PDP-11/70 Boards

Fritz Mueller fritzm at fritzm.org
Sat Dec 4 19:02:54 CST 2021

> On Dec 4, 2021, at 3:57 PM, Rob Jarratt via cctalk <cctalk at classiccmp.org> wrote:
> What kind of load tester are you looking to buy? I have an 11/45 which I need to test the power supplies on too.

I picked up a Kunkin KP184 this last year, and it does nicely for testing those brick regulators and the power control boards in isolation (a bit more convenient than the old light bulb or power resistor approach.)

Some of the large electrolytics in those bricks will likely have dried out.  I disassemble and “reform" these with a bench supply and current limiting resistor on any of these bricks I have not yet personally serviced (Yes, I know the reforming thing is controversial for some.  I think its safe to say it at least doesn’t hurt anything :-)

Check the big bridge rectifiers carefully; I’ve seen a few of those cracked or toasted.

It’s also been my experience that beyond the bricks that are dead or fail at the start, some smaller fraction will also let go after the first few hours of continuous operation.  The don’t tend to fail with fireworks — you’ll generally get a slowly increasing whiff of magic smoke so generally plenty of time to shut things down.

The operation indicator bulbs on these seem almost always dead.  You can sub CM7381 for the +5V regulators, and OL-6003BP for the -15V.

Last, if you have H742 supplies on your 11/45, be sure to check the small muffin fan above the power control board. It is quite common for parts of the internal wiring harness to rub up against these and stall them out, eventually leading to a fried controller board.

Good luck!  Good winter time project :-)


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