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

Wed Oct 23 15:39:24 EDT 2024

Good point.  Most of the ones I've seen in my R390/URR appear to be the stacked type.  That said, I wonder if these would introduce noise in the same way that a SM cap where the silver has migrated.  Without migration, then I doubt it but, maybe?

Thanks,
Barry - N4BUQ

> I think that the assumption has been made that all mica capacitors are
> silver-mica.  If the capacitor is a dipped mica then perhaps this is true but
> stacked mica capacitors, the "domino style" cased units, can be made with
> aluminum or copper electrodes.
> 
> From:  https://madpcb.com/glossary/mica-capacitor/
> 
>   
>   - Stacked Mica Capacitors: These capacitors are designed with thin mica sheets
>   which are arranged one above another. The separation of every mica sheet can be
>   divided through thin metal sheets of aluminum or copper. The whole unit can be
>   enclosed within a plastic box to defend it from mechanical harm s well as
>   moisture. The capacitor’s both the terminals are connected at every end of the
>   capacitor where the mica sheets can be arranged among the metal sheets which
>   work like a dielectric material. This material will oppose the current flow &
>   aluminum or copper sheets perform like electrodes.
> 
> If the capacitor tolerance is equal to or greater then perhaps 5% then I would
> think that this is not a silver-mica capacitor.  Below 2% then the capacitor is
> most likely silver-mica.
> 
> Jim
> Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy
> 
>    On Tuesday, October 22, 2024 at 08:15:45 PM CDT, Barry <n4buq at knology.net>
>    wrote:
> 
> 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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