DEC 3000/600 Alphaserver problems

[W2HX]

A bad tant will show 0V across it and it will show 0V (or very close to it) across all the other tants used as filter caps on that same rail. I would turn it back on and measure all of your rails. See which is affected. Then pull out every card in the system that uses that rail (probably all cards use all rails). See if the supply comes back up. If not, problem is in the supply. If yes, put one card back in at a time and check the rails. It will help you ID the bad card. 

For the bad board, look at the schematic for all the caps (tants) on the affected rail. If there are only a few, pull them all out and see if that solves the problem. The system will likely function just fine missing some filter caps. Take each cap and put it on an ohm meter. If the result is 0 ohms, that's your bad one. Pull the rest back and replace the bad one.

If there are many tants on a board then I like the smoke-'em-out method. Pull the board, put it on your bench. If the bad rail is, say +15V, take a very stout bench supply, set it to 5V and current limited to very low, 50mA. Connect your bench supply to the card and slowly increase current. The supply will be voltage limited to something near zero but not quite because of the short in the tant. You may also need to increase the voltage too (even though it may still be zero). Likely would have to exceed 1A-2A or more amps to get the effect you want. The bad tant will heat up and hopefully let the magic smoke out. Or just get hot to the touch. If you had an IR camera, that would be a much more elegant approach but I just get them to smoke. Do wear eye protection as sometimes they explode. 

Here is a video where I used this technique just a few days ago. You can see me increasing the current and you can see how easy it is to identify the bad cap. There were 15 tants on this board, making it very difficult to locate the bad one otherwise.
https://youtu.be/Fh8fGgh4yKY

Sometimes there is more than one bad one. In this case, you may need to increase the current to many more amps. If you notice one smoking before any others, remove it (don't bother replacing it yet) and continue with the current to locate the next bad one, etc.

By your description, it appears that the rail on the stock power supply is plenty stout enough so it isn't being shorted but it is working to help you reveal the bad component. In case the power supply has any inductors that might suffer from a shorted tant, I would remove and use my own bench supply as described above.

Good luck!

73 Eugene W2HX
Subscribe to my Youtube Channel: https://www.youtube.com/c/w2hx-channel/videos



-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Peter Coghlan via cctalk
Sent: Tuesday, January 11, 2022 11:41 AM
To: General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: DEC 3000/600 Alphaserver problems

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.

I have had similar experiences with one or both of my two tabletop units in the past and I was unable to track down the culprits then either.
Neither unit works properly due to cache failure issues.  I wonder could it be plausable that the caches are not functioning correctly due to prior failure of several supply decoupling capacitors on the system board?

Unless these devices have markings on the underside, I have no idea what their capacitance values are.  There is probably no point in trying to measure them in-circuit if there are several all over the board in parallel with each other.  Unsoldering them and trying to measure them then isn't
going to work either if they have failed.   Any suggestions for something
to replace them with that is less likely to go bang in the future?  Could I get away with using components with leads to replace them instead of surface mounted ones or would that introduce too much inductance?

Regards,
Peter Coghlan.