[R-390] A vs non A

[William A Kulze]

To take a side road off from 390's briefly, I must say I found sweeping to make a difference in aligning my HQ-129x. The first time through it I just peaked everything at 455kHz. I don't own a sweep gen and wasn't sure how I'd even do it, but I found a way by using a urm-25f and an o-scope. I also used a winradio, but only for a visual guide to where the generator was set. I connected the scope to the cathode of the detector, same as connecting to diode load on the 390a. The stronger the signal, the higher negative DC voltage there.

Using this setup I found that just peaking at 455 didn't mean the actual peak for the filter was at 455. I would run the generator back and forth between +6kc and -6kc, looking for where it peaked and how balanced it was on each side. I kept at it until I had the peak at 455 and the +/- points were the same level.

It made a very noticeable difference in audio quality. When I just peaked at 455 the audio was tinny and it was hard to tell if I was centered on a station. After re-doing it the audio was much flatter in response and more pleasant to the ear.

I noticed when tuning it that I could really flatten it out, but at a reduction in gain, so I didn't go too far past peak in the middle and balance at the sides. I may have another go at it and see what happens if I flatten it more. There seems to be a lot of gain to spare, so I'm curious to see what happens.

I may be off target, but the way I see it, if your bandpass peaks 1 kc off center, then you would have an audio peak around 1 kc, but only on the sideband where the peak is. And if it's peaked on center, but not very flat, you would get a real drop in response at higher audio frequencies. Like I said, I may be wrong about that, but that's the picture I get in my mind when I ponder it.

Bill W2NVD

-----Original Message-----
From: r-390-bounces at mailman.qth.net [mailto:r-390-bounces at mailman.qth.net] On Behalf Of djed1 at aol.com
Sent: Sunday, October 14, 2012 11:19 AM
To: rbethman at comcast.net; r-390 at mailman.qth.net
Subject: Re: [R-390] A vs non A

I also just finished looking through the R-390 manual to see what it said about alignment and confirmed there is no reference to sweeping.  To be fair, the description of transformer coupling to achieve the passband is correct, but unless there is a means to adjust the coupling then the best you can do is to align all at the center frequency, and sweeping won't make it any better.  I've tried sweeping my Hammarlund, which does recommend sweeping the crystal filter.  I found that it wasn't much different than aligning by ear. 
Speaking of Hammarlund- they incorporated variable coupling to provide an effective variable IF bandwidth, so the technology was known in 1936.

Ed  W2EMN