[GreenKeys] FSK converter madness

[Nick England]

I was checking out a newly acquired FSK converter (Harris RF-3352). It has
a built-in loop supply that connects via straps on the rear. Hooked up a
keyboard and printer and it worked fine in loopback (transmit audio tied to
receive audio). So I put it in a rack and hooked it up to my TTY patch
panel using the patch panel's loop supply and current limiting resistor.
Transmit audio sounds good and the receive M-S lights are blinking with
ITTY,. So far so good.
Then I plugged a reperf and a MITE printer into the receive loop.....
The printer prints but the reperf doesn't perf - Say What???
It's a loop, the reperf selector magnets are in series with the printer -
What's happening?

I tried a 28 printer (with LESU line relay) - it works but the reperf
doesn't. Tried another reperf (with LESU selector magnet driver) - it works
OK. Turns out that the loop is actually 60ma marking and 10ma spacing - so
I guess the reperf magnets were holding while the printers were cycling
normally.

So why is there 10ma in spacing?? Put it back on the bench and hooked up
the internal loop supply again - 60ma marking and 0ma spacing. Huh?

Well, here's what I think I figured out - the receive loop keyer is not
your typical transistor switch from loop + to ground - it is kinda
complicated with + and - loop keyers so you can configure it for polar or
neutral operation. And that circuit evidently draws 10ma from the loop +/-
terminals. That's no big deal when drawn directly from the internal loop
supply terminals, but because my TTY loop + is actually the p/s + fed
through the patch panel's current limiting resistor AND the reperf/printer,
then the 10ma is flowing through my signal loop and not just the supply
terminals. At least that's what I think is happening.
The manual says you can use an external loop supply, but as I found out,
you can't use an external loop supply with TTY devices in series! What a
strange engineering decision.
If you are curious, the manual is here  -
https://www.navy-radio.com/manuals/harris/rf3352-man-8310.pdf

So, I either need to modify this thing (and I really really hate to do
that) or use its internal supply and bypass my patch panel's supply. Ah
well, I've already done that bypass rewiring for a Dovetron with internal
supply so it isn't a big deal, just this week's annoying puzzle.

Semper TTY,
Nick England K4NYW
www.navy-radio.com
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.qth.net/pipermail/greenkeys/attachments/20240705/101ca92a/attachment.html>