[GreenKeys] TTY Model 33 motor start cap

[Jones, Douglas W]

The motor start capacitor on the Teletype Model 33 in the U of Iowa PDP-8 lab went bad.  Of course, it did so during an open-house demo.  We had the PDP-8 chugging out the alphabet (all 128 chars) over and over, and then it wouldn't start.  Diagnosis that it is the capacitor was straightforward.

The motor start capacitor is shoehorned into a space that barely holds it next to the starting relay.  The capacitor is 60mm long by 28mm dia.

It's an 88-108uF capacitor, which means a 100uf+/-5% should work just as well.  Looking at the available replacements, most were way bigger, but I found, and ordered, a capacitor that is 64mm long by 34mm dia., also with wire leads.  That was the smallest I could find.

So to the question.  Has anyone shoehorned in a capacitor this size into its original spot in the machine?  Interference with the motor fan is my biggest concern.  It looks like bending the tab that currently serves as a capacitor hold-down may be necessary.

I did find one web site where someone got a really big substitute and put it under the paper-tape feed roll.  This Teletype is older, the paper-tape punch is on a bolt-on outrigger, with the cover on the outrigger being a separate bolt-on piece of plastic.  There is no internal space where a wire could go through without me cutting up the cover.

                Doug Jones