Hi Arnold,
At a Dutch forum you mentioned an important finding I think. With "open" 50R resistor and scope set at 1M, 75R and 50R respectively, the DC input resistance appeared to be Infinity, 75R and 50R. With repaired 50R resistor the first figure was 1M. So I guess the 50R resistor is only used in the 1M input stream, with some kind of protection immediately after that resistor. The protection shorts to ground and the resistor gets roasted.
Albert
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On Wed, Nov 28, 2018 at 04:48 PM, <arnoldniemeier@...> wrote:
Hello Tek fanatics!
I have an MSO 3012 and I lent it to a friend. That was the last time it leaves
the house!
It came back with the message: something is wrong... Well you can say that
again!
I've done some tests and I've put the story on the "official" (and completely
dead?) Tek forum:
I don't know how he did it. But somehow two resistors, which are in series
with the
incoming signal, were burned out. I've put in two 47 Ohm resistors for test in
parallel
and the scope seems to work well then. The input impedance was nicely 1Meg
again instead of oc.
In the meanwhile I've checked and confirmed that these resistors need to be
50.0 Ohm.
(At my work was a 4 channel version, and I've checked it there) I've replaced
the
resistors with thin film 50.0 Ohm resistors. All ranges are working normal
again.
But I have some questions left...
What happened here? I think it had a huge over voltage/HF power. Probably
some crowbar construction with the 50 Ohm resistors protected the scope?
If there is a protection circuit, I would like to know if that is still
working well.
Are there any schematics available on these MSO 3012 scopes?
Thanks in advance!
