8714 power supply problem

[Clueless]

The power supply on an 8714ES fails after it gets warm. It takes a couple of hours, then if the system is turned off and on again it will not restart. I believe that it is the 5V output since that is starting out low, and goes lower over time. I noticed the diagnostics reporting it at 4.68 once when it was in a reboot cycle. It is rated at 5.2V, and under light load it regulates to that. I pulled the supply and gave it a good visual inspection. Also spot checked ESR on the larger caps. I did not see any problems visually or with ESR. The +12 is at 11.85 and the other supply outputs look OK. Has anyone had a similar experience and suggestion where to look ? BTW, the unit is one of the later models, built in 2000, and has not been in continuous use.

CL

[TheSteve]

The older models with the same supply are known to have caps that fail. I'd replace all of the electrolytics on the board with the DC supplies on it if you can't find anything else obvious.

[TAMHAN]

If your unit has one of the infamous "Boeschert Cartridges" inside, TheSteve is likely to be right. See here:


Tam

[Clueless]

No Boschert in the 8714 but I do have a 53310A and two scopes with those. None have failed but when I opened up the supply for the 53310A I found one cap that was starting to leak. Thanks for bringin it to my attention. It looks like a pretty big project to fully re-cap these Boschert supplies. As for the 8714, I can't make it misbehave on the bench. It seems to have healed itself. I tried running on the bench with a load and did some further inspection and probing. So far the only thing that was not proper was a 10 ohm carbon comp resistor that drifted up to 14 ohms. As best I recall the red LED was on and green off when the problem occurred, which is a combination not listed in the troubleshooting section of the manual and should be impossible. Oh well. 

[TheSteve]

Is it possible there is something wrong with the rest of the 8714 and the current drain increases as it warms up?

[Clueless]

Yes it is possible and I have not pulled out any of the modules for an inspection, other than the CPU. Since it "healed" itself I measured the actual current draw on the supply. It was well under the supply's rated output:

For 5.2V, rating is 8A, measured 4A
on 15.5V, rating is 3A, measured 1.4A
on -15.5, rating is 2A measured 0.6A
on +12, rating is 3A, measured 1.46A

The impossible LED combination of red, no green would have to point to the power supply if my recollection of the LED state is correct. But I will pull the other boards and inspect.

BTW, this particular supply has an Agilent label on it, and all the PCB have Agilent etched onto them, and I see some components with HP style part numbers xxxx-xxxx. It appears to be a match to the publicly available schematic for that power supply. Pretty decent design I might add. More complicated than necessary, partly due to each output being fully isolated. They all have a common ground at the backplane.

CL

[TAMHAN]

Hello,
actually, I would not worry too much. My father and I got it done in the time from saturday 8h to Sunday 9h - with a lot of quarreling, going to the club (like 3h, as we had another quarrel there who is the bigger VIP) and going shopping for parts twice (2h drive) intermixed.

The main thing you need to do is CLEAN UP the tip of your soldering iron after EVERY SINGLE FUCKING CAP. If not, the oxidation makes heat transfer difficult on desoldering. Also, the PCB of these Boeschert cartridges usually is built tanky...so no sweat re applying a little bit of force. Finally, all of these caps are gonna croak. So no need to be kind to them - rip em out like the cancer they are.

Tam

[Clueless]

If it takes 2 hours per instrument, that adds up. I have at three instruments with Boschert / CA  power supplies, in addition to the 8714. I went to order caps and it took me an hour just to list all the values, lead spacing, diameter, and put together an order for them. I went with Nichicon UPW series for all but the large snap-in caps.

[TAMHAN]

Hello,
the buying was the issue, I understand that. I was lucky, I simply bought some LowESR crappe at the store.

But, re repairing: I would definitely do it. Think of it this way - you do it once, and have 10 years of peace!

Tam