majestic: Safety Capacitor Question for my Majestic 7B09A

I'm working on my Majestic 7B09A, my first full restoration.  As I am going through the unit doing a recap, I have run across a capacitor that I think is the dreaded "death cap", but I'm not sure.  It's C14 (.1 uf, 250 volt), which the Photofact lables as a "Line Isolation" capacitor.  It's tied between chassis ground (as is one side of the input AC) and B-, with a 220K resistor across it.  I'm attaching the schematic for reference and I've circled the section in question.

I'm thinking this cap needs to be replaced with a Safety Cap.  I researching them, it appears that only X2 caps come in the .1 value, which would make this application "across the line". However, the connection to B- is causing me confusion, as this point is the only reference to B- and uses the Earth Ground symbol, which is used throughout the rest of the chassis circuitry.

Is B- actually a DC circuit ground point and the .1 cap is providing noise filtering between it and the AC line?

Man, I've got a lot to learn, but this is so much fun!

• Majestic 7B09A Schematic (288 KB)

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Dear Bob,

Your receiver has no isolating transformer. For that reason the B- can’t be connected direct to the chassis. Otherwise it could happen that the line phase is directly connected to the chassis. For certain reasons the chassis can’t float with any connection at all. Therefore the chassis is 'grounded' by a capacitor and a resistor to B-.That should be the only connection from or to anything in the radio.

Regards,

Rolf

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Rolf, thank you for the quick reply.  Your explaination makes sense.

In my radios' configuration, if the capacitor were to short, I could end up with line voltage on the chassis as well, so it seems appropriate to replace it with a proper Safety Capacitor.

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It may be worth noting that there are two different grades of "safety capacitors".  On one hand the X grade (often marked X2). This grade is intended for use across mains voltage, but not for applications where a failure of the capacitor can cause line voltage at an accessible part. The latter applications should use Y grade capacitors (often marked Y2).

Today, 0,1 uF is considered a way too big capacitance to use between mains and an accessible part, beacuse the leakage current will be too large. If possible, don't use more than 10 nF in case of 110-120 V mains or 4,7 nF in case of 220-240 V mains.  In modern equipment, larger capacitance to accessible parts is found only in heavy equipment that will always be connected to protective ground, and where the large capacitance is absolutely necessary for interference suppression (for example, welders, washing machines or air conditioners).

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Thanks for the input Torbjorn.  I was researching this very topic last night and came to the same conclusion and ordered some Y2 caps for this radio.

I  appreciate the information.  I always learn something new in the these groups!

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