38-116 missing cap - Printable Version
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38-116 missing cap - madsowry - 02-11-2022
Hey All,
I'm working on this 38-116 that is a real nightmare from the word go. When I bought this thing it was about 400 miles away so I asked the guy if had any experience with packing and shipping. He said he just shipped a radio last week, so I assumed he was experienced. Well you know what ASSUME is all about and the radio showed up with 1 wrapping of cardboard around it and upside down with the chassis hanging out of the cardboard. The chassis was bent and 4 of the 5 shafts were broken off. So my challenge started and I have most of the damage corrected. I'm working on reattaching the shafts to their appropriate origins while also recapping and resister checking. While recapping and checking the schematic I noticed my chassis does not have cap #122 which is supposed to be in parallel with choke #119 and attached to 2 of the Ecaps on either side of it. My chassis has this item missing. I've checked all the change to the circuits info I could find but nothing mentions this cap.
My questions are:
Is this cap needed?
If so, what does it do?
If I add it can I just put across the chokes 2 lugs?
Part52-Philco Wiring Diagrams Vol. 2.pdf (philcoradio.com)
Any help would be greatly appreciated.
Thank you ahead of time
Dick
RE: 38-116 missing cap - Mondial - 02-11-2022
That cap is not absolutely necessary. It creates a parallel resonant circuit with the choke to reduce the power supply ripple, thereby reducing hum. They did not have large capacity electrolytics in those days and this was a way around the problem.
I have a 38-116 and the cap #119 was missing on mine also. The filter caps were previously replaced so I don't know it was not there originally or just left off later.
You can just add the missing cap across the choke. If not increase the capacitance of the electrolytic after the choke to compensate. Do not increase the capacitance of the input caps right after the rectifier as it will raise the DC output voltage. Film caps are recommended for the input filter as the ripple current is very high at that point. I used 10 uF 630 V polypropylene films in mine.
https://tubedepot.com/products/solen-fast-cap-10f-630v
RE: 38-116 missing cap - madsowry - 02-12-2022
Thanks Mondial. I've already restuffed the input caps as well as all the rest and installed them so I think I will install that cap and hope the hum is not too bad. If it is, I can always restuff the input caps or just tuck the 10uf polys underneath and only be out a couple new 8uf Ecaps. No great loss.
I appreciate your explanation of how and why that cap was added. I learn things every day on these forums and love it.
Thanks again
Dick
RE: 38-116 missing cap - Mondial - 02-12-2022
Dick, the polypropylene caps will not necessarily reduce ripple more than the same value electrolytics, but they will practically last forever. This is especially important if you restuff the original cans and do not want to ever open them up again in the future.
Modern 8 uF electrolytics are not rated for the amount of ripple current they will see in locations #120 and #121 and will have a relatively short life.
Cap #123 and 123A are a different story. They see relatively low ripple current and electrolytics will last a long time in that circuit location. Their value can be increased within reason to 22 or 33 uF to reduce hum, yet not increase the DC voltage of the power supply.
In any case, adding cap #119 will act as a trap for the 120 hz ripple frequency of the full wave rectifier circuit and reduce the residual hum.
RE: 38-116 missing cap - madsowry - 02-12-2022
Thank you sir for your reply and info. I think since I already have all the Ecaps restuffed and installed, I'm going to install the cap #122 (.3uF) across the lugs of the choke #119 and see if there is any hum. I'm still quite a ways from testing this thing so it will be awhile.
Also getting my left hip replaced on Valentines Day so the restoration will be paused for awhile.
Thanks again
Dick