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General help needed for FM alignment

Ernst Erb Jürgen Stichling Bernhard Nagel 
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Forum » Technique, Repair, Restoration, Home construction ** » Alignement of radios: Superhet and TRF » General help needed for FM alignment
Paul Brady
Paul Brady
USA  Articles: 3
Schem.: 0
Pict.: 0
07.Jul.13 05:40

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Hello Everyone!

I am having problems aligning mono FM tube radios. I am presently playing with a Montgomery Wards Model GEN1710A and a Hallicrafters S-95 Civic Patrol. Both units have fresh tubes, all caps replaced and nothing apears out of the ordinary. It seems the service manuals for both of these give me two alignment options for the FM section. One advises that i use a 10.7 MHZ unmodulated AM carrier, inject it at a certian point, monitor another with a VTVM and tune for max deflection. It also states to align the ratio detector at the center of a max positive reading and a max negative reading using the VTVM. 

I used the above method and it only produces marginal results on both radios. It came down to the point of aligning them by ear for the best sound. The sensitivity isn't realy what it could be, the hallicrafters turning in the best figures of 18uV.

The other method says to use an FM marker generator. This is a new concept to me. I have seen these units before, but i have never used them. Cold someone explain to me how these exactly work and is this the more preferable method for early tube FM alignment?





Bryce Ringwood
Bryce Ringwood
ZA  Articles: 79
Schem.: 17
Pict.: 106
10.Jul.13 13:24

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I think the procedure as given in the manuals is to connect a VTVM or 100uA meter (centre zero 50-0-50uA via as large a value resistor you can find that works.) between the centre tap of the secondary and ground. (Or at the junction of two large equal value resistors x, if its a Foster Seely discr). The primary is tuned for maximum deflection with a 10.7 MHz signal, and then the secondary is tuned for zero deflection of the meter. (It will swing from a small value to a maximum, through zero and back to maximum again as you swing the signal generator either side of 10.7 MHz.) Next you finely adjust the primary so that the swings through the maxima are equal. And so on - you have to have a degree of patience. Sometimes you have to change the discriminator valve to get it right.

There is an instrument called a wobbulator (or wobbuloscope), which allows you to see the shape of the response curve on a CRT screen as the signal passes through either side of the passband. Some of these ave a crystal oscillator circuit built-in so you can see marker pips on the display. I used to use a Cossor instrument, but now I only have a partially working Annecy  wobbuloscope. (The faulty crt trace is a thin slanted line - as if the instrument draws in italic - any ideas anyone?)

I have also tried FM signal generators (Are you meant to just listen till it sounds nice?) - but nearly always end up using the centre zero meter. 

This article was edited 12.Jul.13 09:31 by Bryce Ringwood .

Bryce Ringwood
Bryce Ringwood
ZA  Articles: 79
Schem.: 17
Pict.: 106
12.Jul.13 09:35

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I added a schematic to my previous (confusing) post.