My 7704A does the same thing. I thought it was normal until I got a
R7704 where the readout characters stay put no matter what the
vertical position controls do (It's the first thing I checked on the
scope). I have 7A26's, 7B70 and a 7B71. I also tried cleaning the
connectors in the 7704A but didn't get an improvement. The power
supplies looked ok to me too. I'm also curious if anyone else has any
ideas.
Cheers,
Neil
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On Tue, Sep 23, 2008 at 7:37 PM, fuzzbrain01 <fuzzbrain01@...> wrote:
I have a 7623A which has developed an unusual problem. Perhaps
someone in the group has dealt with this before?
I have 7A18 and 7A26 vertical plug-ins, and a 7B53A timebase plug-in.
The traces for each vertical channel are visible and operating
normally. When I turn on the readout, I can see the expected
characters; however, the position of each vertical channel's amplitude
readout moves with the vertical centering adjustment for that channel.
The readout for the timebase seems to move with channel 1 of the
selected (right or left) vertical plug-in. When I put a sine wave on
one channel, the readout for that channel has an added vertical offset
that corresponds to the AC component of the sine wave input. It's as
if it were dancing! If I use the variable adjustment on the timebase,
I can "zero-beat" the vertical offset error of the character.
I checked the low and high voltage power supplies, which were all
within tolerances. I cleaned the contacts of the plug-in connectors
on the main interface board, and re-seated the plug-ins in the
mainframe. I swapped the vertical plug-ins, and tried a different
timebase plug-in. Still the same problem.
The symptoms seemed to indicate a problem with vertical channel
switching in the mainframe. I read through the theory of operation
for the readout system, and looked through the schematics. I
concentrated on the A4 Vertical Interface board. Using another
'scope, I checked the waveform at the base of Q2390 (X/Y inhibit). I
found normal voltage levels, and pulses of the correct amplitude that
appeared in small groups, that I took to be the character slots.
Checking the emitter and base of Q2390, I also found correct voltage
levels (per the blue voltages given in the manual's schematic). I
noticed that the X/Y Inhibit line also goes to pin 6 of the Vertical
Channel Switch IC, U2340.
Does anyone have a cross-reference or data sheet for 155-0022-00,
U2340? Without knowing what's inside, it's difficult to know what to
expect on the output pins.
Or perhaps I'm on the wrong track. Has anyone seen this before? What
should I look for?
Thanks for your help.
