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Hello Lop Pol,

Well... Attenuator blocks are not really swapable "Plug-andPlay" as they are
usually adjusted to match each 7A26's input capacitance...
But, since the behavior is consistent and the performance is good on the other
attentuator settings, I think you successfully ruled-out the attenuator block
as a possible cause.

With all that in perspective, I can only see 3 possible causes for this
problem (if logic is sound and if Mr. Murphy is not fooling around your 7A26).
1. A poor leaf switch contact(s) of the X2 attenuation section (the B
contacts, as they're named on the schematics), either the one before the X2
attenuator block, or the one after the X2 attenuator block.
2. Or the R106 / LR106, whichever is the case on your particular 7A26.
3. Bad contact (or dry solder joint) at the X2 attenuator block contact pins,
but that seems rather unlikely as those are gold plated "collet-type" socket
pins and seems very reliable to me.

From all 3, the most likely seems to be number 1, the leaf switch contacts...
The problem is that those contacts are the ones on the back side of the
attenuator block PCB (the same side of the attenuator rotary cam which is a
lot of work to disassemble).
If the bad contact doesn't clear with contact cleaner (and be sure you use
only the ones that are safe to plastics), I`m afraid you have some tough work
ahead because, to clean them as per the manual, with IPA and paper, you'll
have to dismantle the whole thing.

Rgrds,

Fabio



On Fri, May 18, 2018 at 06:51 am, lop pol wrote:

I changed the attenuators with a set of known good ones. Still the same 20mV

issue.

20mV /g/TekScopes/photo/49370/4?p=Name,,,20,1,0,0
10mV /g/TekScopes/photo/49370/3?p=Name,,,20,1,0,0
50mV is the same as the 10mV so you get the picture.
Pretty crazy bad.