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 Post subject: Silver mica disease?
PostPosted: Sep Tue 12, 2017 6:52 pm 
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Location: Bristol, Rhode Island
Someone posted a question about it below. I don't generally work on sets of the vintage in which you get SMD. But I have an Emerson DS-365 on the shelf that has been fully recapped and gone over. But the sound is atrocious. Dead spots on the dail, crackling here and there, a few stations coming in loud in clear. Generally just a nightmare to deal with so it's on the shelf.
Question: Does this sound like SMD? And if so, how do I address it?

Thank you!

http://www.nostalgiaair.org/pagesbymode ... 005042.pdf


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 Post subject: Re: Silver mica disease?
PostPosted: Sep Tue 12, 2017 6:57 pm 
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This model, according to the schematic you posted, doesn't have fixed micas in the IF transformers, so that would rule out SMD originating in the IFs.

One capacitor that could be causing static crashes is C30 - from plate of first audio tube to ground. If that's a mica cap and it has not been replaced, you might try replacing that.

Dead spots on the dial would not be caused by SMD regardless, so that has some other cause.

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 Post subject: Re: Silver mica disease?
PostPosted: Sep Tue 12, 2017 7:13 pm 
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Could also be the tuner is shorting out. To test disconnect the antenna wire and oscillator wire from tuner. Use ohm meter to test each side from lug to frame while turning the shaft. Should read open all the time. If you do get a reading there's your problem. Normally it's one of the outside fins that's bent and touching the the stationary fins. Also clean and lube the contacts that ride the tuning shaft.


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 Post subject: Re: Silver mica disease?
PostPosted: Sep Tue 12, 2017 8:18 pm 
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Thanks for the tips. I will try both. It's a beast of a table top (PP 6L6s) and I would like to get it going. Much obliged.


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 Post subject: Re: Silver mica disease?
PostPosted: Sep Wed 13, 2017 4:48 pm 
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Tom Albrecht wrote:
This model, according to the schematic you posted, doesn't have fixed micas in the IF transformers, so that would rule out SMD originating in the IFs.

One capacitor that could be causing static crashes is C30 - from plate of first audio tube to ground. If that's a mica cap and it has not been replaced, you might try replacing that.

Dead spots on the dial would not be caused by SMD regardless, so that has some other cause.

Thanks for the help, Tom. But you may be looking at the wrong schematic? There is no C30 on the schematic ... it jumps from C29 to C31. Very odd. I'm definitely not overlooking it; C30 is also not listed on the parts list. Here is a slightly better scan of the schematic. Any suggestions on where to try? Thanks!

Ted


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 Post subject: Re: Silver mica disease?
PostPosted: Sep Wed 13, 2017 5:36 pm 
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Sorry about that! I was looking at the other schematic on the same page of Rider. In your radio, the mica in question would be C29. For this radio, however, C29 doesn't have much voltage across it, so it seems less likely to be the cause of your problem. Nonetheless, I would try replacing it. Any other micas in the audio section would also be candidates.

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 Post subject: Re: Silver mica disease?
PostPosted: Sep Wed 13, 2017 6:26 pm 
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So, I'm thinking it may be a mechanical issue at this point. The dropping out, silence and dead spots on the dial mostly occur when I wiggle the tuning knob back and forth. Weird, because the shaft is not connected to the gang, only to a pulley which in turn moves the gang with a cord. Neverheless wiggling it must be causing a short somewhere. I've gone ahead and removed the dial plate, tuning gang etc. Next step is to replace the bushings as a few of them are dry-rotted. Still there, but not pliable like they should be. The tuning gang appears to be in perfect shape. No bent plates etc., nothing seems to be shorting out and I'm getting no continuity between oscillator and antenna lugs to the frame. Hopefully reworking the rubber suspension on and around the gang will help things out. If not, onward!


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 Post subject: Re: Silver mica disease?
PostPosted: Sep Wed 13, 2017 10:03 pm 
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Cleaned up the tuning gang and hooked it back up, out of chassis. I'm sure the clip leads aren't ideal but you can get a sense from the video of what the set's doing. Fair bit of motorboating, crackling etc. Reception stinks. Any ideas? Thanks!

https://youtu.be/3o0Wv1e5OCM


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 Post subject: Re: Silver mica disease?
PostPosted: Sep Wed 13, 2017 10:34 pm 
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Another cause of static, crackling etc can be a resistor breaking down. I had an issue similar to yours and found a screen grid resistor failing.

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 Post subject: Re: Silver mica disease?
PostPosted: Sep Thu 14, 2017 2:04 am 
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debug around the micas.

steve

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 Post subject: Re: Silver mica disease?
PostPosted: Sep Fri 15, 2017 3:48 pm 
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Many now replace old carbon resistors whilst doing recap job -- not much trouble and cost just a few pennies.


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 Post subject: Re: Silver mica disease?
PostPosted: Sep Fri 15, 2017 5:01 pm 
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I've changed all resistors more than 10 percent off spec. Perhaps some of the old ones develop issues under power? If so, maybe I'll shotgun them all but for now, just the ones that were off.


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 Post subject: Re: Silver mica disease?
PostPosted: Sep Fri 15, 2017 8:00 pm 
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Location: Santa Clara, CA 95051
Are you able to beat a signal from a signal generator through the receiver? What reaction is there in the tuning eye while the popping is taking place? I found mine as a nearly dead short in the secondary of the third IF can. It was tough to find in one respect given the resistor that in mine is in series with that third coil. I always check the continuity of those cans before I heat things up the first time and while I had a high DC resistance I had continuity so I hoped for the best.
Have you tried simply tapping? Can you get the popping to come alive if you physically tap components? The short in my secondary would give me a spitting sound when I tapped it.

Tomie


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 Post subject: Re: Silver mica disease?
PostPosted: Sep Fri 15, 2017 9:04 pm 
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replace micas.

viewtopic.php?f=6&t=322945&hilit=+rework

steve

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 Post subject: Re: Silver mica disease?
PostPosted: Sep Fri 15, 2017 9:44 pm 
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Since you have the cap out of the chassis, this is a good time to check for shorts. Connect your ohmmeter between the frame of the cap and each stator in turn. Turn the cap through its range. The meter should read "open" at all times. Another point: there are brass spring fingers that contact the tuning shaft. Use some very fine sandpaper on the fingers and on the shaft where they touch, then spray those areas with contact cleaner.

The cap out of the chassis with long clip leads is not going to operate properly.

Did you clean all tube pins with contact cleaner, plugging them in and out of the sockets while pins were still wet?

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 Post subject: Re: Silver mica disease?
PostPosted: Sep Mon 18, 2017 10:49 pm 
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Location: Bristol, Rhode Island
Yes, sockets cleaned.
I did test the tuning gang and the sections tested open as they should, throughout the range.
I removed the gang and washed it but did not try scouring underneath the brass clips. Thank you for that tip!


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