1955 kids record player hum puzzle - diyAudio
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Old 24th July 2014, 12:31 AM   #1
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Default 1955 kids record player hum puzzle

I'm an amateur trying to restore an old record player (Sonic Capri from 1955) that buzzes/hums when turned on. I've replaced nearly everything: capacitors and resistors with new ones of equal value, replaced the 25C5 tube with a good-tested tube, and I replaced the selenium rectifier with the 1N4007 as recommended on many web sites. I cleaned the POTS also. I don't have the schematics so I did the best I could with the photo here (but I'd love to get a copy if anyone has the schematics). The problem is that the system still hums just as it did before I started, and I get no response at all when the needle is touched. Oddly, or perhaps not, pin 5 has no contact with anything on the board, and I don't know if this is normal or not. Any advice appreciated. Thank you in advance.
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Old 24th July 2014, 02:10 AM   #2
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Your old record player does not appear to have a mains transformer. If so, this topic of discussion is not allowed.

See note #2 in the notes section.
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Last edited by cogsncogs; 24th July 2014 at 02:19 AM.
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Old 24th July 2014, 02:10 AM   #3
maton00 is offline maton00  Mexico
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The pin number 2 and the number 5 are the same (Grid 1), but what happened to the main supply capacitor?
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Old 24th July 2014, 02:13 AM   #4
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Yo Scott,
Pin number 5 is a second connection to the control grid of a 25C5. It parallels pin 2 so no connection is normal if pin 2 is used. Regarding the hum, have you checked/replaced the electrolytic filter capacitor(s) in the PS. Also, an open cartridge can cause a hum by leaving the input unloaded and would explain the insensitivity of the stylus. Many of these were crystal and go open as a failure mode. Try momentarily shorting the input to see if the hum stops.
"I regard the brain as a computer which will stop working when it's components fail. There is no heaven or afterlife for broken down computers. That is a fairy story for people afraid of the dark." ~Stephen Hawking

Last edited by HollowState; 24th July 2014 at 02:20 AM. Reason: added
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Old 24th July 2014, 06:52 AM   #5
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Default Possible fix - missing electrolytic capacitors

Someone contacted me and said the two bottom Xs (solder points) in the photo need to be connected by a electrolytic capacitor, and another capacitor needs to go from the right X to the bottom of the green line where the selenium rectifier connects to a resistor.

I'm going to try this and see what happens, as I discovered solder traces at those points where something used to exist... capacitors, I hope.
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Old 24th July 2014, 11:30 AM   #6
DF96 is offline DF96  England
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You understand that the smoothing caps will be polarised? Get them wrong and things go bang (literally!).

You also know that a transformerless circuit is extremely dangerous? You need experience to handle this safely.
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Old 24th July 2014, 02:45 PM   #7
hoff70 is offline hoff70  United States
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I would install an isolation transformer to make it safer. There are plenty of write-ups about doing this.
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Old 24th July 2014, 05:40 PM   #8
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Default cap polarity and UPGRADE recommendations please

DF96 - yes, I was told the negative on both caps will go to the X on the bottom right in the photo. thank you for your concern. I am an amateur but am very safe.

hoff70 - I did not plan to alter the original layout but I'm very interested in ANYTHING that can improve this record player. Can you give me a few more details about adding an isolation transformer? brands, spec recommendations or location would help a lot. I'll also do some reading. Thank you.

ANY other improvement recommendations GREATLY appreciated.
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Old 24th July 2014, 07:00 PM   #9
hoff70 is offline hoff70  United States
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I'm pretty newbish myself but I believe that's a "hot chassis" amp. Do some searching on it and it'll tell you more than I can! The chassis of those are connected to the mains neutral so If you plug it in backwards then the chassis is connected to the hot wire

These are the cheapest of the cheap tube amps but I think it's interesting!

I'm fooling with an old hot chassis amp and picked up an N68-X isolation transformer which seems to be popular with these little amps. I also got rid of the selenium diode and replaced it with a full wave rectifier. The old selenium jobbie is half-wave and that's supposed to cause hum as well as making the iso trans run warm.

Less than $20 worth of parts and supposed to be both performance and safety improvements!

Hopefully someone more informed than myself will weigh in and clarify.
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Old 24th July 2014, 08:31 PM   #10
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Right now both pins on the power plug are the same size. So if I put a polarity plug (one pin thick and the other pin thin) which AC line in the photo should go to the thick plug side vs thin plug side? This makes me nervous. Photo reposted.
Click the image to open in full size.

Last edited by scottusa; 24th July 2014 at 08:53 PM. Reason: linked thumbnail image instead of photo
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