Philco 38-40T Hum - Printable Version

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+--- Thread: Philco 38-40T Hum (/showthread.php?tid=14513)

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Philco 38-40T Hum - Landshark - 05-01-2016

            I only have limited electronic experience and only have recapped one radio, an AC/DC AA5 Marconi set.  I recently got this Philco 38-40T and discovered it can run off 110V AC or 6V DC.  It works well, and I have removed the chassis and done the normal visual checking, powered up with my homemade light bulb voltage regulator.  I have the schematics and am in the process of identifying the paper capacitors to do the recap. The radio looks in good shape, tubes all glow, brings in the one and only AM station very strongly,  only one can capacitor appears to have  been altered, all the rest of the works look original.  Tubes likely replaced as the bottom holders have been cut and bent but the tube protectors hid this.

The radio does have a moderate hum, so when I replace the paper capacitors, I will start with the most likely one that could cause a hum, the filter or electrolytic capacitors. I have already replaced the power cord and rebuilt the dial light as it was shorting out.  I did check to make sure there are no electronic near by causing the hum, it is a constant hum and it across all bands (MW and SW).

My first problem is looking at one of the can capacitors. Can #14 (30-2291X) someone had cut the yellow wire coming out of the metal tube capacitor leaving the black to go to common negative/ground and Green seems unchanged. The “Can” had the specs of what was in it, but appears to have been rubbed out, maybe on purpose. It is #14 on the schematic with 3 wires coming out, Black, Green and Yellow. I think it says on the side of the Can; Green is 8 MFD 350 DCWV and Yellow is 8 MFD 475 DCWV, Black as mentioned, is Common Negative.

My guess is that one of  Electrolytic Capacitors, inside the #14 Can has been replaced with the big yellow 20 MFD Electrolytic Capacitor that has been added under the chassis. This big yellow capacitor is not shown on the schematic, it has a patent of 1933 so it is pretty old.  On it is written "Type PRS, For rectified A.C. or Pulsating D.C circuits only",  (makes sense as this is a duel power radio), also is "Dry Electrolytic Capacitor,  MFD 20 Max SGEV 525, AEE-6  DCWV 450".

My question is; Are the Electrolytic Capacitors special for this radio as it operates with both AC and 6V DC vibrator power supply?  Or can I just replace with a regular EC of similar voltage and MFD?  Why is this large yellow capacitor so much bigger, 20 MFD than the 8MFD it would have replaced?  

I have not hooked up a 6 volt battery yet, the rubber “feet” are all toast and if this thing vibrates it could get interesting without the cushions.


Thanks in advance for any advice.  I have been reading the internet, (big job, I know) so I am building my knowledge.  I live in a remote part of BC Canada so don't know anyone who can help.  I joined the Canadian Vintage Radio Society but they don't get much traffic on their web site so hopefully I can find some one here who can help, I am sure I will have more questions later.


Bob

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RE: Philco 38-40T Hum - klondike98 - 05-01-2016

Welcome to the Phorum!


Repairs to electrolytics were often made by clipping the lead to the old cap and tacking in a new one of equal or larger capacitance (probably depending on what the repair guy had in his shop at the time). Nothing special about the electrolytics so you could use 8 or 10 uF at the proper voltage rating. Some suppliers: http://www.philcoradio.com/resourceb.htm#n

Lots of folks here to help so ask questions when ever you need help. No dumb questions here.

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