[R-390] Filter Bandwidth Question

[Bob Young]

Thank you Larry,

I get lucky every ten years or so.

Bob kb1okl

________________________________
From: Larry H <larry41gm at gmail.com>
Sent: Tuesday, May 12, 2020 12:23 AM
To: Bob Young <bobyoung53 at hotmail.com>
Cc: r-390 at mailman.qth.net <r-390 at mailman.qth.net>
Subject: Re: [R-390] Filter Bandwidth Question

Nice find, Bob.  The resistors R502 and R503 set the bandwidths for the 1 and .1 kc positions, respectively, and are hand picked.  If you need to change both of them, do the R502 first.  If their values are within range, do the adjustment, first.  C503 can also cause a problem in this area.

Regards, Larry

On Mon, May 11, 2020 at 7:32 PM Bob Young <bobyoung53 at hotmail.com<mailto:bobyoung53 at hotmail.com>> wrote:
This may help, I got it from Antique Radios there is more at that address.

The 1KHz and .1KHz filters are based on a single crystal filter cascaded with the 2KHz mechanical filter. I had to do some digging to find useful explanations of the old single crystal filter circuits. What I learned is that the load impedance, because it is series with the crystal, is the main thing that sets the filter Q. In the R-390a for the 1KHz filter, the load is a 47K resistor (R-502). I guessed that mine might have drifted high. Last night I checked and sure enough the 47K resistor was actually 52K. This is one of the few cases I have seen where a relatively modest drift in a resistor value can have a substantial effect on the radio. I replaced it with one that read 48K (the closest I had). The filter response is significantly better and there is a clear difference between the 1KHz and 2KHz positions now.

https://www.antiqueradios.com/forums/viewtopic.php?t=97808

________________________________
From: r-390-bounces at mailman.qth.net<mailto:r-390-bounces at mailman.qth.net> <r-390-bounces at mailman.qth.net<mailto:r-390-bounces at mailman.qth.net>> on behalf of Larry H <larry41gm at gmail.com<mailto:larry41gm at gmail.com>>
Sent: Monday, May 11, 2020 10:21 PM
To: Ed G <ed.n3cw at gmail.com<mailto:ed.n3cw at gmail.com>>
Cc: r-390 at mailman.qth.net<mailto:r-390 at mailman.qth.net> <r-390 at mailman.qth.net<mailto:r-390 at mailman.qth.net>>
Subject: Re: [R-390] Filter Bandwidth Question

Hi Ed,  You are right that the .1, 1, and 2 kc bandwidths use the 2 kc
filter.  Sometimes getting the vfo on the center using the .1 b/w is
difficult, so try adjusting the kc knob carefully.  What can happen is the
455 kc crystal filter in Z501 will drift over time and nolonger be on 455
kc.  That's why the 1st part of the IF alignment procedure is to set the kc
knob on the strongest signal point when in the .1 kc position and set the
bfo to 0 there.  The crystal should be ok as you said that the signal
strength was ok on the 1 kc position.  The Z501 alignment procedure may
need to be done to get the .1 and 1 kc signal levels equal.

Regards, Larry

On Mon, May 11, 2020 at 2:01 PM Ed G <ed.n3cw at gmail.com<mailto:ed.n3cw at gmail.com>> wrote:

> Hi Folks,
>      My Capehart 390A is working mostly FB, but I do notice what would
> appear to be filter loss when I switch down to the .1 filter bandwidth.
> All other filter positions seem to be fine with no difference in
> signal/S-meter readings when going from 16 KC down to 1 KC.  But when I go
> to .1 KC signal level does drop down noticeably. I thought the 1 and .1
> positions used the same mechanical filter, so perhaps the problem is
> something else (load resistor)?  Any suggestions on what to check?  Thanks!
> --Ed—
>
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