MISCELLANEOUS WEPIX-2000-B NOTES
1. When switching from memory bank 1 to 2 the frequency will NOT change until the
channel counter has been incremented manually or automatically at least once. The
switch only selects the memory address. The synthesizer chip must receive a clock
pulse to let it know it's OK to read the EPROM and this it gets from the increment counter
either manually when you hold the red button in or automatically when it's scanning.
2. Since memory bank 2 is intended for use with a GOES down-converter,
it's not advisable to be in the scanning mode since it will lock onto
the first frequency that falls within the IF bandpass. Remember that
the IF bandpass, by necessity, is 30 KHz wide but the tuning increments
are spaced only 5 KHz apart ( in bank 2 only). This means that you
could be locked on at the very edge of IF bandpass resulting in high
distortion. The correct way to use bank 2 is to first make sure your
GOES down-converter outputs at 137.5 MHz. (some do not) , then manually
increment the counter by pushing the red button until you get the best
reception. Custom programmed EPROMS are available from Vanguard for $20
if the current one does not meet your frequency requirements.
3. The EPROM which stores the synthesizer programming is capable of storing data
for 256 frequencies consisting of eight 4 bit words per frequency as required by
the NJ8820. Since the 2716 EPROM reads out in 8 bit words there are 32 spare bits
per frequency available to the user. These will be used in a future model to program a display
reading out the frequency. The address and output lines are available now on the
circuit board ground plane side, however additional circuitry is required to make
any use of them. Note that presently any unused bits will contain only random data
4. The present display reads in a clockwise direction starting from the lower left
exactly as would a rotary switch which it replaces. The last 2 frequencies, 136.77
and 137.77 are put there so that they can be locked out by presetting the counter
to count to 6 instead of 8 (actually it counts to 16, but we divide it in 2 banks), more
on this in a future upgrade. These 2 frequencies are the high speed (4800 baud) data
stream of the NOAA satellites and do not contain picture information. It does contain
an enormous wealth of scientific data which can be decoded wiyh special hardware and
software. You can write to NESDIS in Washington DC for the format. We will supply
it to our customers at a later date when we understand it better. LAST MINUTE NOTE:
137.13, 136.77 and 137.77 have been replaced in the EPROM with other more useful frequencies.
See EPROM programming sheet for explanation.
5. The first frequency, 137.13 is reserved for experimental satellite transmissions.
Picture data has been received on this frequency recently so we included it for
adventurous types. One never knows what one may see.
6. Programming information for the 8820 and the 2617 will be supplied at a later
date. In the meantime if you have any special requirements as to frequencies, we
can supply you with a new EPROM programmed to your frequency sequence for $20 postpaid.
The smallest frequency increment available for this particular synthesizer is 2.5 KHZ
so make sure the frequencies you select are within that range. Example: 137.5025.
Note that the 3rd and 4th places to the right of the decimal must not be smaller
than multiples of 25 which includes 00 and 100 which would then make it 137.5000 and 137.5100.
Normally we program the synthesizer in 10 KHz steps because the smallest increment
also becomes the sampling frequency and 10000 samples per second are better than
2500 samples per second. Of course, if you need 2.5 KHz steps, then there is no choice.
The synthesizer is available as a separate module to O.E.M's for $150 in lots of
10 or more.
7. You will note that we have included a 12 VDC AC adapter .rated at 800 mA . whereas
the receiver draws less than 300 mA. The reason for this is to provide 13 volts
(under light loads 12 Volt adapters always output at least 13.5 volts) to operate
the receiver's adjustable voltage regulator which we set for 11.5 Volts and to provide
enough spare current for you to operate a GOES down-converter powered through the
antenna cable. Note however that you will have to short out the 330 ohm current limiting
resistor normally used for the preamp.
8. Behind the push button switch on the circuit board is a red LED. When lit, it
indicates the synthesizer is out of lock. When scanning or changing frequency it
will flash briefly since it must unlock to change frequency. If it stays lit it indicates
a failure of some component in the synthesizer. If this ever occurs it's best to call
Vanguard for instructions. Because a synthesizer is a closed loop system troubleshooting
can be extremely frustrating unless you know exactly what you're doing.
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MISCELLANEOUS WEPIX-2000-B NOTES (continued)
9. Next to the NJ8820 is a crystal. This is frequency reference for the synthesizer
and all frequencies generated are compared and phase-locked to it (after going through
a series of dividers). Small adjustments in frequency can be made with the small
trimmer capacitor next to it, however remember that for the synthesizer to track all
frequencies correctly the crystal frequency must be set at exactly 4.000000 MHz.
Since any probe you connect at the crystal will add a small amount of capacitance
it will change the frequency slightly , therefore we recommend that you pick off the VCO
( Voltage Controlled Oscillator) frequency instead. This can be picked up at the
mixer injection point which is the first coil closest to the small square metal
cover (on the left side when holding the case with the front panel towards you and the components
facing up which is with the bottom cover removed). The VCO frequency is the signal
frequency minus the I.F. frequency (10.7 MHz). Example: 137.5 minus 10.7 = 126.8
MHz. To read this frequency you will need a frequency counter and a small 1 turn pick-up
loop at the end of a cable. You can then use the trimmer capacitor next to the 4
Mhz crystal to set the VCO frequency to exactly 126.800000 MHz. Of course, the red
LED indicating 137.5 must be lit before you do this or you must recalculate the VCO frequency
corresponding to whichever LED is lit. Remember however that even a frequency error
as high as 1 KHz will not affect the performance of the receiver. We have included
these instructions as a point of interest for experimenters only. Normally there is
never a need to make these adjustments.
NOTE TO TECHNICIANS
: If you intend to make repairs or modifications note that all the
holes on the circuit board are plated through. To remove any component
that goes into or through a hole it is necessary to remove as much
solder as possible from both sides of the hole by using a de-soldering
wick and a temperature controlled iron. Without a temperature
controlled iron you run the risk of removing the pad as the bond will
usually fail above 600 degrees F. After the solder is removed you
should reheat the lead and carefully pull out the component. To replace
a component you will first have to clean out the hole with a hot
stainless steel wire. Ordinary copper wire will get soldered into the
hole unless you keep moving it as you heat it. Removing surface mount
devices requires heating both ends of the component simultaneously.
This can be done with special adapters on soldering irons or with 2
small irons (one one each end of the component) . Re-using surface
mount components is not recommended as their terminals are easily
damaged. Soldering new surface mount components should be done with
special solder that contains a small amount of silver. If you use
regular solder it will leach the silver from the chip terminals and
cause the terminal to fail. All of the above should only be done only
by technicians with some previous experience with surface mount
IMPORTANT NOTE TO ALL:
The squelch operates on signal-to-noise and works very nicely when properly adjusted.
Since there is a squelch delay and hysterisis due to the action of the Schmitt trigger
circuit (necessary to keep the squelch from rapidly opening and closing) there is
a tendency to overshoot the correct setting. The squelch should be adjusted when there
is no signal by turning the squelch slowly clockwise until the noise suddenly stops
and no further. If the squelch control is advanced too far the squelch will not open
even on a stronger signal. This is because it operates on signal-to-noise rather than
carrier strength and since the receiver uses a very low noise GaAsFet RF stage the
signal-to-noise will not change after .5 microvolt since it will already have reached
its maximum full quieting. The hysterisis of the Schmitt trigger gives the effect of
backlash on the squelch control however this is normal and is akin to the differential
on a thermostat. The advantage of a noise operated squelch is that it can operate
on as little as .1 microvolt.
We do our best to make sure the WEPIX-2000 gives you trouble free
performance. Electronics and Murphy's law being what they are though,
it's always possible for some malfunction to occur. There are a few
simple tests you can make to make sure it's not something you're
overlooking or doing wrong. If none of the red LEDS light, check that
the power adapter is plugged into the wall socket and the receiver's
rear panel. Check the receiver's fuse. If the plug that goes into the
12 Volt DC receptacle on the rear panel is frequently plugged and
unplugged, the contacts may loosen. If the front panel LED's scan but
the frequency doesn't appear to change, check the memory bank switch.
In bank 2 the frequency only changes slightly centered around 137.5 MHz
for use with down-converters. Don't use bank 2 for APT satellites. If
everything seems OK but you're not picking up any satellites when
they're due, check the red LED on the circuit board behind the
frequency push button. If it's always on, the synthesizer is out of
lock. Sometimes it's something that can be easily corrected but it's
best to give us a call so we can tell you what to do.