[time-nuts] LaVoie LA-800D WWV Comparator is Working -- Update
dave at veratek.com
Sat Feb 6 22:14:07 UTC 2010
Late last year I was stuck on getting my LA-800D WWV timebase comparator
operational and badly needed a schematic. A kind soul on this list
(thanks Thomas!) provided me one via email. Using that schematic, I
found the major problem with my LA-800D on Dec. 31st., and it got to
bring in the new year via WWV on 5 Mhz. It must of felt like
Rip-Van_Winkle going to sleep in the 1960's and waking up in the year 2010!
The major problem it had was the 90 degree phase shift network going to
the XY scope display was way off tune. This network fed both the display
and later audio stages so if it was off, then everything was off. I
just tuned to it back on frequency and everything else worked fine. I
actually pulled the phase shift assembly out of the unit and swept it
with a signal generator to convince myself that's all it was. Since the
tuning adjustments on the network still had intact factory paint dabs, I
assume that one of the mica caps in the phase shifter drifted over the
40+ years this unit had sat around. I did go thru the various stages
with my scope but pretty much ended up not adjusting anything else.
I had to do a few other things such as change the line cord, and bypass
a leaky AC filter network, but the unit is pretty much intact and
functional with original tubes, capacitors, etc. I did change the weird
front panel antenna connector to a standard SO-239 which mounted in
exactly that same holes.
The unit is very sensitive on 5 Mhz requiring less than .5 uv of WWV
signal to get a nice circle on the XY display. The audio is sounds
pretty good too with the original speaker. The ticks and time
announcements are clear. I was fooled at first about how it compares
the external local timebase to WWV. Instead of putting the timebase on
one set of scope deflection plates and the WWV signal on the other for a
lissoujous pattern, it just splits the 5 Mhz WWV signal and delays one
path by 90 degrees using the before mentioned phase shift network.
(Both the timebase and WWV signals are converted to 10 Mhz first) When
the 0 degree WWV signal and 90 degree WWV signal are applied to the XY
channels of the scope display you get a nice little circle.
The size of the circle is dependent on WWV signal strength but limits
out at 1 or 2 uv of signal to a fixed diameter. The timebase signal is
used to intensity modulate the beam on the scope so that you get a
rotating half circle assuming a 5 Mhz timebase is applied. The speed of
the rotation tells you how close the timebase is to the WWV reference
frequency. If you use a timebase at another sub-multiple of WWV you may
get more circle arc segments rotating on the scope. The idea seems to
be that you adjust your timebase so that the rotation speed is as close
to zero as possible.
The LA800D has 5 Mhz WWV reception and 15 Mhz WWV reception. However,
to use the 15 Mhz reception, you have to provide a timebase signal
because the unit uses your timebase signal to mix the 15 Mhz WWV down to
5 Mhz. There is a multiplier chain in the timebase path so that in
theory, all you have to do is provide some sub-multiple of 5 Mhz as a
timebase anywhere between 500 Khz to 10 Mhz. The unit did work the best
with a 10 Mhz timebase so I suspect the multiplier chain may be out of
I appreciate the comments that I received earlier on this unit and know
that I am repeating some of what you have already told me. However, it
is pretty cool to actually see the unit work on discover what it does
myself. Hopefully someone will find my information useful also.
This is a pretty neat little device, but I need to find a home for it
since I could use both the space and some money. I'll post the manual
with schematics to BAMA if anyone wants to look at them. The unit
itself needs more cleaning and probably a more serious alignment. It is
missing the bottom cover. I don't think it ever had a top cover since
it is intended to be rack mounted. I may end up placing it on Ebay but
if anyone here is interested, please email me. I would not mind at all
seeing it going into a museum of some sort.
Oops -- BAMA is off-line for uploads. Here is a link to the manual...
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