Microelectronics | Hewlett-Packard 6624A System DC Power Supply

Synopsis

This system DC power supply has two issues to fix. First there are the dreadful RIFA film capacitors that are waiting to explode. And second there is the very noisy AC motor fan.

Otherwise the power supply appears to be working correctly. I just made some adapter cables since this supply exposes all connection terminals at the back of the unit. With these adapters I can easily connect any device that requires power.

Repair and Maintenance

On each power supply printed circuit board, there are five RIFA film capacitors. Two are rated 22 nF / 250 V, one is rated 15 nF / 250 V and the remaining one is rated 2.2 nF / 250 V. I replaced all of them with WIMA metalized paper (MP) safety capacitors of the same ratings. Believe it or not, RIFA capacitors are still available. I hope they have changed the yellow molded resin composition. Otherwise they'd just end up like the old ones, in twenty years from now.

The small 2.2 nF parts are decoupling to the chassis ground.

Quite a weird capacitance if you ask me. I have rarely seen 15 nF capacitors in the gear that I repaired.

I find a bit ugly the font used by WIMA for the capacitance markings on these MP parts. But the fact that they're new and from a reputable manufacturer surpasses all inconveniences.

RIFA parts that are more or less cracked.

That's it for the capacitor replacement so far.

I replaced the 115 Vac fan with a Noctua S-12A FLX part. This fan should be spinning at 1200 rpm, moving 63.2 CFM at a 17.8 dBA noise level. The Noctua fan has half the airflow of the original fan. But I did some tests with the power supply on close to full load, and the ventilation system assures a good operating temperature. At least on my well ventilated workbench. If you want to make sure the power supply will work in a tightly packed 19" rack, then you must find a replacement fan that complies with the original specifications.

I investigated the schematic diagram and found out a non-populated 2-pin header on the GPIB PCB assembly. I just soldered a 2-pin connector and checked the voltage. I measured 13 Vdc unregulated. This should be good for the Noctua fan. The modification looks original and unobtrusive. This appears to be the new revision of the GPIB PCB assembly with SMD components. The old one is constructed using through hole components.

Here is the new connector that I soldered. It is designated P210.

And the solder side.

I made a connection cable from the fan to the new connector and everything appears to be good.

Now, if I listen carefully, I can hear the mains power transformer 50 Hz hum. But overall the unit is very quiet compared to when it had the original cooling fan.

Conclusions

This is a good power supply but a bit unfriendly in terms of quick adjusting of voltage and current settings. At least if using the front panel keyboard. If using the computer to control the power supply settings, then things are better.

I have looked for other possible failure points in this supply. I measured the ripple filter electrolytic capacitors and they appear to still be in good condition. There is no evident failure prone component despite these electrolytic capacitors.

In the end, I'm glad the updates were easy to implement.


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