ASR-33 Questions

O. Sharp ohh at
Mon Mar 7 16:31:06 CST 2005

Well: I finally took a leap of faith, and bought an ASR-33 from someone 
who wasn't anywhere near Seattle. After a previous (extremely bad) 
Teletype shipping experience (read: TTY dumped in box and packing peanuts 
poured on top; unit largely destroyed in transit; seller refused to 
refund and claimed damage was somehow _my_ fault; story actually got 
worse from there if you can believe that, but maybe that's enough for 
now), it took a leap of faith to try buying another one I couldn't 
personally pick up. But the seller took a _lot_ of extra care packing it, 
including bolting the typing unit to the chassis, and wonder of wonders 
it actually arrived completely intact.  :)

But in spite of that it needs some help, so I thought I'd ask a couple of 
questions here.

-The one big problem it has is its lack of a reader power supply. The last 
owner got it from a government surplus program, but they neglected to 
supply anything but the unit itself... and so no reader supply. Does 
somebody have the schematic of one of these so I can build a new one? 
(Better yet: does somebody have a spare one I can talk them out of for a 
reasonable price?)

-I'm looking at some DEC diagrams saying how the ASR-33 gets modified for 
use with a PDP-8 series interface (thanks once more to Mr. Kossow!). It 
says two "6RS205P484 Thyractors DEC #1100106" should be added to the unit. 
Okay, I admit it: I don't even know what the hell a "thyractor" _is_, 
much less what ratings I should be looking for re: these two. Can 
someone shed a little light on this?

-Right now the unit is doing a lot of chattering on both Line and Local 
when powered on. Initially I thought something might be mechanically wrong 
due to being shaken around in shipping, say one of the clutches on the 
main shaft locking into an active position, but so far I haven't found a 
culprit. I'm also wondering if I'm even looking in the right direction 
for the problem, sooo here's the question: could the lack of a power 
supply for the reader be causing this behavior? (The reader itself is 
obviously inert right now, but I'm wondering if there's a missed 
connection or somesuch which could explain it.)

...At some point I'll probably ask someone with a copyholder to take some 
tracings and measurements for me so I can fabricate a new one, but maybe 
I'll just see if I can get it working first. :)


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