Author Topic: RF signal generator for AM/FM radio build ??  (Read 432 times)

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Offline isobig

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RF signal generator for AM/FM radio build ??
« on: February 04, 2018, 09:25:13 am »
 Hey all...  getting back into doing some electroncs and I purchased a Elenco AM/FM radio kit to assemble recently. Instructions say to use a RF generator to align the IF stages and in sections it states to use modulation. I have seen many generators are able to do AM for the IF alignment on the AM side of the radio but it seems difficult to find something that does FM for a reasonable cost. The kit wants the generator to cover up to the FM band (88-108MHz) and they say to set it for 1kHz modulation with 22.5kHz deviation.

Just doing this as a hobby so I don't need anything costly. I know the kit can be built without a generator but I prefer to do so if possible as to be more accurate. Already have a scope to measure the signals.

Any recommendations on low cost new or used gear that would meet this need?

Thanks!
 

Offline Chris56000

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Re: RF signal generator for AM/FM radio build ??
« Reply #1 on: February 22, 2018, 09:31:39 am »
Hi!

Whilst I've never tried one personally myself for that purpose, one of those cheap.DDS synthesised sinewave generator modules with an  amplitude modulation facility might be of use, but you'd need to completely house it in a screened diecast box to prevent radiated "spurii" interfering with the alignment, with the generator o/p connected to a coil of wire about six turns as a radiator loop if the AM side has a ferrite–rod aerial.

However, it may be worthwhile reading up your radio kit instructions thro' carefully, as there may be an "Alignment With No Signal Generator" procedure given – it's very often possible to do this because the IF transformers and oscillator coils are "pre–tuned" at the factory that made them and only a very slight adjustment will be needed to bring them into final alignment of the radio has been assembled correctly!

If you're making a number of these kits, then a vintage valve–operated signal generator is your saviour  – (please don't laugh, these instruments were made to a very good quality and stability specification and even a typical piece like an Advance E2 will be found to be surprisingly accurate, providing you allow adequate warmup time first!) – let us know how you get on with the kit instructions first!

Chris Williams

PS!

If you download the March 1968 issue of "Radio Constructor" Magazine from the American Radio History Website, and turn to the "In Your Workshop" feature, "Smithy" the featured "serviceman" goes into some depth on restoration and use of a typical service workshop type RF Signal Generator.

If the radio is monophonic only on FM, you can line up the FM Discriminator Transformer with a simple Unmodulated c.w. RF carrier at. FM IF frequency, two 47k matched resistors and a dmm with a low current (not more than 2mA range) – FM alignment generators tend to go for silly prices either from the States or worse, all end up in bloody Israel!!!
« Last Edit: February 22, 2018, 09:50:51 am by Chris56000 »
It's an enigma that's what it is!! This thing's not fixed because it doesn't want to be fixed!!
 


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