Dominique,
I agree with Rod’s suggestion to check the current, but one other
possibility is to check for small resistances, maybe considerably less than 1
Ohm, somewhere between the 5V regulation and the delivery of 5V to the logic
boards. If a connector has corroded or a solder joint has developed a crack,
there could be such a resistance. Then the regulator would actually be
controlling the output to 5V but the logic boards would only be getting 4V, 3V,
etc. off of the bus. The difference would scale with the number of boards in
the way you describe.
Hope this helps, apologies if you have already looked at this.
- Mark
210-522-6025 office
210-379-4635 cell
On Aug 18, 2017, at 12:25 PM, Dominique Carlier via cctalk
<cctalk@classiccmp.org> wrote:
> Some news !
>
> Following a risky way (but I did not see how to do otherwise), I deactivated
> the Power Fail by hiding the contact number 23 of the two power supplies.
> The idea was to avoid automatic protection by lowering the regulated voltages
> (+5V and 15V) and see first which unit was involved (G1 or G2), and also
> which voltages became weak, at what level it is lowered, and according to
> which board (model or number of connected boards).
>
> Results of the observations:
> - This is definitely the regulated +5V of the G2 power supply. More I add
> boards more the + 5v level goes down. +5v, +4.8v, +3.6v, +2.9v. It remains
> stable however with just the CPU and the three core memory boards, it becomes
> difficult for the power supply when I add boards in addition to these.
> - This is definitely not a problem at the level of the Power Fail circuit.
> - The big capacitors are not in fault (I rechecked twice).
> - So this maybe a problem at the level of the regulation itself, the +5V
> balancing system ?
>
> Question: a faulty voltage regulator can behave in this way? I always thought
> it worked or it did not work, but not between the two states depending on the
> charge.
>
> Anyway, suggestions are always welcome ;-)
>
> PS : I'm starting to want to put another power supply for that regulated +5V,
> and bypass the +5V regulated of G2, but it would be a shame and not in the
> spirit of a restoration in my opinion.
