Crawls Backward (When Alarmed)

IconProjects, musings about guitar builds, guitar repairs, vintage tube amplifiers, old radios, travel, home renovation, and other stuff.

FM Alignment on the Zenith H845 Radio

With the AM alignment done, we move on to the FM alignment.

To accurately align the FM circuit, you will need an oscilloscope along with an FM sweep generator.  It's possible to use an AM generator and VTVM - many old alignment instructions have directions on how to do this.  But the most accurate alignment will be with a scope.

In this case, I'm using my Tektronics 453 scope.  This scope was introduced in 1965 and was state of the art - two channels with a 50 mHz bandwidth.  Even today, it's plenty of scope for old radios and amps.  This one is ex-IBM.

The process for FM alignment is not too different than the process for AM.   The real difference is that we measure and observe voltages differently.

But the process of connecting a signal at specified points, reading the output, and adjusting IF transformers is the same as for AM.

One of the differences is that the IF frequency is much higher - in this case it's 10.7 mHz, aka 10,700 kHz.  And it's frequency modulated rather than amplitude modulated. 

Here's the HP 5314A reading the output from the Hickok generator.  One of the shortcomings of old tube test gear is that it takes a while to stabilize.  I give the Hickok 30 minutes to warm up.  At that point, it's reasonably stable, but you still need to keep an eye on it.  After an hour it's pretty good.

Here's a typical readout.  The scope is reading the shape of the wave passing through an IF transformer.  By turning the slugs, we try and get the most symmetrical wave with the highest peak.

We do this for each IF transformer in the set.  Just a small turn of each slug will affect the waveform.

Another downside of old gear is that the signal isn't always 'clean.'  You can see small ridges in the wave - that's DC in the output.  But it's good enough for our work.

After the alignment(s), I run the set for a while on the bench.  My defacto test for AM reception is a local sports station.  It's not the strongest station in the area, and some sets have difficulty picking it up with a strong signal.  Not so the mighty Zenith!  It picks it up easily.

Most sets have a hard time picking up much on FM down in the dungeon.  The Zenith did quite well again - it picks up a lot on just the tiny factory antenna.

And it sounds great, with plenty of volume.

 
 
 
 

Post a Comment 0 comments:

Post a Comment