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

Wed Oct 23 16:32:01 EDT 2024

Barry
AFAIUI, The IFT in consumer radios uses a sheet of mica with a sprayed silver coating that is not protected.  Silver migration causes a type of spark gap transmitter that overloads the received audio and makes the AGC go very negative.  This cuts off the signal so all you hear is the lightning crashes as the spark varies in intensity.  Collins did not go this route.  

I think what is happening is that the stacked mica capacitors are finally reaching their end of life.  Moisture is penetrating the plastic coating and causing the dielectric to become more conductive.  We can replace all of the mica caps with film caps.  Long before they reach end of life, we will!

Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.  Murphy 

    On Wednesday, October 23, 2024 at 02:39:24 PM CDT, Barry <n4buq at knology.net> wrote:   

 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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