Replace mica caps in Bakelite blocks?
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Hello, everyone,
I'm re-capping a 37-116X and ran into part #72 on the schematic, a Bakelite block (8035DG) containing two 110 pf capacitors. What is the consensus on replacing the micas? Do they need to be replaced or can they be left alone? The electrolytics have been replaced and the set is working but all of the paper caps were original and are now being replaced. Most of the resistors have been spot on!
Audio is distorted but that might be because the speaker cone's adhesive has failed where the cone's rim is attached to the basket. 
Mike, K9UW
Amherst, WI
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I replaced one of these in my 1936 116PX chassis. I was told afterwards that it could be left alone. I pulled the parts out of the garbage and checked them with my meter, and they were out of spec. So, in hindsight, it was a good idea to replace them. I'm not an expert, but, my take would be: check them and if they are still within 20% of stated value, leave alone. If not, replace them. 
Blessings,
Jeff W.
Jonesboro, Arkansas
http://jeffsradios.weebly.com
God loves you as you are, not as you should be, because none of us are as we should be. - Brennan Manning
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When I was restoring mine, the consensus here was that the 100pF in blocks were not necessarily micas, and so I replaced them.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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In the vast majority of 8035-xx bakelite blocks I have restuffed, the original caps were paper and not mica. Only a few have had mica caps inside. You're better off by going ahead and restuffing now while you are working on the chassis.
--
Ron Ramirez
Ferdinand IN
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I guess (purely out of interest) one way to find out whether they are mica or not is to measure them. It might be a bit tricky with a regular meter as it will show about 100pF when not connected so it needs too be zeroed first; if it shows close to the nominal value, it is then a mica; if it shows a different value (paper caps migrate as fas as 2x to more the value) then it is paper.
Has to measured out of circuit, with the wires around clipped.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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The very first Philco I repaired that had cap blocks in it had one with two 110pf caps that I thought certainly must be mica with that value... so I wisely left them alone. And when completed the radio worked great.. well, for a little while then it developed this intermittent problem that I traced to.. guess who, the only caps in a block I didn't change. When I pulled them out of the block I found they were paper not mica. So in this case since you don't have a visual to confirm what they are... why take a chance.. change them out.
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Thank you all for your advice. New 110 pf mica caps have been ordered. Since posting my inquiry, another 110 pf dual Bakelite has presented itself. I'll try to measure the originals to see what values they currently have.
Now to re-attach that speaker cone!
Mike, K9UW
Amherst, WI
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(09-17-2018, 10:28 AM)morzh Wrote: I guess (purely out of interest) one way to find out whether they are mica or not is to measure them. It might be a bit tricky with a regular meter as it will show about 100pF when not connected so it needs too be zeroed first; if it shows close to the nominal value, it is then a mica; if it shows a different value (paper caps migrate as fas as 2x to more the value) then it is paper.
Has to measured out of circuit, with the wires around clipped.
Good suggestion, Morzh! It took a while to isolate the leads from one of the caps inside the Bakelite block but, after adjusting the meter to compensate for capacitance in the leads, the cap that had been 110 pf had jumped in value to 245 pf. They must be paper, and they're coming out!
Mike, K9UW
Amherst, WI
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Yep, the increase is consistent with the paper caps: I regularly measure those I replace and it is almost always 2x - 2.5x increase for the 30-s - 40s caps.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
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Sometime you can't even read the capacity on a paper cap, the leakage throws it off, even at low voltage.
Regards
Arran
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