Re: My 2465B has arrived + A5 Board leaky caps cleanup

Chuck Harris

Hey! Don't muck with the DAC adjustment, unless you are prepared
to do a full calibration!

It is not supposed to be +1.25 and -1.25 volts, but rather a 2.500V span.

Sure, if you are a perfectionist, and diddle with the 10K reference
resistor values, and put in perfect opamps, it can achieve those values,
but that is not a necessary goal: 2.500V span when doing the calibration
routine is the necessary goal. -1.25 and +1.36V references that are *stable*
is the goal, as any errors in them get stored in the calibration constants,
and as such calibrated out, when the scope is calibrated.

Any tweaking, or replacement of DAC parts that you do will require a
full calibration of your scope. Why? Because you have no idea what they
were like when it was last calibrated.

If they were on the low (or high) side of spec when last calibrated, and
you adjust them to be right on, with a DVM that was on the high (low) side
of spec), they errors could add up to be wrong enough to perform poorly.

-Chuck Harris

tekscopegroup@... wrote:

Hi Tom, thanks very much for your input. Bellow just a quick follow up.

On Wed, Oct 31, 2018 at 04:08 PM, Tom Gardner wrote:
Highly unlikely the 10MHz is incorrect - but you can always measure it on your
other scope :)
Is there a test point designated on A5 to check on the 10MHz oscillator?
Just for fun and while I am at A5, might as well check that, although after reading Chuck's detailed explanation on how the scope actually performs the frequency measurements, the 10MHz is not used in the process, and I suspect that it will be within spec.

What's more likely is that the A5 voltages and/or calibration values are
invalid. Before tweaking anything, make sure the reference and DAC output
voltages are as specified in the service manual.
Yes, at the time of making those measurements both the positive and negative voltage reference values where/are still off. Did not want to touch the adjustment until the two 10.0K 0.1% resistors that where damaged by the leaking caps are replaced with the correct parts. They should arrive sometime this week, and once they are in, I will tweak R2010 to set the reference voltages to +1.36v and -1.25v.

Thanks again.


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