ASR-33 problem

From: Tony Duell <>
Date: Sat Jan 24 17:14:54 2004

> 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.

Received on Sat Jan 24 2004 - 17:14:54 GMT

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:36:47 BST