[Info-vax] SBB power supplies failing slowly

[chris]

On 03/29/22 01:36, gah4 wrote:
> On Monday, March 28, 2022 at 3:00:05 PM UTC-7, Scott Dorsey wrote:
>
> (snip)
>
>> The filter stuff on the primary side seems to last much longer, except for
>> the kickstar cap and I don't know why that one seems to fail so often.
>
>> NOW... I will say that I have only experience with these supplies used on
>> 120V and if you run them on 240V then the two big stab-in capacitors on
>> the high voltage input will have twice as much voltage across them and this
>> may shorten their life. So you may wish to replace those pre-emptively if
>> you are in a 240V country. I have never seen one fail in the US.
>
> The popular design some years ago (as in when I was actually looking at
> how they were built) is a bridge rectifier in 240V countries, and voltage
> double in 120V countries.  So the primary side capacitors run
> about 300V in both cases.  That is, for ones with a 120/240 switch.
>
> Also, it used to be usual to run the oscillator at about 20kHz.
> Some now might run much higher.
>
> When working with the computer museum, we would replace all the
> electrolytic capacitors before powering up the first time.  That is,
> for ones that might be 30 or 40 or 50 years old.
>
> They used to make "computer grade" capacitors.  I don't know
> if they still do that.
>
> In any case, the thing that electrolytics least like is being
> reverse charged. I don't know about the kickstart, but it
> is possible that it gets reverse charged, maybe just a little.
>
>
>

A serious interest here is older workstation restoration and
power supplies are usually the first thing to look at. The
capacitors dry out over time, dues to high temperatures and many
do need to be replaced. Input capacitors often test ok, but the
output rail caps seem to be underspecified and get so hot that
they leak electrolyte all over the pcb as they fail. That can be
fixed by washing and drying, but only the best caps should be
used for replacement. Choose 105 C and low esr rated caps,
as these modern types are often far better than the
originals.

Many of the older systems were poorly designed for airflow and
internal sleeve bearing fans should be replaced with ball
bearing types of higher output if possible...

Chris