[R-390] Low gain 0.5 MC through 7 MC

[Barry]

I noticed small white spots that had formed on some of those in the RF deck.  I wasn't sure if that might be some indication of a problem and, other than wiping it off, I didn't go further.

There's a Japanese(?) website where the fellow that was working on his R-390/URR found a LOT of bad molded micas.  It very well could be the problem in mine.

Thanks,
Barry - N4BUQ

> Don't forget to be suspicious of all molded mica caps in your radio.  While they
> are by no means the least reliable parts, they are no longer above suspicion.
> I've seen parts go off value and develop high loss (dissipation factor), which
> will harm sensitivity even if they don't test leaky.
> 
> Dave Wise
> ________________________________
> From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of
> Barry Scott <72volkswagon at gmail.com>
> Sent: Sunday, September 22, 2024 8:07 PM
> To: Jacques Fortin <jacques.f at videotron.ca>
> Cc: R-390 Mailing List <r-390 at mailman.qth.net>
> Subject: Re: [R-390] Low gain 0.5 MC through 7 MC
> 
> Jacques,
> 
> It's not a short 6C4 - just a standard height version.  I'm not sure of the
> manufacturer but I do have some spares I can try.  The output of the 1st
> crystal oscillator appears strong on an oscilloscope and is about the same
> magnitude as the 2nd crystal oscillator's output.
> 
> You have made me realize that I did not align the two variable IF
> circuits!  I completely neglected to do that as I was focusing on the RF
> sections. That very well might be where the problem is.
> 
> Thanks!
> Barry - N4BUQ
> 
> 
> On Sun, Sep 22, 2024 at 9:25 PM Jacques Fortin <jacques.f at videotron.ca>
> wrote:
> 
>> Barry, the troubles of your R-390/URR reminds me a lot of similar things
>> I've gone thru with mine....
>> Yes, it can be the first mixer tube: I just hope that is not a "dwarf"
>> Raytheon 6C4W used there.
>> Or it can be something around (any component in the plate circuit) Z221
>> again ???
>> The problem can also be the first crystal oscillator (V401) which do not
>> have enough output...
>> Check every component around and swap the tube for a known good one if you
>> have spares.
>>
>> 73, Jacques, VE2JFE in Montreal
>>
>> -----Message d'origine-----
>> De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
>> part de Barry Scott
>> Envoyé : 22 septembre 2024 20:37
>> À : R-390 Mailing List <r-390 at mailman.qth.net>
>> Objet : [R-390] Low gain 0.5 MC through 7 MC
>>
>> Sensitivity on the lower 7 bands is significantly lower than on 8 MC and
>> up.
>> I can see this very readily with the signal generator as input and, with
>> the
>> output level remaining the same, switching from any of the first 7 bands to
>> 8 MC or higher shows a significant (approximately double) rise in the
>> radio's dB meter.  It's also noticeable just listening to stations.
>>
>> The common factor seems to be V203 (1st mixer) and I suspect that tube
>> might
>> have low gain (possibly gassy?).  Is that a reasonable thing to start with?
>> I don't see how something affecting the AGC would affect all the paths to
>> V203's grid and so I was thinking the tube itself might be the problem.
>>
>> Thanks,
>> Barry - N4BUQ
>>
>>
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