Re: 7904A and X-Y Display -- Help !


 

Good idea. I would have suggested it but I didn't know you hand another
timebase. If you switch the CH2 Polarity to Invert it should switch the
angle of the pattern 90 degrees. Alternatively you can stick a few
microfarad capacitor in one of the two signals going to the vertical plugin
to generate a phase delay. It will lower the amplitude and you will probably
have to increase the frequency a few orders of magnitude but you will
eventually get an oval at some combination of frequency and capacitance
since the capacitance is in series with the 1Mohm of the vertical amplifier.

I don't know what relays Tek used in the 7904A but often they are in
sockets. If they are in sockets and they are the same try swapping them. If
they are in sockets and they are not the same try removing them and applying
a voltage to each coil and listen for the click. The coil voltage and coil
contacts should be marked on the relay. If they are reed relays try holding
a magnet next to each one and you will hear a very, very faint click as the
contacts close or open.

If the relay's only have a Tek part number on them someone on the user group
will be able to tell you what the pin outs and coil voltages are.

Dennis

-----Original Message-----
From: Jerry [mailto:jsternmd@...]
Sent: Monday, April 15, 2013 5:04 PM

Hi Dennis,
I made notes below next to each of your check steps. I am pretty certain
it's the XY Delay board problem. I had some success see my notes to you
below.
Jerry

<snip>
I WONDERED WHAT WOULD HAPPEN IF I HAD TWO 7B80'S IN THE A AND B SLOTS. I
HAVE OTHER 7B80 BUT IT DOESN'T HAVE THE XY SO I PUT THE XY ONE IN SLOT 'A'
AND THE OTHER IN SLOT 'B'. I TOOK A 1KZ SINE AND SPLIT IT TO VERT AMP CH1
AND CH2 AND SET BOTH TO THE SAME VOLTS/DIV. I SET THE TRIGGER TO CH1 AND
DISPLAY TO CH2. WHEN I PUSHED THE 7B80 DISPLAY MODE TO XY - THE SCOPE
SWITCHED TO THE B CHANNEL BUT I GOT
THE ANTOICPATED LISSAJOUS PATTERN !!!! I WANT TO CONFIRM AND GET TWO
SIGNALS OUT OF PHASE TO DOUBLE CHECK. THEN I WILL CHECK HOW THE SCOPE
BEHAVES WHEN I CHANGE THE TWO SWITCHES ALBERT MENTIONED ON THE XY DELAY
BOARD. I STILL BELIEVE THEY DON'T WORK AS DESCRIBED IN THE MANUAL IN THE
NORMAL TIMEBASE SETUP. I THINK IT'S A BAD BOARD AND ONE OF THE RELAYS IS
NOT SWITCHING CORRECTLY BUT I MAY HAVE A WORK-AROUND. ANY THOUGHTS?

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