VAX 4000-500 PSU Overload?

[Glen Slick]

On Tue, Dec 15, 2015 at 12:38 PM, Robert Jarratt
<robert.jarratt at ntlworld.com> wrote:
>
> Tonight I replaced the leaking capacitors on the +12V/-12V board, and also
> replaced the two similar ones that looked and measured fine. I put the PSU
> back together, put in a couple of less important boards and drives, and the
> machine now seems to power on OK. I measured the ripple using the power
> connector for the front panel and that looks OK too. Unfortunately though,
> that connector only sends out +12V, +5V and -12V. It does not have output
> for +3.3V, so I have not, so far, been able to check the ripple for this. It
> is a bit awkward to test on the bench with a dummy load.
>

Nice work and good outcome. Gives me some hope I might be able to get
the H7874 supply from my BA430 VAX 4000-200 repaired and running
again.


> As I did not replace the capacitors on the +5V/3.3V board, because the
> underside is very hard to access, and I am reluctant to pull them off and
> solder from above, I would like to be sure there is no ripple on the 3.3V
> supply. So, I was wondering if anyone has any neat tricks for probing the
> +3.3V supply with the PSU installed in the machine?

On the BA440 pedestal chassis lift off the two front door panels and
then remove the 5 screws on the left side and the 6 screws on the
right side that hold the front door mounting brackets in place and
then you can lift off the outer skin panels in one piece. After you
remove the outer skin there are 18 screws (if I counted correctly)
that hold the rear metal skin over the backplane area. After you
remove that you'll have full access to probe the square pads on the
backplane next to the power supply connectors. The pads are not
labeled on the back so make a note of the voltages when looking at
them in from the front when the power supply is removed.

It might sound like a lot of work but it's really not bad, assuming
the BA440 chassis isn't buried under stuff where you can only get at
it from the front.