Re: TDS 380 calibration and vertical scale problem

Siggi
 

Hey David,

If there were an existing schematic of these attenuators, the odds are the
member of this mailing list would know of them and/or have found them
already.

I think the mission we're trying to push on you, is to reverse the
schematic of the attenuator by inspecting and measuring the original you
have in your possession. Here's an EEVBlog video on how this is done <
https://www.youtube.com/watch?v=lJVrTV_BeGg>, though Dave doesn't do a
super job of it :). Thankfully your attenuator is MUCH simpler than the
front-end he's trying to reverse.

If you take a look at the attenuator schematic in the 2465 <
http://www.ko4bb.com/manuals/download.php?file=Tektronix/Tektronix_-_2465_Oscilloscope/Tek_2465A_Service_Manual_070-6019-01_May87.pdf>
you'll see a high-level picture of what's inside. Here <
https://groups.yahoo.com/neo/groups/TekScopes/photos/albums/501546574> are
some pictures of the attenuator hybrids from my 2430 with the relays
popped. If you compare the schematic to the thing, you e.g. see where the
AC coupling cap is, where the 50Ohm resistor is and where each of 10X
attenuators are. We know each one of the attenuators will have pair of
resistors, and we know there's a 1M termination resistor there. Squint a
bit, and you can see all of those, plus some other passives that have
(presumably) to do with the frequency response of the attenuator.
If you look at the service manuals for older scopes (Tek 485 is my fave)
you'll see full schematics for their all-discrete attentuators.

What I expect you have in your attenuator is similar to any other
attenuator, e.g. there will switching for AC/DC coupling, for the CAL
signal, and there will be resistive dividers for the attenuation ratios
(possibly 25/100) each with 1M resistance. There may also be some series
resistance in the input path to allow for current limiting on overload, and
there is some evidence that you have an open series resistor on your CH2.
There may also be multiple resistive dividers dividing the input by
different ratios into different inputs of the preamp IC, in which case the
dividers will have 2M resistance.


Siggi

On Wed, 10 Feb 2016 at 11:29 david.wilson92@... [TekScopes] <
TekScopes@...> wrote:



Thanks for that info,

I uploaded a attuenuator schematic showing the hybrid's connectivity to
the mainboard, no it doesn't show the schematics for the attenuators
themselves but it is information to review at least. If you haven't
already, useful information can be found on a post I made on eevblog.com
http://www.eevblog.com/forum/repair/tds-380-calibration-error/, click
that link to take you directly to the post.


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