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Old December 2nd 05, 04:10 AM posted to sci.electronics.repair
sck0006
 
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Default tek 2465a, again, pleeease help, cal procedure help needed desperately

short intro
Same problem as before, tek 2465a... In B swp, the cursor readout
delta t reads wrong when using superimposed waveforms (using the
single vertical line cursors reads correct). Everything else on the
scope works fine, just the cursor readout is off in B swp. I've
replaced many parts on the scope with the same exact outcome. I
replaced ALL caps in the sample and hold circuit, in fact I replaced
all caps that were that same type on all the boards. I replaced the
delay loop with one out of a working scope (it has hv problems that we
can't get rid of), I replaceed the ic, can't think of the number but
it's the custom tek one that usually goes bad, still nothing (you
know, the dip looking ic w/ the metal tab hanging out the end). I'm
starting to wonder if it's an adjustment issue.

So I ran through all the adjustment procedures except Cal 07. The one
that has me wondering is in cal 1, steps 17 through 29.

The manual says to hookup cal generator to CH1 on the IUT(instrument
under test, of course), and also to CH1 on an additional bench scope.
It then says to hook up B Gate out from IUT to Ch2 on bench scope.
Ok, so far no problem. Then on bench scope, view CH1, trigger off
CH2, both to 50ohm coupling (assuming dc?). So, you're only viewing
CH1? It then says to put in time markers, for instance in step 17, of
1uS. Ok, no prob. This is where I get confused.

Step jj.
For each step in Table 5-3 (except step 28), adjust the delta control
for roughly the listed number of markers over the center 8 divisions,
then superimpose markers on bench scope screen. Manually set SEC/DIV
setting of bench scope to keep a usable time mark as listed in Table
5-3. Use IUT DELAY POS to bring markers on screen. Some sweep speeds
might require adjusting holdoff to see both markers. When markers are
superimposed, press and release the trigger coupling switch. blah
blah blah

Ok. So from the manual, you're only viewing CH1. Ch1 is the time
marker input from the cal generator, triggered by B gate out from the
other scope. So, what are you superimposing. Adjusting delta adjusts
how many marks per div. That works fine. Delay pos adjusts something
on B gate out. I honestly don't know what to be looking for in these
steps. Just by viewing CH1 triggered by CH2, adjusting Delay Pos will
move the time marks left and right on the scope screen. I viewed both
channels, and there are two pulses, looks somewhat like this,

__ ________
| |__| |_______

with time marks on CH1 like:

| | | | | | | | | | | | |

Adjusting Delay Pos adjusts pulse width of the first pulse. I really
don't know how to describe it. The marks will move left and right.
just to finish up the cal, I turned the knob full ccw until no further
movement, full cw until no further movement, and then lighed up the
mark that was between the two marks at the two extremes with the
rising edge of the first pulse. I performed all the steps this way,
but I still have an offset on the cursors. I'm sure I performed this
step wrong, and am desperately seeking help for this situation,
because if this doesn't work, we're going to have to buy a new (or
newer) scope.

Has anyone performed these steps before, and if so, would you please
tell me what I'm supposed to be looking for on this bench scope? I
will gladly clairfy anything I can, I know this is probably not
explained very well. Thanks very much for reading, and all comments
always appreciated.

Thanks, Steve


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