[Info-vax] MIcrovax 3100-98

[Scott Dorsey]

Bill Gunshannon  <bill.gunshannon at gmail.com> wrote:
>On 11/6/19 8:40 AM, Scott Dorsey wrote:
>> Bill Gunshannon  <bill.gunshannon at gmail.com> wrote:
>>>
>>> As long as we are talking power supplies, does anyone know anything
>>> about the power supply in the VXT 2000+?  Mine has died and I was
>>> wondering what the chances were I could replace it with a PC power
>>> supply.
>> 
>> I've never worked on one of those, and I don't even have prints for them.
>> What is it doing?  Totally dead or is it crowbarring?
>
>It made a loud pop and all the magic smoke escaped.  No sign of
>damage other than one possible electrolytic.  I don't want to
>start butchering it without documentation.  But, if the outputs
>are the same as provided by a PC PSU I have a number of them that
>would fit inside the box.

If it smelled like electrolyte, it probably was.  If it smelled like
burning FR4, it is likely to be something far more alarming.

In your place, I would look for the failed electrolytic and shotgun out
all the electrolytics on the secondary side of the transformer as well
as whichever one clearly failed.  But FIRST I would look for damage to
the big power transistor and I would check said transistor with a meter
to make sure it's not wiped.  If the power transistor is good, there is
likely little to no collateral damage.  If the power transistor is wiped,
everything on the gate side of it is suspect (and the pwm controller IC
is almost always involved in the failure cascade).

Switching supplies all pretty much follow the same model.  The schematic
is useful, but the number of possible schematics is limited and most are
based very heavily on the sample circuits for the PWM controller used.

Switching supplies are not all that hard to repair once you get the hang
of it.  There is a lot more going on inside the box than with the simple
linear regulator of the PDP-8, but the same thing is going on inside all
of them and once you can figure one out they are all okay.

Your main worry is that once the electrolytics start to leak, they start
to cause corrosion on the board and then you get bad traces and bad vias
and that becomes a nightmare to fix.  So get the board clean and get
the suspicious caps out before it's too late.
--scott
-- 
"C'est un Nagra. C'est suisse, et tres, tres precis."