Service manual
has schematics - it's not overly complex and is relatively
easy to fix.
Has anyone here diagnosed a "no power" condition on these?
What was the most likely point of failure?
The maintenance manual goes into all kinds of detail about the logic
board but has nary a thing to say about the power supply, AFAICT.
The power supply is a very simple transformer/regulator linear supply.
It is described in section 4.3.18 on page 4-21 of the ADM3A Maintenance
Manual, which is page 37 of my PDF.
The full schematic of the supply is located near the end in drawing
#129471 which is page 103 of my PDF.
Are you sure the terminal is completely power-dead, and not just
"not working" - Measure the power rails of the logic ICs and also
look for the filiment glow from the CRT - these indicate the 5v and
12v supplies (respectively) are working.
Assuming it is power supply:
I'd begin by disconnecting P3 and P4 and measuring the output of
the transformer windings and confirm that they are close to the
values listed on the Schematic. If no voltage on any windings,
check the fuse, switch and associated wiring between the mains
and the transformer. A good fast check is to measure resistance
across the AC mains plug (not plugged in of course) - it should
read infinite with the switch OFF and very low with the switch ON
.
If voltages are present but low in all or some windings, check the
connections to/from the transformer. If only one output is affected
and connections are good, the transformer may be shorted or open.
Powering through 40w light bulb may reveal shorted windings - with
P3/P4 disconnected the Xfmr should draw almost no current - if the
light bulb glows there may be a hort. Open windings can be checked
in non-powered state with an Ohm-meter.
If AC voltages are OK, I would reconnect P3/P4 and check the outputs
of the full wave bridge rectifiers - they should be at least 2V above the
rated output of the connected regulators (and will probably be higher).
You can check them at the bridges, or at the input side of the regulators.
If voltages are low, rectifier may be bad, or you may have a shorted
regulator. A scope can be handy here as examing the shape of the
signal input to the regulator it will let you see if one leg of the rectifier is
shorted/open.
If inputs to the regulators are OK, check the outputs - these should be
very close (ie: almost exactly) the rated output voltages of the
regulators. If too high, you have a bad regulator (and may have damaged
other components). If too low, either the regulator is bad or the circuitry
beyond it is drawing too much current and causing it to shut down.
A thermal probe can be very handy, most of this stuff heats up if there
is excessive current draw. Once you are well away from the mains,
fingers can be used, but be warned that things can get *VERY* hot,
enough to cause burns. Your nose and ears are also good tools, as
"hot smells" and "sizzling sounds" are things you should pay
attention
to.
Powering the terminal though a high wattage (100w or more) light bulb
can minimize damage if components are shorted - the bulb lights and
acts as a current limiter. If the bulb remains glowing brightly after the
terminal is switched on, power it off and find the cause of the excessive
current draw.
WEAR EYE PROTECTION WHEN WORKING ON POWER SUPPLIES!
I've had shorted regulators explode with enough force to penetrate a
wall beside my workbench.
Dave
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools:
www.dunfield.com
com Collector of vintage computing equipment:
http://www.classiccmp.org/dunfield/index.html
