Tek 442 / jumpy trace on ch1


Ahlbrecht, Andreas <ahlbra@...>
 

Hello members,

I have a Tek442 oscilloscope with a strange problem.

(The Tek442 is a two channel scope / 35MHz bandwith /
single timebase / very basic scope without any fancy features)

The trace of channel 1 is very unstable. It jumps up and down,
sometimes less, so it looks as if there was some low freq. noise
on ch. 1.

At first glance I thought it might be due to some dirt in the
vert. pos. pots or vert. sens. rotary switch. But not so.
I can operate all the pots and switches without any effect on
the jumpy behavior.

OK. Then it might be a bad solder joint, loose or dirty
connector or something like that. I opened the scope, pulled
every connector within reach, inspected it for dirt or other
obvious problems(none found) and firmly reseated them back into
place.
No success.
So I powered it up and tapped on nearly everything, carefully
watching the trace. No success either. The trace keeps jumping
up and down and does in no (noticeable) way react on any tapping,
rocking or whatever mechanical 'impact'.

I have to say that the scope has been dropped by it's previous
owner, so badly that the metal front frame has cracked in the
lower right hand corner and the BNC connector of ch.1 is slightly
bent to the right. However, the internal mointing points for the
various PCBs and stuff seem not to be affected and no boards seem
to be cracked.

Everything else works just fine and ch.2 is not at all affected.
It really looks like some intermittently bad electrical contact
of some sort, somewhere. But I just can't find where.

I'm stuck, not knowing where to look at.

Any ideas, anyone?

BTW, I performed various google searches to look for any info
reg. the 442 and didn't find much. Is the 442 a very uncommon
scope? Rare? 'Bad' or inferior in some way so it wasn't very
successful on the market?

Any comments are greatly appreciated!

Best regards,
Andi

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