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Figure 1. The Luven radio station KWS-1 and 75A-4 transmitters and receivers stacked on moveable platforms. This was the configuration in Switzerland apparently, except that the ATU's were located on a narrow shelve that was attached to the wall to the left of the radios. I will do the same to keep the station as true to the original as possible.
Collins Radio KWS-1 and 75A-4 transmitter and receiver assembly
Collins Radio KWS-1 and 75A-4 transmitter and receiver assembly
I finally unpacked the crates of Collins Radio KWS-1 and 75A-4 equipment. There was unfortunately only one microphone in the crates, a Shure Model 444. This would have been a very good match with the KWS-1 transmitter as the Shure Model 444 audio response declines rapidly below 300 Hz and above 3 kHz, with some peaking towards 3 kHz; this would match the frequency response of the Collins mechanical filter used and transmitter audio response (200 Hz - 2.9 kHz) of the KWS-1 very well. The Shure Model 444 was first sold in 1964. A brochure that contains specifications of the Shure Model 444 can be found at Links and literature
I therefore have no idea what microphone was used with the other transmitter. Probably another Shure Model 444? Perhaps I will be able to see what was used once I have unpacked and worked through all the notes, drawings and photos. I stacked the equipment exactly as the Luven station had it. The two ATU's were mounted against the wall on a wooden shelve, I will do the same. There were specially made up steel inserts (with a rubber layer on one side) under the rubber feet of the receivers to spread the load to the frames of their steel cases.
There is a large assortment of cables and two relays that need to be connected. It would seem that the two speakers were connected to four receivers simultaneously, two receivers to a speaker. This would not bother the audio output transformers of the 75A-4's.
The operators used a small wooden table that was placed in front of the KWS1/75A-4 assembly. This table did not reach South Africa, so I will have to make a similar table. When placed in front of the equipment, it provided a convenient operating platform for a CW key, microphones, notepad and logbook I imagine. From a sketch I have, the table also had a small night lamp used by the operator, probably used when writing and reading.
A higher resolution image can be accessed below for those interested. I will add more details and photos here as the project continues.
KWS-1 set-up Luven.jpgProgress
Progress
Of course, one can not just plug in these sets and switch on the power. That would be a bit rash, as they have been in storage for a long time. I have started a set by set inspection, and it is clear that as a start all power cords will have to be replaced. The 75A-4's original power cord was a rubber twin flex cord with molded two pin 115 volt plug. It would make sense to replace these with a 3-core flex cable with molded plug. This will make the receivers a bit non-standard, but safer. The strain relief bushing for a flat cable will have to changed to a suitable round cable version. One can argue about this, originality vs. safety but I won't. The receivers consume about 85 watts each, so they do not require thick conductors. The KWS-1 transmitters however require up to 1.5 kW peak input each, so their 115 volt input cables should have a suitable surface area. If one aims for a maximum of 1% voltage drop, planning for a cable length of ~9 ft, stranded wire of at least AWG 14 (2.5 mm2 cross section area) is required.
The KWS-1 and 75A-4 set-up will be attached to a dual transformer configuration. A 500 watt 230 to 115 V transformer for the receivers and a 2.44 kVA 230 to 115 V transformer for the transmitters. It is conceivable that one may want to switch on receivers for monitoring only, it would be a waste of power to utilize a 2.44 kVA transformer to produce 85 watts for a 75A-4. The Luven station was of course off-the-grid, just as the replica station is off-the-grid...so electricity consumption is an important and continuous consideration. The positive side to producing you own electricity is that the power is more stable, and the 115 V line is in fact 115 V. This is quite important when using equipment designed for 115 V such as the KWS-1 as higher or lower AC supply voltages can play havoc with tube heater voltages, and some components may be stressed if supplied with too high voltages.
Capacitors
Capacitors
Capacitors in vintage transmitters and receivers can be a source of problems. However, in my experience, capacitors do not fail frequently, and I do not replace capacitors because they are old and "might" fail. Neither do I replace them because there is a list somewhere of "deadly capacitors". If however, there is obvious risk in leaving a particular capacitor in circuit, or whenever there is a damaged, out of spec, or good enough other reason, I will replace it with an appropriate similar type or better capacitor. However, when it comes to radio receivers and transmitters, I do not fix what is not broken.
With that out of the way, what did I find when I started to physically inspect the capacitors? Well, it is clear that I have to replace some of the Sprague bumblebee paper (0.1 μF) capacitors, as they are cracked. This could lead to moisture absorption and current leakage. Often a bit of leakage is no problem, and many old capacitors may leak a bit of current but the functionality of the radio will not be altered. I removed a cracked bumblebee capacitor and it has a value of 0.11 μF . It also leaks current a bit and has an internal resistance of ~70 MΩ. So, no real disaster, but it is better to replace it. One should always consider the purpose of the capacitor in its circuit and evaluate what an effect its out of spec or other ailment will have. If it can lead to disaster, replace it. Often times however, it is a case of leave well alone!
Update July 2020
Update July 2020
I have installed and tested one of the KWS-1 transmitters. It is fully operational and on-the-air tests have resulted in good reports. Before testing the KWS-1, I did a component check to ensure there were no obvious bad resistors, capacitors or wiring problems. I had to replace the power cable to both KWS-1 transmitters as the originals were cracked in places and were obviously not safe anymore.
The second KWS-1 unit has an rf drive problem, the drive is so weak that no drive current registers on the mA meter when in CW mode. It was also intermittent in that the weak drive levels would disappear and then re-appear at some later point. I cleaned the wafer switches and relays with contact cleaner, this improved things a bit, but the weak drive problem still persisted. Consequently I swapped existing tubes with other tubes to check whether there was a weak tube somewhere, however this did not improve matters either. So, as it stands, this KWS-1 is a bit sick at this time, and will need further attention to get it fully functional.
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