On Fri, 26 Oct 2018 at 01:13 <tekscopegroup@...> wrote:
Comments and suggestions very welcomed about any follow up tests to make
sure I have possibly not missed anything on the A5 cleanup, any know
suspect VIAs to check, or hidden traces to verify they have continuity to
another part, etc. Any other obvious possibilities besides the A5 mess on
why the screen is non-op?
A5 problems are a sufficient condition for no screen. On my 2467 a clamping
zener on the -1.25V reference had gone short, causing no display at all.
You've probably read up on this already, but the resistor network that sets
the DACs reference current is your prime suspect for the no display
condition. In your shoes, I'd buzz out all the traces and the resistors
involved there - this is everything connecting to pins 14 and 15 on the
DAC. Note that if you need to replace R2012/R2013 that they're precision,
low tempco resistors, so ideally you'd want to replace them with like -
especially if you intend to calibrate the scope. The little trimpot is a
frequent casualty of capacitor leakage, so give that a good look. I
wouldn't worry about jiggling it or replacing it, you'll have to go through
the DAC calibration procedure either way. It's a simple enough procedure,
and I think it's safe to assume that bringing the DAC into spec will bring
you nearer calibration than wrecking it.
There was a recent thread here discussing the +1.36, -1.25 reference
voltages and how they derive directly from the DACs reference current. I
don't think on your A5 board they will be brought to a test point, but if
you hunt them down, they allow you to assess the wellness of the DACs
reference, which is otherwise difficult to measure.
