Re: 485 No Trace - some progress, question

Robert Simpson

The difference at Pin 5 was just a front panel button. When set the same, both scopes have the same value. Although Pin 15 comes from the delayed sweep control, both scopes are set the same. Now trying to figure out how that circuit works.

--- In, "Robert" wrote:

Funny, as I was thinking about this while falling asleep, the thought of a stuck front panel switch came to mind, like being in single trace mode or something. It will take me some serious manual reading to figure out the switch/signal circuit effects.
--- In, David wrote:

I suppose that is good news since U1560 would probably be difficult to
replace. On the other hand, that just moves the problem into the
inscrutable oscilloscope logic.

I would take a close look at the horizontal and vertical mode switches
because they are subject to mechanical failure and those pullup
resistors. Those are easy to check.

Sometimes a pushbutton switch can conduct a static discharge into the
circuits and damage them so I would check any logic ICs directly
connected to them like U1530, U1535, and U1580.

On Thu, 31 Jan 2013 05:39:01 -0000, "Robert"

Quick test: I tried IC U1560 in the working scope and got a trace, so it looks like some input is keeping it in blanking mode. On to more circuit analysis and tracing.


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