W9DDD,

The TT-98 was a "deluxe" TT-4. The TT-98 had a plug-in box in the rear left corner that had all the connections and a rotary switch for 60ma, 20 ma, polar, & local test. I worked on GRC-46s in Viet Nam. We typically used TH-5 & TH-22 modems instead of the radio.

We have a cut-open AN/GRC-46 in the AWA Communication Technologies Museum.

It may be that the M28s were mainly used in fixed (or naval) installations and were typically set up for one type of current loop and rarely changed. The Kleinschmidts, however were used in more tactical installations and were smaller and easier to move around, so they needed to be more flexible in adjusting for different current loops.

73 & have fun,
Duncan Brown, K2OEQ
USASA 31J30

Antique Wireless Association
Communication Technologies Museum
Asst. Curator, Commercial Equipment
(also chief TTY op & repairman)

i-Telex: 212503

www.antiquewireless.org
AWA Youtube channel: https://www.youtube.com/channel/UCX55peBhzeX1qps_VYXdLBA

On 11/15/2024 4:52 PM, John, W9DDD wrote:

Re: the M28, yes has a hard to get to terminal strip accessed from the bottom of the LESU. I remember the switch on the TT-76. My GRC-46 was missing the TT-98 for the whole year I was assigned to it. So I don't know about it. Never saw TT-4 while on active duty. But you have explained what at the time seemed to be a strange way to label the switch.

On 11/15/2024 11:34 AM, Duncan Brown wrote:

The 60 ma loop comes from the DC telegraph circuits which could be miles long. In the 1940-50s, when more TTYs were used with voice circuits (telephone modems & radios), the distance to the modem or radio converter (TU) was measured in feet, not miles, and a lower loop current could be used.

All the Kleinschmidt models (designed for the military, starting in the late 1940s) had easily accessible 20/60 ma switches. The switch on the TT-4 was labeled "DC LINE / VOICE FREQ LINE"

In the M28 line, there was typically an "Electrical Service Unit" (ESU) which contained a polar relay that isolated the selector magnet from the external line. Since the SM was on an internal loop, it did not need to easily switched between 20 & 60 ma. I'm not as familiar with the M28 line, but I think there as probably a wiring choice on the terminal strip on the rear wall for the 20/60 ma choice.

Have fun,

Duncan
K2OEQ

On 11/15/2024 12:07 PM, Tom Hunter wrote:

W9DDD,

I have a holding magnet 15 with a 20/60 mil switch. So they did indeed put them on some 15s.

Tom N3CRK

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