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 Post subject: Re: Admiral 24C16, 20B1 chassis
PostPosted: Sep Sun 05, 2021 1:45 am 
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Posts: 5427
Location: Woodinville, WA USA
analog.tv wrote:
That lack of sound may F.M.'s version of 'silver mica disease'. You might open the IF can and look.
I encountered that problem in two of my Admiral TVs. This article explains how I replaced the bad mica caps in the audio IF transformer cans of my Admiral 24C15 TV (20B1 chassis):

https://antiqueradio.org/Admiral24C15Te ... asInIFCans

Read the section titled, "Replacing Mica Capacitors Inside IF Transformer Cans."

In that TV, I removed the transformers to replace the caps. In another Admiral project (model 24A12), I replaced the audio IF caps while leaving their transformers in place on the chassis:

https://antiqueradio.org/Admiral24A12Te ... ansformers

Read the section titled, "Replacing Mica Capacitors in Audio Transformers."

Either method will fix the problem, but you might find one way more convenient than the other, depending on the layout of a particular chassis.

Regards,

Phil Nelson
Phil's Old Radios
https://antiqueradio.org/index.html


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 Post subject: Re: Admiral 24C16, 20B1 chassis
PostPosted: Sep Wed 08, 2021 2:03 am 
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Joined: Jun Wed 05, 2013 1:48 am
Posts: 187
Location: ON, Canada
analog.tv wrote:
That lack of sound may F.M.'s version of 'silver mica disease'. You might open the IF can and look. Craig


OK, I have succeeded in opening one can - here's how it looks inside:
Attachment:
IF_Cans.jpg
IF_Cans.jpg [ 388.69 KiB | Viewed 265 times ]

Seems like there are two (2) 30 pF capacitors on a double-slug coil - adjustments are from the top and from the bottom... The caps are not identified on the schematic though...
Attachment:
L18.JPG
L18.JPG [ 25.85 KiB | Viewed 265 times ]

Do these mica wafers look OK, or have they succumbed to the "disease" you mention?

Also, how does one open the other can? I do not see the clips on the sides of the shield, and undoing the bolts from the bottom seems to lift the entire black can base, i.e. it does not release the shield...

If schematic is correct, there should be another open mica wafer under there - of an unknown value at this point...
Attachment:
L19.JPG
L19.JPG [ 27.65 KiB | Viewed 265 times ]


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 Post subject: Re: Admiral 24C16, 20B1 chassis
PostPosted: Sep Wed 08, 2021 8:32 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 1565
Location: Belrose, NSW, Australia
Mica caps across windings don't normally suffer from silver migration, aka Silver Mica Disease. it takes a DC voltage and many years for that to happen.
That's not to say there isn't another failure mechanism in play there, causing probably an open circuit.

You say you have no data on the value of the caps with the base pins through them? If so, I may be able to find some coil winding details (with caps) in some old magazines I have. The audio coils I have details of will be for 5.5MHz, not 4.5MHz like yours, but a suitable scaling factor applies.

Easiest way to replace those caps would be to cut through them with a pair of cutters.

Edit - I found this in a constructional article for making a TV out of WW2 surplus parts. The capacitor values should work for you, within the range of the tuning slugs. I hope it's readable.
Attachment:
RTV&H_5inch_IF.png
RTV&H_5inch_IF.png [ 781.6 KiB | Viewed 246 times ]

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 Post subject: Re: Admiral 24C16, 20B1 chassis
PostPosted: Sep Wed 08, 2021 3:21 pm 
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Joined: May Fri 29, 2009 4:35 am
Posts: 2274
Location: Chicago, IL USA
The values are 30 pF as marked. Seems like the silver corrodes from exposure to the air causing the value to change or leads to leakage. Clip them out of the base and tack in some new 30 pF micas.

Image

As for that second can, it looks like the plastic as the top has deformed. Remove the nuts underneath the chassis, the pull the can off. The black plastic base and shaft should slide through the can. You may need to trim that plastic with a sharp blade.

It will look something like this. Note this is a video IF transformer - not audio, but the form is the same.
Image


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