Model 90 Cathedral stopped receiving - Printable Version

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+--- Thread: Model 90 Cathedral stopped receiving (/showthread.php?tid=21586)

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RE: Model 90 Cathedral stopped receiving - zboltman - 08-30-2020

Wow, this is all a lot to take in.
You Gentlemen have been so very helpful.
Please give me a couple days to catch up on things.
(Replace vol pot, inspect mica, measure resistors, etc)
After a couple days, expect to hear back from me with all my new woes....
Thanks again!!!

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RE: Model 90 Cathedral stopped receiving - zboltman - 09-08-2020

Hello to all,
I replaced the volume control and good semi good results.
I can now hear oscillator pitch sounds which vary when I tune, the cool part is they happen at a station where I hear faint fuzzy voices. (I promise, the voices are not in my head...)
Over the whole range of tuning, I get this about 4 places.

Now I am very interested in the comment made about the MICA. So I took some pics for further evaluation.
Here is where I need help.
1: What is the purpose of the MICA
2: IS the MICA damaged to the point of needing to be replaced.
3: Can the damaged MICA cause any of the issues that I am currently having, (see above)

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RE: Model 90 Cathedral stopped receiving - morzh - 09-08-2020

These are trimmer capacitors. The mica is the insulator between the two plates. The capacitance is changed through tightening/loosening of the nut that gets the plates closer/further from each other thus changing the capacitance.
The capacitance is small and is used to trim the band limits to be exactly where they are wanted after they are roughly adjusted.

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RE: Model 90 Cathedral stopped receiving - Eliot Ness - 09-08-2020

Good, progress!  I wouldn't worry about the mica just yet, but would start measuring resistors and replacing capacitors.  Replace any resistor that measures higher than 10 or 20% of their original value.  I'd do one resistor and one capacitor at a time and then recheck how the radio plays.... that way if you make a wiring error you'll know right where to look.

I think I posted a link on how to restuff the little Bakelite blocks, but you don't really 'need' to do it that way, it is just neater.  BUT, what you need to do is look at the #'s on each block and make sure you use what is supposed to go in there, and not every block has been replaced or jumpered yet.  If you decide not to restuff the blocks you need to cut ties to the old capacitors still in there.  In the picture below you can see tiny wires coming out of the eyelets:

   

Use a small drill bit to break that wire before you jumper in a new capacitor.  If you decide to restuff then you can ignore this step.  Below are two methods, but Mike's is easier, but use whichever method works best for you:

https://philcoradio.com/library/index.php/shop-talk/techniques/bakelite-capacitor-rebuild-without-desoldering/

https://www.philcorepairbench.com/how-to-rebuild-philco-bakelite-blocks/

Link to a chart showing block #'s:

https://philcoradio.com/library/download/service%20info/service%20bulletins/Philco%20Service%20Bulletin%20289.pdf

I think the best way to find a resistor value is to find it on the schematic, but there were some changes Philco made.  If the following gets confusing we can answer any questions:

https://philcoradio.com/library/index.php/shop-talk/general/how-to-decipher-philco-resistor-part-numbers/

One good source for capacitors:

http://www.tuberadios.com/capacitors/

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