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

Barry n4buq at knology.net
Wed Oct 23 16:59:08 EDT 2024 

Jim, 

Many of the ones in my radio have small white spots on them like some sort of chemical residue. I don't know if that's an indication of moisture or something migrating out of the encasing. It may not be anything to worry about but it "worries" me nonetheless. If I have to replace that many caps, especially some that are buried, this may change from a hobby into a job. 

IFT? 

Thanks, 
Barry - N4BUQ 

> 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 < [
> > mailto:n4buq at knology.net | 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 |
> > 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 : [ mailto:r-390-bounces at mailman.qth.net | r-390-bounces at mailman.qth.net ] <
>>>>> [ mailto: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 < [ mailto:jacques.f at videotron.ca | jacques.f at videotron.ca ]
> >>>> >
>>>>> Cc : R-390 Mailing List < [ mailto:r-390 at mailman.qth.net | 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 : [ mailto:r-390-bounces at mailman.qth.net | r-390-bounces at mailman.qth.net ] <
> >>>>> [ mailto: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 < [ mailto:r-390 at mailman.qth.net | 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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