Re: 7a26 rise time


Brendan
 

On Fri, May 18, 2018 at 02:24 am, cmjones01 wrote:


On Thu, May 17, 2018 at 10:16 pm, lop pol wrote:
Messing around today after i THOUGHT i had things straighted out, I was
looking at the rise time again and 50mV is good 10mV is fine but now 20 mV
only has horrible rise time. Do you guys think that means another cap is
open
somewhere?
If it's only happening on the 20mV/div setting, it's probably something to do
with the attenuator compensation capacitors. These are also prone to problems,
though I've found that it's not so much the capacitors that cause trouble as
the solder joints to the pins of the attenuator module. They need a careful
hand and a fine iron to tidy up.

You won't be able to set the attenuator compensation correctly without an RC
normalizer on the input. I made my own with a little box containing a 1Mohm
resistor and a trimmer cap (set the trimmer cap like you would a scope probe
on a known-good input). Without the normalizer, the attenuator will appear to
set up correctly though some adjustments have very little effect, but in fact
the result is badly wrong. The adjustments on the attenuator modules interact
because they're cascaded, so it's important to follow the procedure in the
manual. I found that the 100mV/div setting (the first 10x attenuator) was hard
to get right so that the square wave looked right both with and without the
normalizer, so I had to make gentle tweaks adding and removing the normalizer
to get the best setting.

The point of all this is to get the frequency compensation and input
capacitance the same at all attenuator settings.

The procedure I use if I can't be bothered to find the manual is this:
- set attenuator to 10mV/div (straight-through, no attenuation)
- connect fast rise square wave generator to input with in-line 50R terminator
(to make sure there are no aberrations due to the cable), *no* RC normalizer
- set up LF/MF/HF trimmers on amplifer board for optimum settings
- insert RC normalizer between square wave gen and input. You can remove the
terminator if it makes it easier for your generator to produce the right
output, because we no longer need such fast edges for the rest of the
adjustments
- adjust normalizer trimmer for optimum square wave
- set attenuator to 20mV/div. Adjust 2x attenuator trimmers for optimum corner
and flat top.
- set attenuator to 50mV/div. Adjust 5x attenuator trimmers for optimum corner
and flat top.
- set attenuator to 100mV/div. Adjust first 10x attenuator trimmers for
optimum corner and flat top.
- check 200mV/div and 500mV/div settings on the way past
- set attenuator to 1V/div. Adjust second 10x attenuator trimmers for optimum
corner and flat top.
- check 2V/div and 5V/div settings
- go back and check all attenuator settings again
- remove normalizer
- check all attenuator settings again
- repeat until it's good enough

Don't do this without the screening cover over the attenuators because it
makes a significant difference to the adjustments. I wasted enough time with
that!

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

20mV https://groups.io/g/TekScopes/photo/49370/4?p=Name,,,20,1,0,0

10mV https://groups.io/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.

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