Re: A 2465 teaser...

 

Here is what I think, and I'm pretty confident.  The channel switch (U400) design "leaks" Ch 5 into Ch1 and Ch2.  Tek decided (for some reason) to fix this by adding logic external to the channel switch.  That external logic is implemented in U485, U475 and U975.  These parts function so that any time horizontal A or horizontal B (A sweep or B sweep) are operating the readout into U400 is clamped to zero at the emitter outputs of U475C and U475D.  In your case, they are not functioning, most likely a fault in U975.  So, the readout vertical reaches U400 and leaks into Ch1 and Ch2, just like Tek always knew it would.

On ‎Friday‎, ‎November‎ ‎10‎, ‎2017‎ ‎11‎:‎47‎:‎44‎ ‎AM‎ ‎CST, machine guy @Mac [TekScopes] <TekScopes@...> wrote:

 
Chuck, I think its from the readout.  Most likely the readout DO circuitry.  Probably in U485, U475, or U975.  There is a complicated switch using emitter coupled logic implemented by U485, U475, and U975.  If this switch and logic fails, readout is coupled to the vertical channels of the channel switch.  I have never tested to see if it affects only Ch1 and Ch2.  But I have seen it.  Check waveforms 50 and 51 on U975.  
Mac

On ‎Thursday‎, ‎November‎ ‎9‎, ‎2017‎ ‎04‎:‎51‎:‎43‎ ‎PM‎ ‎CST, Chuck Harris cfharris@... [TekScopes] <TekScopes@...> wrote:

 
A hook is a trace that goes from vertical to horizontal
with a little curve between. If you could see the whole
thing, it would look like a shark's tooth, or a lazy
saw tooth pulse.

With display off, it looks like this:

(_____________________________________________CH1

(_____________________________________________CH2

_____________________________________________CH3
_____________________________________________CH4

When you turn display on it looks like:

(__(____(____(______(________(_______(((___(__CH1

(_(__(_____(________(___(_____(_((_____(_(((__CH2

_____________________________________________CH3
_____________________________________________CH4

The "(" sections are actually a sharp rise with
a slower fall, and look like random grass. They
appear only on CH1 and CH2, not at all on CH3 and CH4.

The "(" sections after the trigger are at least
0.75 div high, and unaffected by vertical input
condition, or attenuator setting. The initial pulse
hook, is undoubtedly just one of the "herd"... only
it appears with or without the display being turned
on.

I haven't tried replacing the CH1 and CH2 preamp
yet. It would surprise me to find that both failed
in exactly the same way at the exactly the same time.

[Note that CH1 and CH2 are otherwise performing normally,
and in calibration, and that the "(" stuff does not
appear in the CH2 output connector on the back panel.
I would think that would rule out the preamps.]

It would seem more likely to me that it would be
a bad bypass cap on a filter section that feeds both
CH1 and CH2 in parallel. But the power looks pretty
clean, and nothing larger than 1 or 2 mv.

-Chuck Harris

hahi@... [TekScopes] wrote:
I am puzzled. Anyone have any ideas?
Not sure exactly what you mean by hooks ...

This may not be the same but I have seen a few scopes with noise
on the trace(s) looking like about one minor div of digital noise
randomly across the trace(s). All fixed by replacing the preamp(s).

/Håkan

[Non-text portions of this message have been removed]



------------------------------------
Posted by: hahi@...
------------------------------------


------------------------------------

Yahoo Groups Links








[Non-text portions of this message have been removed]

Join TekScopes@groups.io to automatically receive all group messages.