DWS MK119A

DWS MK119A Transmitter /Receiver

I will add some technical details and photos of the Luven station MK119A set here. The MK119A needs some restoration work, but a quick inspection revealed that the transmitter, receiver and power supply sections are original and complete. After an initial clean-up, both the transmitter and receiver sections were tested and found to be working well. There is a bit of a hum in the receiver audio, so some attention will have to be given to the power supply. The spares compartment lid is a bit tatty and needs some work as well as the plywood box. The DC cable is missing so I will have to make another. There are some photos of an original that I can use as example.

Initial Tests

I performed some preliminary tests on the MK119A. There is a bit of drift after initial switch-on as can be expected, but after a 30 minute warm-up period, the drifting starts to settle down and is barely noticeable in a practical sense. The background hum in the audio output is high enough to be a bother and will have to be sorted out. This is probably a power supply DC filter problem, the DC smoothing capacitors are most likely the culprits. The vernier frequency (fast/slow) dial is quite impressive, and for such a relatively simple receiver circuit contributes much to resetting the receiver to a specific frequency. I will have to re-align the receiver to ensure best performance. The receiver VFO stops oscillating close to 10 MHz, and although the frequency read-out is fairly accurate, tracking is out at the top and bottom edges of the five wavebands covered.

The frequency-reset ability of the MK119A is close to 1 kHz after a 30 minute warm-up period (not from a "cold" start). There is no provision for netting the transmitter and receiver. However, this poses no problem as one can write down the receiver vernier setting after determining the transmit crystal frequency and the receiver frequency setting using the BC221 (the BC221 receives and transmits on the same frequency). In practice, one would use a small number of crystals, therefore it would be an easy task to determine the exact frequencies of the crystals, determine the frequency dial and vernier log setting, table these on a card and store this card in the spares box.

Tests conducted from a cold start showed an initial fast 4-5 kHz drift, then a slower drift after 30 minutes warm-up (VFO frequency drifting lower), followed by a very slow drift for 2 hours, and then a very slow random drifting in the sub-kHz region. How far the set is off-frequency at initial power-up depends to some extent on the ambient temperature, and whether the set had been in use recently (i.e. if the wireless set temperature has been raised above ambient).

Set up to produce a heterodyne signal from the BC221, the receiver was still producing the audio heterodyne (although shifted in audio tone) after 5 hours. Bottom line is this: the MK119 can be pre-set accurately enough to an operating frequency using the frequency readout and logging scale vernier. It it's a cold start, one may have to tune ~ -3 to -10 kHz to find the station you are trying to contact; if the set has been warmed up, frequency stability is adequate to place the VFO within 1 kHz of the required frequency. So, the operator who is familiar with the set and knows its frequency drift and stability characteristics should have no problem to set the receiver on frequency. At some stage I will add a plot here of frequency versus time, which will clearly illustrate the wireless set's stability characteristics. It is not a set for an inexperienced operator, as it requires some fiddling and adjustment during operation, but it is a pleasure to operate. To be clear, it is a real radio transmitter/receiver not a large cellphone...