DEC 3000/600 Alphaserver problems

[Jon Elson]

On 1/11/22 10:40 AM, Peter Coghlan via cctalk wrote:
> I have a rackmount DEC 3000/600 Alphaserver containing a H7816-BA power supply
> which doesn't work.  As I mentioned before, this power supply is a real pig
> to work on.
>
> I also have two tabletop DEC 3000/600 Alphaservers which contain H7816-AA
> power supplies which are similar but not identical.  The main difference
> seems to be that the H7816-AA power supplies contain four integrated fans
> (and lots of dust) while the H7816-BA relies on one large fan external to
> it.  I am hoping that comparisons between a working H7816-AA and the failed
> H7816-BA will help diagnose the cause of the failure.
>
> So I dug out one of the tabletop machines, took the cover off and powered
> it up to ensure the power supply actually works before I go any further.
> The green LED came on, the fans turned and the diagnostic LEDs lit up.
> After running for less than a minute, there was a cracking/popping noise
> and a small spark visible from somewhere around three unmarked orange
> surface mount components on the I/O board (not in the power supply) plus
> a whiff of cooked electronics.  The power supply kept running and the
> diagnostic LEDs remained lit but I powered off quickly and poked around
> with my finger to try to find a hot component without success.  I examined
> the area with a magnifying glass under a strong light but I was unable to
> find anything damaged.
>
> I wonder would these orange components be tantalum supply decoupling
> capacitors?  They seem to have a small pip in the solder at the positive
> ends.  After powering back on, two of them had 5.0V across them and there
> was 4.87V across the third slightly smaller one, there were no further
> fireworks (so far anyway) and the SROM mini-console works.

The orange ones are usually Sprague Oxi-Caps, designed to replace Tantalum with Niobium Oxide, and they generally do NOT have the classic Tantalum issue with catastrophic failure after periods of disuse.  When the do fail, they generally turn black.  Some Tantalum caps were made black, some yellow.  Note, the bar polarity marker is usually on the PLUS side on Tantalums and Oxi-Caps.
Yes, failed decoupling caps could lead to intermittent failures.
Jon