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The Restoration of the PDP-8/I Minicomputer (PAGE 9)

The Mighty Hunter Returns with Capacitors -- 2003 07 12

Well, isn't life just grand? Our local "Computer Recyclers" in Ottawa had a wide assortment of capacitors:

Assortment of Computer-Grade Capacitors S M L XL Here are the capacitors that I was able to scrounge. From left to right, top to bottom: 90mF @ 50V, 64mF @ 30V, 150mF @ 10V, 160mF @ 15V, 150mF @ 15V, 66mF @ 30V, 150mF @ 15V, 75mF @ 40V, 130mF @ 15V, 78mF @ 40V, 58mF @ 25V, 120mF @ 15V, 120mF @ 15V, 100mF @ 20V, 100mF @ 20V, 43mF @ 30V, 69mF @ 55V, 75mF @ 40V. The two in the front are, left-to-right, 7uF @ 660V and 8uF @ 650V (I bought these just in case the capacitors in the "resonating" circuit get hosed.) There are so many, not only because I don't expect all of them to work, but also because my PDP-12 will be next on the restoration list, and the H724 power supply takes identical capacitors as the PDP-8/I's H704 power supply, and since those machines are of the same vintage (with the PDP-12 probably being powered off about the same amount of time)... :-)

My next trick will be to reform some of these capacitors in preparation for swapping the old ones out of the power supply. Here's what I'm thinking of swapping:
Designation Original Replaced by
C2 57mF @ 50V 69mF @ 55V
C3 160mF @ 20V100mF @ 20V
C4 160mF @ 20V160mF @ 15V

None of these values seem too far off, with the exception of C3; I'm hoping I can squeeze by...

New Power Supply Capacitors Installed -- 2003 08 04

No, you didn't miss anything over the past three weeks -- I was off doing contract work. Today I installed the three new capacitors. Pictures will be up shortly; I spent three days moving hard disks around the lab, and my Windoze box isn't back up yet (that's the one I need to get the pictures off the camera).

Using an oscilloscope, I verified that the +5V is rock solid. The unregulated voltages (measured right at the capacitors) look fine too -- nice, low-ripple, saw-tooth waves. Perfect!

Now where was I? :-) Unfortunately, I haven't had a chance to work on it for a while now. First of all, I got gout in my ankle (and that's a wonderful experience, let me tell you!) then I had to complete my year-end accounting, and now I'm buried in finishing my next book. (As of 2003 10 24, the book is almost done; I've cleaned out the garage, painted the floor, and moved the PDP-12 into the garage in preparation of having some time to work on these machines again! The other "secret agenda" with moving the PDP-12 out of storage and into the garage is that many of the cards are the same; I'll be able to swap cards around in order to better diagnose the 8/I.)


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