--- In TekScopes@...,
David Sligar wrote:
> Hello, Group --
> As possibly many new members do, my first post is a
plea for help. Once my very nice 532 is working again, I
might decide that I prefer "reliable" to "elegant" -- and
trade this one in for something a little newer.
> I've been using this scope on a daily basis for about
a month. On Friday morning I tuned it on and suddenly --
no sweep. I've been going through the beast as carefully
> as I can, looking for the bug.
> Tubes all check good, no visible damage / exceptions,
"all" voltages measure
> correctly. I suspect one might need a (working)
scope to figure out
> where the sweep waveform disappears. I decided I'd
try running my
> function generator as external sweep -- it worked, so
that says at least
> the horiz. amp is OK. I've now spent a couple of days
> and I'm not sure I'll be able to find what's wrong.
> Amazing how helpless we are without tools! I hope one
or more of you will be able to suggest where I should
> Thanks very much,
> David Sligar
> Grants Pass, OR
A second scope would be very helpful, but you may be able
to do some things without it. Selecting a very slow sweep
speed (1 div/sec) would allow you so see the sawtooth on a
First you need to isolate the main area not working. It
could be at least three – trigger, timebase, or blanking.
Turning the Trigger Stability control to the far right
should force the scope to free run. This does not isolate
the trigger circuit however. It only biases the
multivibrator in a way to have it reset when the sweep
length comparator trips at the end of the sweep.
With another scope, you can trace all of the trigger
circuit fairly easily. I usually use line trigger mode
when doing this, as I know this gives a reliable signal,
eliminating the possibility of a problem in the trigger
pickoff circuit in the plug-in. Trace the signal through
the comparator(V308), and verify the Trigger level control
moves the grid voltage around (pin 9 for all + polarity
settings, pin 2 for all – mode positions).
I was hoping to disable the unblanking pulse to rule that
out, but I don't see a way to do that simply by pulling
tube, as the signal is generated in a dual triode (V430B)
that is shared with the trigger multivibrator.
Unfortunately, the AC coupling to differentiate the pulse
occurs before these tube stages, so there is no place you
can easily short to ground or a supply to get a stable DC
If the trigger is going through OK, then you need to focus
on the time base. Use the circuit description in the
manual to guide your troubleshooting.
Let us know your progress. Your scope looks really clean
and is worth preserving.