Re: Another A5 board repair attemp - help needed


Siggi
 

On Thu, Nov 12, 2020 at 11:32 AM Rogerio O <rodd414@gmail.com> wrote:

Hi folks,
I think I have posted my previous reply on the wrong post again, since I
can't find it here. Sorry for that.
Here are my findings.
After another careful inspection I found out that I have swapped R2011 and
R2014. Sorry for wasting your time with another silly mistake.
The negative and positive references are now -1.15V and +1.22V (probably
low).
Yup, they're low.


The voltage drop around R2016 is 1.032V so the current through it is about
1mA.
Since the voltage drop across R2012 is very close to zero, it is
reasonable to assume that this current is going to the DAC and it is the
expected value for it (see Siggi's reply).
The voltages at the DAC output have a ppk value of 2.52V on both outputs.
This is low by roughly the same ratio as your references above.


Siggi, on another post mentioned that they should be close to 2*1.36V =
2.72V (this is again low, but I can't say for sure).
I am hoping that these readings mean that the DAC is working properly but
I have no experience to say so.
But there is something wrong with this circuit since it is impossible to
finish the DAC calibration procedure successfully.
When delta is pressed and the cursor is moved to the left, the minimum
negative voltage that I can get is -0.98V and the maximum is +1.24V.
The trimpot adjustment is already fully CCW so no further adjustment is
possible.
My next attempt will be to replace U2420, but I wouldn't like to start
changing components at random.
Any suggestios?
Measure the voltage drops over all the resistors in the DACs network. If
you find one has zero voltage drop, there's your problem (though R2012
should be roughly zero).
I believe the error you have would roughly correspond to R2015's current
contribution, but I'm too lazy to math at the moment.

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