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 Post subject: RCA CTC-7 recap discrepancy?
PostPosted: Apr Fri 03, 2020 9:33 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 182
So, I ran into a bit of an odd situation today as I was plugging away at recapping my CTC-7N. Cap C735 in the color oscillator section is shown on both the Sams and RCA schematic as a .47 MFD / 200 V. What’s actually in the circuit is two caps in parallel: A .047 / 200 V and a 40 MFD / 25 V. Now, I’m observant enough to tell where previous work has been done, but clearly both of those caps are original factory work based on the soldering.

What’s the deal here? I’ve seen cases where two caps were used in parallel as a substitute during component shortages, but a 40 MFD and .047 don’t make a lot of sense here to replace a .47. I would normally opt to use what the schematic and service manual indicate, but I also know chroma circuits are finicky and am inclined to replace it as it was originally wired (it obviously worked that way).

Thoughts?

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 Post subject: Re: RCA CTC-7 recap discrepancy?
PostPosted: Apr Fri 03, 2020 9:51 pm 
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Joined: Apr Wed 10, 2019 3:48 am
Posts: 281
Interesting...
Appears production experienced a shortage, and had to make do.
Like the pony express, those radios must go through. Lol


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 Post subject: Re: RCA CTC-7 recap discrepancy?
PostPosted: Apr Fri 03, 2020 10:10 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 182
Even more strange is that I happened to remember that I had another junker CTC-7 chassis around here and just dug it out to compare. Lo and behold, it’s set up the exact same way! I could definitely see a temporary substitute if they happened to be short at the time, but if so, awfully coincidental that I would happen to have two chassis that were both impacted. I guess I’ll replace it with the same combination now that I’ve confirmed it wasn’t a one-off fluke.

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 Post subject: Re: RCA CTC-7 recap discrepancy?
PostPosted: Apr Sat 04, 2020 10:39 pm 
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Joined: Aug Thu 12, 2010 6:25 pm
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Location: Durham, NC
With two chassis showing the exact same replacement, I'd go with it!
From an engineering standpoint, the "replace with what's available" didn't seem likely, since the values are SO different from the schematic. More likely, I think, is they found that the vertical info in the applied signal was screwing up the decoding occasionally, so a bigger bypass (40uF - probably available overkill) was needed, but large value caps usually don't have good high-frequency characteristics, so the parallel .047 was still needed. Just a guess.

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 Post subject: Re: RCA CTC-7 recap discrepancy?
PostPosted: Apr Sun 05, 2020 1:14 am 
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Joined: May Sun 07, 2017 11:35 am
Posts: 850
Location: Belrose, NSW, Australia
Mark I'd concur with that assessment. It's been a running change to fix a problem that's shown up in early production units.

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