PDP-11/04 - Power Supply Part 6

The failed capacitors are replaced so the regulators are ready to test on the upcoming weekend.

The 11 has three regulator modules.



2 x H7441 5V regulators are switching supplies that use the 28V from the main transformer. The main 22000uf capacitor in one of these was faulty. Replaced it with a 20000uf modern one (notice the size comparison between the two pictures). If there is still ripple on the output then another one can be paralleled up. The other 5V regulator looked like the 22000uf capacitor had been replaced sometime at some point and that tested out OK. In both 5V regulators I replaced with small 560uf and 1200uf capacitors in the CLC filter that cleans the switching noise from the output as they looked to be leaking.

5VDC Power Supply

5VDC Power Supply

1 x H745 15V regulator which is an older linear design. This had a massive 33000uf capacitor but it was also faulty. The smaller 4700uf capacitors on the output where fine. 33000uf/50v capacitors similar to the original are not available at any reasonable price so I replaced it with 3 x 10000uf capacitors in parallel. The three caps stack well within the existing case. The packing shown in the picture ensures they don't short together. That can be replaced once it is tested.

-15VDC Power Supply


PDP-11/04 - Power Supply Part 5

An interesting study in the changes in capacitors over the last 25 years.  The new one is 2000uf less (22000uf v 24000uf) than the original but otherwise the specs are the same as best I can tell.

5VDC Power Supply

Different mounting but nothing solder lugs and cable ties couldn't solve.

5VDC Power Supply


PDP-11/04 - Power Supply Part 4

Only three modules left..... the next 2 are H7441 5VDC regulator modules.

5VDC Power Supply

Finally something faulty... it proves that the slow and steady approach works.  

One of the two 24000uf 50VDC caps is faulty.  It appears to be shorted and won't hold a charge. 

5VDC Power Supply Cap

The other is fine (at the moment anyway). Time to find replacements


PDP-11/04 - Power Supply Part 3

With the power control, main transformer and fan assembly operational the next module in the power supply is the 15VDC regulator.  This mounts on the other side of the power control box and is powered from the 28VAC output of the main transformer.

15VDC Power Supply

As well as 15VDC, the module also provides AC and DC Low signals and a 50hz signal for the real time clock on the DL-11W serial board.

There are two electrolytic capacitors on the board that needed testing before I was willing to connect it to the main transformer.

I built the capacitor test jig described by on http://dustyoldcomputers.com/pdp12/pdp12/restore1.html using junkbox parts.  

Capacitor Test Jig

Combined with 3 batteries saved from the UPS at work I was able to test and both capacitors on the 15VDC regulator board and they seem to be serviceable.

Capacitor Test Jig


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Contact Andrew Quinn

jaquinn@ihug.co.nz http://twitter.com/jaquinn