard)

The kit was complete.  All of the parts were jammed into a couple of plastic
bags. (resistors in one, cap's in another, coils in ...)  The quality of the
components was good.  The PC board was crude, though, I found that I couldn't
use 67/37 solder, because the alloy it made with the board's plating behaved
terribly.  60/40 solder was ok though.  The parts all fit, and the board was
either G-10, or FR-4.

There were problems with the oscillator capacitors used on the motorola IF
chip.  They had to be changed back to motorola's recommended values in order
to make the 10.245MHz crystal oscillator work.  The channel crystal formula
given in the documentation was wrong.  (was xtal = (f + 10.7)/3... should be
xtal = (f - 10.7)/3)  The crystal type in the documentation was also wrong,
it was quoted as being:  "The receiver uses THIRD OVERTONE, parallel resonant
crystals with an 11 pf load capacitance, in HC-25/u holders.  Crystals operate
in the FUNDAMENTAL mode."

An obvious mistake.  I am still puzzled as to why they chose 11pf over
a more standard 15pf, though.

The GaAs FET front end tends to oscillate if there is no antenna connected. The
layout in this area looks fine, I think that I need to load the antenna coil
down a bit to stablize it.

As to the performance, the receiver almost meets the design sensitivity when
it is sitting on my workbench.  I suspect that in a chassis it will perform
better. The finished product looks very good and is quite professional in
appearance.  I am a little disturbed that the GaAs-FET front end oscillates if
you don't have an antenna connected.

The staff was friendly and helpful, but I couldn't help but think that I was
the first person to ever build this receiver.  Some of the mistakes that I
found should never have gotten into print/manufacture.  I probably won't buy
any more kits from them though.  The amount of work that I had to do to make
this receiver work correctly was only marginally less than if I did a custom
design myself (and then I could sell y'all the kits!).

73,

Chuck Harris - WA3UQV
Newsgroups: rec.radio.amateur.misc