13 May
2014
13 May
'14
2:05 p.m.
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