Reviving the old FM band..

[jonathan dollimore]

Has anyone experimented with the various converters to play the modern FM band through the old pre-war one? I've done a little internet searching but inconclusively. I understand the one made by Scott worked quite well, and I was wondering if it would be worthwhile to build one to the Scott spec. All thoughts welcome, as always. Regards, Jonathan.

[Kent King]

Jonathon -

A couple years ago, there was a fellow who was looking into that exact project. My experience with the Scott converter (I've had a couple of them) is that they tend to drift. They must warm up a good bit and a good set of the 717A tubes is essential. Perhaps with modern capacitors and resistors, the drifting could be reduced.

I don't know what ever happened to the fellow's project to build a converter.

Kent

[jonathan dollimore]

Thanks Kent. When we first had vhf/fm over here in the 1950’s, drift was also a problem on the average domestic set, presumably because of the lack of AFC. Philips in Holland did much better from the outset. I was also wondering if it might be possible to use one of those little transmitters to beam a signal of one's choice into the old FM band? I have used one on the AM band and it works quite well (an SSTRAN AMT 3000). These musings are prompted by the poor quality of most stations on the AM band over here - poor both in terms of the transmission and content...
Jonathan

[Kent King]

I am afraid that within a few years, we will have to use local transmitters for all our radio enjoyment, AM or FM. And I agree, the content and quality are bad too, which will only accelerate their demise. I also have an AM transmitter, preparing for the future!

Kent

[Norman S Braithwaite]

I use a broadcast quality FM signal generator from the 1950s to provide a signal for my pre-war FM receivers.  Plug your source into the external modulation input and a short antenna into the output jack.  The signal generator does not produce much power but it is enough to cover all points in my building.

Norman

[jonathan dollimore]

Thanks Norman - very interesting. I'm afraid I don't know much about signal generators, but ignorance hasn't stopped me in the past...... Am I correct to assume that not all vintage generators will cover the obsolete FM waveband?
Jonathan

[Norman S Braithwaite]

Many general purpose RF signal generators will but they will not have the bandwidth to transmit a wide range audio signal.  They are suitable for alignment of FM receivers but not of broadcasting a decent quality signal.  A broadcast quality FM signal generator used for alignment of FM broadcast equipment such as Boonton Radio 150A, some versions of 202 (others with 207 downconverter), or Marconi TF1066B/6 should work well.  There are probably lots of later solid state models that will do the job as well but I am not familiar with them.

Norman

[Joseph W Strickland]

Frequency drift was a common problem with older equipment at higher frequencies including the original FM band as well as the one we use now. It the early days of these units there were very few available capacitors with a negative temperature compensation available. In many cases even the older equipment can be made much more stable by application of capacitors in their oscillator circuits that will compensate for the changing frequency of the tuned circuits. Look at the capacitors that were used in the better FM tuners of the late 1950s and into the 1960s tube equipped models. My old Dynakit FM-3 is stable enough that if turned off at night and turned on again the next morning the unit is still on frequency close enough that there is no discernible distortion in the sound. The Dynakit FM-3 did not use AFC and did not need it. The same was true of some tuners and receivers made by Fisher, H H Scott, Sherwood and others.

Joe