On Sat, 25 Aug 2018 at 16:15 <Sscandizzo@...> wrote:
I went to J119 and checked the voltages. Two points failed:
J119-5 (-4.965 to -5.035) measured -4.53
J119-11 (-7.88 to -8.12) measured -6.30
Oh, goody, maybe you just have some power supply issues to work through.
I don't think I am properly measuring total p-p ripple; my values seem
significantly less than (a factor of 10) the tolerances.
Is this using the settings specified in the service manual (see below)?
AND I have no clue how to measure the p-p at 2x line frequency. I suspect
it's obvious, but I'm at a total loss.
The service manual specifies using a 1X probe, AC coupling, 2mV/DIV, LINE
trigger, 5ms/DIV and 20MHz bandwidth limit on. This is what the LINE
trigger is for, pretty much, this way the ripple on the rail will stand
still on your trace.
I'm guessing total ripple would be the largest excursions you see on an
analog scope of the time - not sure how to reproduce that on your cheap
digital scope :).
All the boards appear to be from 1987 and populated with late '88 chips &
hybrids. Not a single smd to be found (correction, a small daughter card
about 0.5" x 2" near the top right of the main board).
I've never heard of the DAC reference going bad in anything but the SMD
version of the A5 board, so scrap that whole angle :).
You still may want to measure the 1.36 & -1.25V reference voltages, as they
can indicate trouble elsewhere as well.
