Need Help Fixing a Teletype KSR 33

[durgadas311]

Regarding current loop, I may not have tracked what you're looking for but there are plenty of legacy schematics out there of current loop interfaces (done back when current loop was actually used!). One such is the Heathkit H8 serial boards (H8-4) with this. I have no idea about the availability of opto-isolators anymore.

 

[durgadas311]

From what I recall, The LOCAL position simply disconnects the serial data in/out from the interface board (current loop or whatever), and then connects OUT to IN (internally). I seem to recall that the keyboard (and optional reader) and answerback drum are pretty much always connected to the distributor, such that pressing a key will always activate the distributor and you should hear the distinctive "buzz-whirr" of it "sending". I also seem to recall that the typing unit and optional punch are also connected to the receive mechanics. When the START bit begins, it should engage all the mechanics to assemble the data bits into some mechanical positioning of the bars and then a "print" is done on the STOP bits. The noise of the printing usually drowns out any noise from sending.

So, to my recollection, if the receive line is stabilized (so no noise is present) you should hear nothing but the spinning motor in LINE mode, and pressing a key should result in that buzz-whirr noise of the distributor. If it is not working in LOCAL mode, you might need to go to LINE mode and first determine that sending is working, then trouble-shoot receiving.

 

[djg]

So, to my recollection, if the receive line is stabilized (so no noise is present) you should hear nothing but the spinning motor in LINE mode

All 33's I've worked on will chatter in line mode if loop is open. Won't hit print wheel or advance.

 

[durgadas311]

All 33's I've worked on will chatter in line mode if loop is open. Won't hit print wheel or advance.

Sure, that's just "noise" causing false START bits and most of the time the character "received" ends up being unprintable. That's why something needs to be done to stabilize the receive line. I'm not sure what the appropriate method is for current loop, though probably just a correctly-sized resistor across the inputs. Although, if current is required to cause a MARK line condition, then something more may be needed (SPACE line condition will look like a START bit). For RS-232, that's not usually required, but a pull-up resistor (or maybe just to ground) might be needed if the input has too much noise.

This Teletype schematic makes me think that just a modest resistor across the inputs should quiet it down, unless I have MARK and SPACE reversed. I assume that the selector magnet is energized to produce a SPACE, as otherwise it would be burning current the entire time the unit is idle.

 

[djg]

Wikipedia
A digital current loop uses the absence of current for high (space or break), and the presence of current in the loop for low (mark).[1] This is done to ensure that on normal conditions there is always current flowing and in the event of a line being cut the flow stops indefinitely, immediately raising the alarm of the event usually as the heavy noise of the teleprinter not being synchronized, something that would not have been possible if the idle state had been no current flowing.

20mA will stop it. Per earlier posting short will also stop it on mine due to how selector magnetic is driven.

 

[johnx993]

Nice, but at $175 (DigiKey, if ever back in stock) rather over-priced. There have been a number of projects by Greenkeys members over the past decade that scratch this itch for both 60mA and 20mA current loop interfaces. Recommend checking them out. See earlier post: https://forum.vcfed.org/index.php?threads/need-help-fixing-a-teletype-ksr-33.1253508/post-1453496

Oh of course you can always DIY. Lots of schematics for that from the 70's Byte, Kilobaud, MITS Computer Notes, etc.
Getting a 20ma TTY to talk with a serial computer was the thing to do back then.

 

[MauriceH]

Indeed first start with the line mode, see if that proper work.

attached a simple RS232 converter schematic.
Build it and see if that will work.

First see if your local power is present, Fuse F3
and power on local battery supply current-loop pcb.

Check Local-Line-Off switch

Read manual; 574-100-103TC - Circuit Descriptions.pdf
Page 31

Read manual; Model_33_Schematic_Jun75.pdf

Read all of it in those manuals.
Also the references to the symbols and what they mean.
Its only way to get a hand how to read those schematics.

A key press has to be finished to stop the machine from rattling.
Also at startup think a reset loop is made, only can be finished local mode if send-receive is connected.

Indeed first start with the line mode, see if that proper work.