Nick, was this based off my conundrum last week? If so I was still curious as to what happened on my end, in that I know what I did (grounded loop), but how it affected things is what Im curious about. I did do a little testing before fixing it.

In my case, I grounded a jack in a (badly mutilated) patch panel, as I saw continuity to ground on the loop terminals (going to converter). It sorta worked, but as you know, it goofed up with other stuff connected. In this case Im directly connecting jacks, theyre not looped at the panel (everything cut).

While I still showed current (as my grounded jack was after the current meter and loop feed on the loop panel)...somehow, it broke the loop because the ground of the looping jack "closed the loop" so that any further jacks now had nothing or very little really.

Testing:
I checked the loop supply I was using and had no ground on negative or positive. I -did- have ground at the loop patch panel cabinet, but no jack sleeves rung to ground.

I also verified that the inputs and outputs on my high\low converters also had no grounds, on sleeves, BUT they are grounded (metal case), and the stuff is at least 30 years old, so they could have leaky bypass caps inside too (as well as the cv-2460) (and the loop supply possibly).

One thing I forgot to check was what voltage I get between positive and ground on the loop supply.

As it turned out, that patch panel was still connected to another loop supply, which had negative grounded. Go figured they didnt cut that. I disconnected the loop supply in the panel and connected my converter jacks directly to the terminals.

On the low level side youre right in that the negative is grounded. In the ships low level patch panels, all wiring runs were literally bolted to ground at the rack. The only signals on the patch cables were data (tip) and step (ring). No ground on sleeve at all. (I wired ours to have ground on ring).

Jeff KC3GJX






On Dec 3, 2023, at 2:36 PM, Nick England <[email protected]> wrote:


As usual, I'm sure I'll screw something up so additions and corrections are welcomed.

When hooking up multiple units to a multi-loop patch panel it is possible to inadvertently short something out because a connected device has one side of the line grounded.
Typically the minus side of the loop supply will be grounded and that is a return for all loops. Each loop is fed by the positive side of the supply through a current limiting resistor and then through the LOOPING jacks and then the SET jacks (TTY printer, TD, reperf, etc.) and then to the LOOP DEVICE (keyer/converter/etc.) and then back to minus/ground. This arrangement works if NO LINE TO A TTY IS GROUNDED

A note from hard-won experience - some TTY sets are polarity sensitive - M28 RO, KSR, and ASR may have a LESU with a line relay or selector magnet driver - you must apply proper loop polarity to the line inputs. Western Union devices might have one line grounded and you'll need to isolate that.

Some RTTY converters and modems have isolated loop inputs and outputs but many do not. Here's a quick list of what I have found -
FSK Receive Converters (with internal loop supply)
Dovetron - negative side of loop is ground

FSK Receive Converters (no internal loop supply)
AN/URA-8A, 8B (CV-89) - switches loop (pin A) to ground (pin B)
AN/URA-17 (CV-483) - switches loop (pin A) to ground (pin B)
AN/URA-17A (CV-483A) - switches loop (pin A) to ground (pin B)
AN/URA-17B (CV-483B) - switches loop (pin A) to ground (pin B)
AN/URA-17C (CV-483C) - switches loop (pin A) to ground (pin B)
AN/URA-17D (CV-483D) - switches loop (J8/A) to ground (J8/B)
                     (or for a negative loop switches J8/C to J8/B)

FSK TU receive/transmit (no internal loop supply)
AN/SGC-1 -
     SEND/RCV loop isolated (TTY+, TTY-)
AN/URA-17E (CV-3510A)
     SEND loop isolated (+loop J5/A, -loop J5/B)
     RCV loop isolated (+loop J8/A, -loop J8/B)
AN/URA-17F (CV-3510B)
     SEND loop isolated (+loop J9/A, -loop J9/B)
     RCV loop isolated (+loop J6/A, -loop J6/B)
CV-2460
     SEND loop isolated (+loop TB1/6, -loop TB1/7)
     RCV loop isolated (+loop TB2/5, -loop TB2/6)

AN/URC-32 transmit keyer (+loop TBD/1, ground TBD/16
AN/URT-23 transmit keyer isolated (+loop J7/B, -loop J7/C)

Note - I believe that all low level devices have one side grounded and the other is signal with +/-6v signaling.

Nick England K4NYW
www.navy-radio.com
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