PDP-11/70 Boards

Rob Jarratt robert.jarratt at ntlworld.com
Sun Dec 5 03:37:42 CST 2021

Thanks for the tips. The transformer that drives the bricks is a real beast. Did you find an alternative more convenient way to power them on the bench?

I don't see the Kunkin unit for sale except for a couple of items on ebay from China. I am really reluctant to buy anything at all from China. I couldn't find a manufacturer site for them, and I was wondering if there are alternatives since they don't seem to be particularly available. I did find this which is a bit more expensive (I know Rigol is Chinese, but it seems to be decent quality kit) https://www.rigol-uk.co.uk/product/rigol-dl3021-programmable-dc-electronic-load/



> -----Original Message-----
> From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Fritz Mueller via
> cctalk
> Sent: 05 December 2021 01:03
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at classiccmp.org>
> Subject: Re: PDP-11/70 Boards
> > 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 :-)
>     —FritzM.

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