14 May
2014
14 May
'14
8:16 a.m.
Date: Tue, 13 May 2014 19:05:03 +0100 (BST)
From: ard at p850ug1.demon.co.uk (Tony Duell)
To: cctalk at classiccmp.org
Subject: Re: DEC ASR-33 "reader run" debugging
Hi all --
Finally have the time (and space) to work on my PDP-8/L and its
associated ASR-33. The 8/L I've had running for a while now, but I've
only recently taken the time to go over the ASR-33 it came with. It was
a bit dirty but it's cleaned up nicely and I've got it humming along
nicely now. (With the exception of the paper tape punch, which is going
to need a bit more work.)
At any rate -- this ASR-33 was outfitted with the DEC "reader run"
modification, and it does not appear to be functioning -- with the
teletype in "Line" mode, the reader switch set to "Start," and with
the
8/L running the RIM loader the reader does nothing. (It also does
nothing when set to "Start" in "Local" mode, but I'm unsure if
that's
expected or not...)
With the cover off, if I manually toggle the "trip coil" at the rear of
the unit the reader runs and the 8/L receives data (and it looks to be
correct). I've verified continuity between the RELAY+ and RELAY- lines
from the W076 in the 8/L all the way to the reader run board, and I'm
measuring 15V between the two.
It's about here where my naivete around analog electronics starts
kicking in and I'm unsure where to go from this point -- any suggestions
are much appreciated.
FGirstly, 'digital circuits are buitl form analogue parts' so I really do
wonder how you can debug a digital system without understanding analogue
stuff.
But secondly, this is digital. It's a relay. It's a 2-stae device.
OK, from what I remember about this modification, the relay has 4
conencitosn. 2 areh coil. 2 are the contacts. When sufficient
currentpasses through the coil, the contacts close.
The coil is conencted to 2 pins on the mate-n-lock so that the PDP8 can
control it. The cotnacts are wired in series with the reader control
circuit that operates the trip magnet at the back of the typing unit. Be
warned thath the cotncts are in cirucitry that is directly conencted ot
the maisn (pwoerl ine) in most versions, the relay, of course, provides
isolatiuon betwene this and the PDP8.
Assuming you can idtenify the connecitons, start by conencting a jumper
wire betwene the 2 'contact' terminals of the relay so that it appears to
be operated. Does the reader run now (jn local mode, you don't need to
have the PDP8 conencted). If not, then the problem is not with the the
reder run modificsiton. it might be the reader control PSU. Or the trip
magnet (unlilely) Or the conact wires in the rreder that are operaed by
the manual cotnrol elver or tape out sensor (very liekly). Look at th
Teletype Model 33 docs to figure it out. It's a very simple circuit,
although the official diagrams are designed to give yo a headache!.
Once you have got the reader to work like that, you now need to test the
relay. Remvoe the jumper across the cotnact terminals that you just
fitted. Now measure the resistance of the coil (everything off, PDP8
disconencted). Expect a few hundred ohms. If there is a protection diode
in parallel with the coil, check that too.
IF that checks out, conenct the PDP8 and try to read a tape. What voltage
do you get betwene the coil terminals? I think it's about 15V when the
reader is supposed to be running. If that's not right, the problem is the
relay drive cirucity in the PDP8. Or the cable/connectiors.
It might be the relay itself, If the coil is beign energised the contacts
may still not be closing.
-tony Date: Wed, 14 May 2014 06:36:29 +0000
From: tony duell <ard at p850ug1.demon.co.uk>
To: "General Discussion: On-Topic and Off-Topic Posts"
<cctalk at classiccmp.org>
Subject: RE: DEC ASR-33 "reader run" debugging
But
secondly, this is digital. It's a relay. It's a 2-stae device.
Sure, surrounded by some interesting things like 3 or 4 "thyractors,"
the characteristics of which I don't really understand all that well,
for example :). I jumpered a wire across the contact terminals of
the relay and still no
joy. The contacts on the reader control lever are OK (I cleaned those
thoroughly awhile back) I'll trace through the wiring tonight / tomorrow
(if I can find the time) and see where I can get with it.
If you get the 1975 Teletype Model 33 schematics from bitsavers, I think you need to look
at page
8 of the pdf -- sheet FS6.
It appears the reader trip circuit reduces to the trip coil (the one you worked by hand)
and the reader
contacts connected across the mains. So the problem could be a bad trip coil (let's
hope not!), bad
connections, or those contacts in the reader. I've had a lot of problems with those
in the past, and that's
where I would check first. With the machine unplugged from the mains, pull the reader
connector at the
back of the call control unit and see if you can get continuity between the pins of the
cable-mounted part
with the reader switch on, tape loaded, etc. If I'm reading the diagram right,
that's connector J6,
pins 14 and 15
Thanks as always for the advice; someday I hope
not to have to ask dumb
questions on this list :). (Don't hold your breath.)
It has often been said the only dumb question is the one that's not asked. I
generally go along with
that, although I do feel it's silly to ask a question when you know the chap you are
asking won't know
the answer... But that does not apply here. Ask away!
-tony
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