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 Post subject: SMD strategy
PostPosted: Mar Tue 06, 2018 3:15 pm 
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I'm in the habit of fixing obvious issues before power-up. Most of the time, this pays off. Then there is this Magnavox I'm working. IF transformers are suspect for SMD, so I open up #1 for a look. Once out of the can, it looked suspicious, so I disassembled and there it was.

On to #2. Same overall construction except the tuning caps a bit smaller, and the two built-in filter caps in a sandwich......a total of 4 caps spread across 3 different mica wafers interleaved with 6 separate contacts. Not possible to see if there is SMD without disassembly, so apart it goes.
It had zero hint of any silver tracks. The only reason I can think of is that the gaps are a bit wider (or maybe the primary and secondary caps were on different wafers?)
Water under the bridge......the point is that that IF could have been left alone.

Next time, the SMD check will be after the set is done and working.

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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 5:03 pm 
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As I understand it a lingering charge such as a bias cell, residual charge in filter capacitors, or dielectric effect is necessary for development of silver hairs. Perhaps the IF transformer in question was not subject to a lingering charge.

I work on very few post WWII radios and therefore have not encountered problems with silver migration in IF transformers but I have to wonder if the silver hairs can be burned off by disconnecting the trimmer from the IF coil and discharging a small electrolytic capacitor across the trimmer. My only silver migration experience was rescue of a Fluke 301C 1000-volt, 500-ma variable regulated power supply. The power supply was from the USBR at Shasta Dam and, after not working, had been donated to a local community college. Some attempt to locate and repair the instrument had been made at the college without success before they gave it to me likely out of concern for injury should someone attempt to use the instrument at the college. The symptom was lack of regulation and overvoltage. Essentially the voltage would rise to the maximum possible regardless of the voltage setting. I checked and tested the feedback loop from the high voltage end all the way around to the synchronous detector, a Stephens Arnold chopper which would be very difficult to replace. The chopper includes a vibrating silver reed type switch in a vacuum across a standard cell. After testing the contacts for a short with an ohm-meter and finding no short, the power supply worked! On a second try it didn't work. Removal and testing of the Stephens Arnold chopper a second time resulted in the power supply working again and so far no subsequent failure. The only reasonable explanation is that the chopper was shorted by silver hairs that developed across contacts in the vibrating reed switch and when tested by a VOM on the Rx1 scale, the battery voltage in the VOM burned the silver hairs.

Norman

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Last edited by Braithwaite on Mar Tue 06, 2018 5:07 pm, edited 2 times in total.

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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 5:04 pm 
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Damned if you do... damned if you don't.

I try to steer clear of USA-built radios produced in the decade when the seeds of SMD were sown... I suppose that you could say that my way of coping with this problem is to avoid it. Although I've handled hundreds of radios (mostly 1950s European-made sets, but also quite a few USA sets built prior to WW2) over the past 45+ years, I can't think of a single instance of SMD that I've encountered.

That would probably be quite different had my interests being aligned with the "known afflicted subgroup" of radios.


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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 6:09 pm 
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I thought I read that Zenith was terrible for smd in 1936 or 1937 with the black dials like 5S127 , 6S222 etc
Henry


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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 6:47 pm 
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Braithwaite wrote:
As I understand it a lingering charge ........

My understanding is that the silver migrates in the presence of an electric field. Every one that I have fixed had a distinct track from primary to secondary (IOW, from B+ to something near ground) Here is the most recent example:
(opposite sides of the mica wafer)
Attachment:
SMD_IF-1--rear.jpg
SMD_IF-1--rear.jpg [ 39.45 KiB | Viewed 575 times ]

Attachment:
SMD_IF-1.jpg
SMD_IF-1.jpg [ 41.64 KiB | Viewed 575 times ]

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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 6:58 pm 
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radiohenry wrote:
I thought I read that Zenith was terrible for smd in 1936 or 1937 with the black dials like 5S127 , 6S222 etc
Henry

That doesn't seem possible; at least not "classic SMD". Pre-war Zeniths used fixed IF coils and variable compression caps for tuning the LC pairs; sets that manifest SMD symptoms use variable (slug-tuned) coils and fixed capacitors of a specific construction that makes them subject to the continuous formation/destruction of conductive paths through which voltage discharges can take place.

Slug-tuned IF coils were around in the 30s (RCA liked them) but AFAIK Zenith didn't use them in that era.
The RCA filters weren't susceptible to SMD due to the nature of their construction.


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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 7:01 pm 
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Silver migration is a plating process. The metal mostly moves from positive to negative voltage areas. This was a problem with transistors and integrated circuits until they no longer used silver plated leads. Tin and other metals also migrate in presents of voltage.

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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 7:13 pm 
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pixellany wrote:
On to #2. Same overall construction except the tuning caps a bit smaller,
Never heard of an IFT with variable capacitors having SMD.

I thought SMD was only found in permeability-tuned IFTs.

- Leigh

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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 7:32 pm 
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Leigh wrote:
pixellany wrote:
On to #2. Same overall construction except the tuning caps a bit smaller,
Never heard of an IFT with variable capacitors having SMD.

I thought SMD was only found in permeability-tuned IFTs.

- Leigh

Yes, sir...
Sorry for the semantic side road.....somewhere I read or heard about capacitors "tuning" a coil.

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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 9:20 pm 
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Greetings to Mark and the Forum:

Fabulous photos, Mark! Outstanding documentation of what really happens. Kind of scary to look at, but should be required reading for anyone who suspects the problem.

Regards,

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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 9:38 pm 
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pixellany wrote:
...somewhere I read or heard about capacitors "tuning" a coil.

Huh??? What the H is that supposed to mean?

As lorenz said, permeability-tuned coils change the resonant frequency of an LC circuit by varying the inductance by means of a movable iron slug.
lorenz200w wrote:
sets that manifest SMD symptoms use variable (slug-tuned) coils and fixed capacitors...

That's the same thing I said.

I never heard of an IFT with variable capacitors exhibiting SMD.

- Leigh

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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 10:47 pm 
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I am well aware that SMD shows up only in variable inductance IFTs.......let's just accept that whatever got into my head about a cap "tuning" a coil did not get into YOUR head. Maybe it's because I learned electronics in Calif............:)

lorenz??--- he said he doesn't DO SMD......

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 Post subject: Re: SMD strategy
PostPosted: Mar Tue 06, 2018 11:03 pm 
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Mark,

You definitely should NOT skip your meds.

- Leigh

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 Post subject: Re: SMD strategy
PostPosted: Mar Wed 07, 2018 3:05 am 
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I've had a handful of the late 30's RCA radios that used octal tubes and had the permeable IFT's in them.

these IFT's had discrete mica capacitors inside the can either with the dot color code or they had no molding around them.

although they were permeably tuned, they were not prone to SMD.

the type of RCA cans i'm talking about are seen here in one of my 86K7 sets:

steve


Attachments:
32. RCA 86K7 Restored & Finished.JPG
32. RCA 86K7 Restored & Finished.JPG [ 119.85 KiB | Viewed 462 times ]
39. RCA 86K7 Restored & Finished.JPG
39. RCA 86K7 Restored & Finished.JPG [ 116.31 KiB | Viewed 462 times ]

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 Post subject: Re: SMD strategy
PostPosted: Mar Sun 11, 2018 1:49 am 
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There is a radio web site that discussed the silver migration in Zenith IF cans in the 36-37 series I am sure .When our e-mail service was updated I lost a lot of saved information and sites however and maybe there is another term for it .I am not as well versed in this as you gentlemen but it seems I have read also about the hairs shorting out plates on tuning caps as well. What I remember was that you had to remove the mica sheets completely and replace that with a suitable capacitor around 120pf or clean the mica to remove the tracking.
I had saved the site to read further in depth when I intended to work on some Zeniths that had a lot of "crackling " in them.
Henry


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 Post subject: Re: SMD strategy
PostPosted: Mar Sun 11, 2018 2:08 am 
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I don't seem to be able to find the particular site now . There was information on all makes of radios. I think the background was black or green and each manufacturer's name was inside an oval. There was a good explanation of how to tell the different models of the Philco 16 tombstone.
If you can point me in the right direction I would appreciate it.
Thanks,
Henry


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