[R-390] Low sensitivity on 7-8 Mhz band

[Bill Feldmann]

Hi All,

I'm also having a problem with low sensitivity on my Stewart Warner R-390A. 
Mine was really dead on some bands, especially below 8mc even after checking 
all the tube, replacing a few weak ones, and aligning it per my year 2K 
manual copy.

I found a lot of the trimmer caps in the RF transformers at maximum setting 
and the receiver not aligning good on those bands.  After pulling the RF 
cans, check caps with my GR bridge and replacing about 12 mica ones reading 
low or with high loss the receiver came to life and aligned perfectly.

However I'm now beginning to notice after the receiver has run a few hours 
and fully warmed up, the sensitivity drops 15 to 20db on the carrier meter 
for a 100uv signal on all or most of the bands below 8mc where it is triple 
conversion.  So there is most likely some bad caps or other components under 
the RF deck, most likely in the first mixer and oscillator circuit.

I'm not looking forward to pulling the RF deck but after reading my 2K 
manual copy doesn't look too bad.  I wonder if there is a list of bad caps 
or components on the RF deck published anywhere to help me be sure the RF 
module is in good shape after I work on it?  Don't want to have to pull it a 
second time.

I'm new to the R-390A, had more experience with my earlier Collins non-A 
which had all good caps in it from the factory.  Surprised I found so many 
bad ones in the later Stewart Warner and no sign of any bad ones in the 
non-A Collins which tracks perfect on all bands and it's carrier meter 
reading is very close between all bands.

Bill N6PY



----- Original Message ----- 
From: "David Wise" <David_Wise at Phoenix.com>
To: <r-390 at mailman.qth.net>
Sent: Monday, April 17, 2006 10:25 AM
Subject: RE: [R-390] Low sensitivity on 7-8 Mhz band


I would like to reinforce Tim's point about the S-meter.
When you switch from triple to dual-conversion, the first
mixer, which *has AGC on it* is removed from the signal path.
This will change the receiver's AGC response curve.  Don't
expect your S-meter to read the same, and don't expect the
same audio level.  To really get the picture, you'll have
to measure input for a given S/N.

As he alludes, if .5-6 are okay and it's just 7 that's weak,
that's a different story; either switching troubles, bad RF or
1st VIF top-end alignment, weak 2nd crystal, or misaligned 2nd
crystal oscillator.

73,
Dave

> -----Original Message-----
> From: r-390-bounces at mailman.qth.net
> [mailto:r-390-bounces at mailman.qth.net]On Behalf Of Tim Shoppa
>
> > I checked my Amelco and saw a difference of about 3 dB in calibrator
> > signal between 8.000 and 7.999+.
>
> Well, that is the switchpoint where the radio goes from being triple
> conversion to double conversion. You switch out a mixer stage,
> switch out the 17.5-27.5 Mc tunable IF. And the crystal oscillator
> frequency jumps
> a lot (27Mc output on the 7Mc band to 11Mc output on the 8Mc band).
> With all those changes, I'd say a change in 3dB of the meter
> reading is
> entirely normal (and it's in the range of what I see with my EAC RF
> decks.)
>
> So, not only check the RF stage peaking, also check the 17.5-27.5Mc IF
> peaking
> if you really think you have a problem.
>
> That said, S-meter reading is not the same as "sensitivity" (which
> is usually quoted as uV at a given S/N).
>
> If the local oscillator output for the 7Mc band were weak, it would
> also be weak on the 24Mc band. Not much to listen to on 24Mc right
> now. (Guys, check me if I'm right, I'm not sitting at the
> radio with my
> frequency counter right now...) The base crystal is 9Mc, and harmonics
> are picked off to make 18 and 27 Mc. 18Mc is used on the 15Mc band
> and there's stuff to listen to there in the daytime. 27 Mc is used for
> 7 Mc (triple conversion) and 24 Mc (double conversion).
>
> If I look at the crystal oscillator output on my scope, I see
> roughly a
> factor
> of two in output voltage between different bands. To
> over-generalize the
> higher osc frequencies are lower in amplitude than the lower
> frequencies,
> there is also some clear change between different crystals, although
> I think it has more to do with strength of harmonic.
>
> If the peaker capacitor for the crystal oscillator is out of
> whack (say
> you only
> get one peak instead of two) that's not a good thing
> (although the radio
> will
> work). According to my reading about mixer design, having the
> oscillator
> input
> to a mixer down by a factor of two can really cut conversion
> efficiency
> by a lot
> more than a factor of two.
_____________________________________________________________
R-390 mailing list
Home: http://mailman.qth.net/mailman/listinfo/r-390
Help: http://mailman.qth.net/faq.htm
Post: mailto:R-390 at mailman.qth.net
Unsubscribe: http://mailman.qth.net/mailman/options/r-390