FSL-4 Doug Hensley, W5JV, provides this description for the FSL-4 device.
"The item was made for the Royal Canadian Navy and is a pass though audio limiter (clipper) made for radio teletype signals going to a transmitter feed point. FSL means Frequency Selective Limiter, an audio clipper which truncates any level above a set point. These settings were done with a non conductive (phenolic or fiberglass) screwdriver inserted into the guide holes labeled "Output" and "Limiter". There are no other operator adjustments as they rarely had to be used.
a) The "Forward / Reverse" switch: Frequency Shift Signals are composed of high and low tones which define the shift and compose the Baudott characters. The shift was most likely 850 cycles while the Mark and Space frequencies would have been something like 2125 kHz for Mark and 2975 kHz for Space. Receivers and demodulators differed so the forward/reverse switch was necessary at times to flip the tone pair when needed.
b) The transformer labels: BURNELL & Co. made their living producing quality telegraphic instruments like telegraph keys and sounders as well as specialized machined goods. As WW2 got underway, their product line was broadened to also produce transformers, a natural demand since they already produced the fine wire sounder coils.
The three screw terminal strip on the right was, we are guessing, the DC loop input from the teletype work station or operator's machine and once processed would have been fed to the transmitter input via a shielded cable connected to the left side Amphenol jack. Since there has to be an input and an output, its a fair guess."
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The example in this web page is serial number 18 which suggests a very low production run. This device has never turned up in any equipment listing for any RCN ship. So where could the FSL-4 have been installed? Perhaps they were fitted to the transmitters which sent RATT (radioteletype) from shore stations.Vintage is around 1946. (?)
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FSL-4 overall view Left side of front panel Middle of front panel. Right side of front panel. Top view National Company warranty statements. All photos via E-bay Credits:
1) E-bay https://reverb.com/item/20122454-vintage-national-tube-limiter-compressor-amplifier-nos-rare
2) FSL-4 owned by: Ervin Stellar [stellarcellar@ervinstellar.com]
3) Doug Hensley W5JV [w5jv(at)hotmail.com]
Apr 14/20
