23 Jan
2004
23 Jan
'04
5:01 p.m.
Hi,
My ASR-33 has recently started to apply 20V pulses to my TX+ line. I
suspect something has gone south on the selector magnet driver board. Any
ideas on where to start looking?
Thanks!
Vince
23 Jan
23 Jan
6 p.m.
Hi,
My ASR-33 has recently started to apply 20V pulses to my TX+ line. I
By Tx+ do you mean the keyboard loop? In other words, is it Tx on the
teletype or Tx on the computer?
If it is on the keyboard loop, I'd look for shorted contacts on the
line/local selector relay (it's burried in the call control unit, not on
the selector driver module though). Maybe the internal loop supply (used
in local mode) is somehow getting onto the line connections.
-tony
6:18 p.m.
My ASR-33 has
recently started to apply 20V pulses to my TX+ line. I
By Tx+ do you mean the keyboard loop? In other words, is it Tx on the
teletype or Tx on the computer?
9:41 p.m.
My ASR-33
has recently started to apply 20V pulses to my TX+ line. I
By Tx+ do you mean the keyboard loop? In other words, is it Tx on the
teletype or Tx on the computer?
10:10 p.m.
> Sorry, I mean Tx on the computer. The 20V
appears on the + side of the
loop
> going to the TTY's printer, whenever the
printer is actually printing.
The
computer
doesn't like it much, either :-).
I guess I'd start by unplugging the 9 pin connector on the selector
magnet driver bracket. If you still get 20V on the loop, then there's a
short somewhere in the wiring on the call control unit (probably on the
relay). If not, then there's a problem in the selector magnet driver.
There aren't that many components in there, so it can't be too hard to
test the lot!
24 Jan
24 Jan
5:14 p.m.
I found a 9 pin connector behind the selector magnet
driver. I had to
That's it.
remove the selector magnet driver and bracket to get
at it. I disconnected
it, then put the selector magnet driver back in it's socket (left bracket
off for now).
Doesn't matter. The selector magnet driver is a little module consisting
of a power transformer, the PCB, a power transistor, and not much else.
All the external connections go through that 9 pin connector (2 of which,
IIRC, are mains to the transformer, so be careful). You could have left
the PCB out.
I no longer get the +20V spikes on the line, so I guess the problem lies on
the selector magnet driver card?
Sounds like it.
BTW the thing chatters even in local mode now, probably because the selector
magnet is never doing it's thing?
Yes. With that 9 pin connector unplugged, the selector magnet is
disconected. The machine will therefore 'space out' (a telegraph term for
a teleprinter that runs continuosly because it's receiving continuous
'space' -- often due to an open-circuit current loop).
So, is there a theory of operation somewhere for the selector magnet driver?
I can't follow what the schematic is doing at all.
IIRC, it's strange. The selector magnet driver applies a current to the
Rx Loop that is _cancelled_ by the normal loop current.
Now, I can't really remember the schematic, but is there some kind of
clamping diode across the input, I wonder. If that was open-circuit, the
selector magnet driver could provide the 20V open-circuit voltage at the
loop terminals.
-tony
6:16 p.m.
> So, is there a theory of operation somewhere for
the selector magnet
driver?
I can't
follow what the schematic is doing at all.
IIRC, it's strange. The selector magnet driver applies a current to the
Rx Loop that is _cancelled_ by the normal loop current. Now, I can't really remember the schematic, but is
there some kind of
clamping diode across the input, I wonder. If that was open-circuit, the
selector magnet driver could provide the 20V open-circuit voltage at the
loop terminals.
I'll go check that it really does clamp the negative bias to 5V like it is
supposed to.
I am still unsure where the +20V is coming from, since the +20V side is
apparently connected to the "-" side of the computer interface. Seems like
everything else should be more negative than that.
Hmm. Maybe I just partially answered my own question. When the transistor
in the interface is on, the "+" and "-" ends are
"connected". That would
surface the 20V+ on the "+" side of the TTY interface.
But that would mean something tied the "-" end of the TTY's 20V supply to
ground. Aren't those supplies supposed to float?
Vince
7654
days inactive
7655
days old
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