good idea to add line caps to model 52?

good idea to add line caps to model 52? - Printable Version

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good idea to add line caps to model 52? - gribbled - 12-01-2018

I'm finishing up the electrical work on an original 1934 model 52. The radio as designed had no caps on the AC input line; the power cord went directly to the transformer through the power switch.

I'm replacing the cord, and planning to add an inline fuse (84 year old transformer), and was wondering if it was a good idea to add line capacitors? My first thought was to add one across the line to filter out incoming "hash" on the power.

I see multiple threads where people have added a cap from each line to chassis, to eliminate (or reduce?) the need for a chassis ground connection? The radio has spring clip terminals for antenna and ground.

Not that I want to do it, but would a 3 wire power cord accomplish the same thing?

What would you experts do here?

PS - any idea how to clean the volume control? It looks pretty close to totally sealed?

Thanks!

dave

RE: good idea to add line caps to model 52? - morzh - 12-01-2018

Yes, 3-prong cord will do the same as Y-caps (chassis to lines). Its just that the option wasn't available at the time.
However X-cap (from L to N) does help. If using large values (0.1uF to 1uF, which is unnecessary and is typically used for switching supplies to filter out the outgoing, not incoming, noise) installed between the outlet and the power switch then a bleeding resistor (200K or so) is a good idea, abd so is a NTC thermistor of a few Ohms cold resistance.
Otherwise 10-30nF is quite enough.

Use ONLY X or XY rated caps.

PS. Whether it improves the performance or not, depends on your Mains characteristics. Won't hurt anyway.

RE: good idea to add line caps to model 52? - amptramp - 12-01-2018

If you have a 120 VAC line and you have two equal capacitors from line to chassis, the chassis will remain at 60 VAC because you have a voltage divider between the neutral and hot side and the neutral is grounded. If you have a three-prong plug with the ground connected, the chassis will be at ground potential. The capacitors are there to avoid noise conducted from the AC line getting into the circuitry and to avoid emission of noise (especially oscillator noise for the superheterodyne sets).

RE: good idea to add line caps to model 52? - Steve Davis - 12-02-2018

I would by all means add the line capacitors. The main reason for line capacitors is to provide an RF ground path for the antenna circuit. Without them, a chassis ground is required. The line capacitors do create a voltage divider that applies about 60 volts to the chassis. The available current is very low and is no danger.

Steve