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

[Barry]

This is the website where the fellow replaced so many SM caps:

https://vintageradio.michikusa.jp/data/13-4.html

Barry - N4BUQ

> Hi Dave,
> 
> It definitely could be SM caps.  I've seen a website recently where a fellow
> replaced about 20 of them in his R390/URR but I think the problems there were
> more open, short, or out of tolerance.
> 
> I started through the STAGE GAIN procedures and I think I've discovered that my
> first mixer isn't performing correctly at all.  I had checked the output of the
> first and second oscillators and they looked correct then but things could have
> changed with all I've been doing to the radio.
> 
> Thanks for the reply,
> Barry - N4BUQ
> 
> 
>> 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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