[R-390] High Noise Floor in 0-1 MC Band

[David Olean]

Hello Barry

I have also had noise appear from SM caps in the tuned circuits. What is 
interesting is that the noise can be dependent on how strong the signals 
are. I have tested defective SM caps and found that they do not fail at 
a low voltage (weak signal), but will start making noise as the RF 
voltage increases.  I am sure there are many variations on this 
scenario. The caps can start leaking at around 8 volts or less!!  When 
the RF volts goes up, they start making noise.

Dave K1WHS

On 10/22/2024 1:07 PM, Jacques Fortin wrote:
> Hi Barry,
>
> The common crystal, yes (Y401).
> The other possible suspect is the silver migration disease in one of the tuning coils capacitors.
> Meaning in Z213, Z207, Z201, and even T201: all those ones are used on the 0.5 - 1MHz band only.
> And the prime suspect capacitors are those with a working DC potential (even if it is a small one) across.
> Like C233, C254...
>
> 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
> Envoyé : 22 octobre 2024 11:24
> À : Jacques Fortin <jacques.f at videotron.ca>
> Cc : R-390 Mailing List <r-390 at mailman.qth.net>
> Objet : Re: [R-390] High Noise Floor in 0-1 MC Band
>
> Hi Jacques,
>
> Shorting both antenna jacks eliminates most of the noise; however, there's still a small noise increase when switching from the 1-2 MC band to the 0-1 MC band.  I even tried putting the top cover back in place to help eliminate stray noise from getting in through the top but I couldn't notice any difference with it on as opposed with it off.
>
> The 6-7 MC band has the same noise level as the other bands.  I think you were probably looking at the common crystal for that, right?
>
> Given this, perhaps there just a high level of atmospheric or other noise at my location.  I experimented a bit with my Grundig Satellit 800 and, while the two receivers aren't nearly in the same class, I don't notice that much difference in, say 600kHz and 1200kHz so still not sure if it's atmospheric.
>
> Thanks,
> Barry - N4BUQ
>
>> Hi Barry,
>> Does it behave the same (higher noise on 0-1 MHz band) when the
>> antenna input is shorted ?
>> Just try to understand if it is internal to the receiver or just
>> dependant on the antenna conditions.
>> And.. is it also noisy on the 6-7 MHz band ?
>>
>> 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 : 21 octobre 2024 22:26 À : R-390
>> Mailing List <r-390 at mailman.qth.net> Objet : [R-390] High Noise Floor
>> in 0-1 MC Band
>>
>> For the past few months, I've been trying to determine why the noise
>> floor on the 0-1 MC band in my R390/URR is so high.  If I tune to a
>> strong station, the noise disappears so, perhaps, "noise floor" might
>> be the wrong term for it but in places where there's little-to-no
>> signals, the noise is very noticeable and all but drowns out the weak signals.
>>
>> If I switch from 0-1 MC to 1-2 MC (or virtually any other band), the
>> noise is reduced significantly.  The difference can be on the order of
>> 15-20 dB on the carrier level meter.
>>
>> I presume this isn't normal but maybe it is.  I think someone
>> mentioned that the noise on the bands under 8 MC are typically a bit
>> higher than the other bands but that doesn't seem to account for why
>> the lowest band is by far the worst of them all.
>>
>> There are only passives that change from 0-1 MC to the other lower
>> bands and I'm wondering if it's even possible for those to be the
>> problem but I don't know of anything else that could be causing it either.
>>
>> I think someone suggested that the 1st crystal oscillator's output
>> might be low for that band but I seem to recall checking that on the
>> scope and output for the 1st and 2nd positions seems to be very close
>> to the same amplitude so I'm thinking that might not be the problem.
>>
>> Has anyone seen this and possibly have a cause for it?  Like I said, I
>> presume this isn't normal but I don't know that for sure.
>>
>> Thanks,
>> Barry - N4BUQ
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