[R-390] Okay, don't laugh at me but...

[Barry]

Hi Bill,

Okay on the explaination.  I do have a general understanding of FM - I suppose I was just being a bit self-depracating.   It does make sense and is sort of what I thought.  I was just trying to get my head around the idea that "modulating" the 8640B with a triangle signal would "pull" and "push" the frequency that way and I don't understand how that works (given it's using a tuned cavity for the center frequency and not a VCO).

The primary on that last IF peaks at a very odd position (core is quite a bit further out of the can that I think it should be).  Furthermore, tuning is "touchy".  If I don't get the stating centered just so-so, I get some distortion.  Definitely something's amiss.  I would suspect another bad cap but there isn't one across the primary of that transformer.  There are some in the secondary so that still could be part of the problem.

According to the alignment sheet, if using a scope and a sweeper, one should see a double-S curve with the S-es crossing zero at the center frequency of the IF.  That makes sense to me and I suspect that if I could see the pattern, it would not look at all like it should and that's what I'm trying to verify.

Thanks for the reply,

Barry - N4BUQ

 On Wed 09/03/11  9:37 PM , "Bill Hawkins" bill at iaxs.net sent:
> Barry,
> 
> That last transformer that goes through zero is the discriminator.
> It demodulates FM to audio.
> 
> Maybe this is too basic, but FM modulates the frequency with audio
> like AM modulates the amplitude of a fixed frequency. So an FM
> transmitter broadcasts a band of frequencies at a more-or-less
> constant level. Broadcast FM uses about 100 KC which is 1% of a
> 10.7 MC carrier, not much of a strain on the bandwidth of the IF
> strip in the receiver. Then there's narrow band FM, but never mind.
> 
> The FM IF strip has too much amplification for AM. It deliberately
> limits (clips) the amplitude to get a uniform level across the 100
> KC band. This top must be flat.
> 
> If you can get a DC connection to the discriminator output, and
> have a high impedance voltmeter or DC scope, it doesn't matter how
> fast you sweep it. A stable signal generator capable of fine
> adjustment (.01%) is all you need to slowly sweep across the 100 KC
> band and verify that the signal level to the discriminator is flat.
> If it isn't, the discriminator output will vary with the IF
> amplitude, which causes distortion.
> 
> The discriminator is designed to linearly change output across the
> frequency band. The adjustment is to center it (zero VDC) on the IF
> center frequency. If you can't get it to zero then something in the
> IF strip is oscillating at a spurious frequency.
> 
> At least, that's how I understand FM.
> 
> Bill Hawkins
> 
> 
> 
> -----Original Message-----
> From: Barry
> Sent: Wednesday, March 09, 2011 3:46 PM
> 
> I am aligning an old FM radio and I want to sweep the IF transformers but
> Idon't have a sweep generator.  I know that my function generators with
> VCOinput can function as a sweep generator, but they don't cover the
> 10.7MHzrange.
> 
> Here's the dumb question:  Can I use my HP-8640B (in FM mode) and feed
> themodulation input with a triangle waveform from a function generator to
> essentially cause the 8640 to sweep the center frequency?
> 
> BTW, the reason I want to use the scope and sweep it is something
> doesn'tseem quite right using the VTVM method.  It's adjusting the secondary of
> thelast IF transformer is supposed to cause the measured point to go through
> azero point (e.g. +V on one side, through zero, and then -V on the
> other).The procedure is to set it zero volts but I'm only able to get a
> non-zeronull point so I suspect something else is not right and a picture might
> helpme confirm what I'm seeing on the VTVM.
> 
> Again, please don't laugh at me.  FM is just weird to me...
> 
> Thanks,
> Barry - N4BUQ
> 
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