External Scan Control Notes
The external scan control lock operates as follows:
1. The receiver detects a carrier and un-squelches both audio amplifiers.
2. The FAX board detects the 2400 Hz sub-carrier and phase locks to it.
3. The sub-carrier detect line on the FAX board goes low.
4. The receiver scan control line detects the low and locks the squelch
and scan enable lines.
5. If the signal momentarily fades the squelch would normally be enabled
but is held back by a TTL low. This is possible because the phase-locked-loop
on the FAX board will maintain a locked condition for 1 or 2 seconds after
losing the signal.
6. If the carrier signal does not return, the 2400 Hz phase detector on
the FAX board goes out of lock and the sub- carrier detect line goes high.
This enables the receiver's squelch and scan control lines and scanning
NOTE 1: Since the Russian Meteor satellites have an unstable or suppressed
sub-carrier the phase detector on the FAX board is normally turned off (usually
under software control) to prevent a wavy picture. This means that the "hold"
will not operate but the receiver's squelch and scan control will operate
NOTE 2: Starting with February 1992 the WEPIX-2000's have an additional
trim pot for the output amplifier gain (located closest to the speaker rim
near the LM386). This was done because some fax boards have so much gain
that it required the input trimmer to be set almost at zero. By adding another
trimmer to set the gain of the amplifier the input trimmer can be set near
midrange where it is easier to adjust. You should still adjust the gain
with the trimmer mentioned in the instructions which is the first one located
in line with the end of the fuse holder.
TECHNICAL NOTES: When the push button for changing channels is pressed there
could be as much as a 1/2 second delay or it could change immediately. This
effect has caused some users to think that the switch is defective whereas
the reason this happens is that the circuit that drives the channel switching
is driven by a clock with a constant rate. Depending on where in the timing
cycle you press the button will affect the delay. The correct way to change
channels is to hold the button in until the channel changes rather than
just giving it a tap.
About tuning: It sometimes happens that some users with
technical knowledge get the urge to check the tuning of the circuits and
attempt to peak them up. This usually ends up with the user having to return
the receiver for repair. The reason this happens is that the RF stages are
not the typical type that can be tuned to a peak. The RF transistor load
is a resistor coupled to high Q filters.Tuning high Q filters causes a "suck
out" of the preceding stage and places the frequency marker in a trough
between 2 peaks. This is how it should be, however if you try to retune
it to bring a peak to the marker you will actually be detuning the filters.
This will bring on several problems which may not be immediately noticed.
Until we prepare a detailed tuning procedure with graphs it best to leave
the tuning alone.