I just rebuilt the only bakelite condensor block on this radio, a Philco 40-130. It now has two Y type safety caps. But the schematic (see attached) shows them located between the AC plug (now polarized) and the power switch. That means that these caps are always "on" with voltage across them whether the radio is on or off. Wouldn't it be better if they were after the switch?
I looked at a couple of other radios to see what they did. The GE E81 has their AC line caps in between the AC and switch. Some Atwater Kent and a Majestic 451 have it after the switch. I guess there's no convention.
Yes, it's been discussed here before, and, yes, there are cons to this arrangement, like a potentially charged caps when the plug is out.
Then again, when you use a switch with the caps behind it, there is an inrush current and a spark.
Plus, caps do not draw power. Even when conducting current.
So, on average.....it does not matter much. I would leave it as it is, which I always do when restoring Philcos.
People who do not drink, do not smoke, do not eat red meat will one day feel really stupid lying there and dying from nothing.
I think the main reason the capacitors were wired before the switch was to limit the stray voltage on the chassis when the radio is turned off. When the radio was made, they didn't have polarized plugs, so there was a 50/50 chance there would be line voltage on the chassis when the radio is off. With the capacitors before the switch, the chassis will only have one half line voltage. Either way the current is low and not dangerous.
This set doesn't need a polarized plug at all, it's an AC set so the power transformer provides isolation. The power line caps are there to filter out power line noise, or some amount of it, they are ahead of the power switch but even if they were after the switch one is permanently connected between one side of the line and the chassis anyhow, which probably implies that you need to connect a ground to this receiver. On many sets the power transformer has an electrostatic shield built into it so no line capacitors are needed, on others there will be a single line capacitor connected across the line with no connection to the chassis. With a pair of .01 uf caps there won't be much current flow anyhow, unlike the B- to chassis coupling cap found in many AC/DC sets which are a .2 uf, where you can feel a bit of a tingle sometimes when you touch the chassis.
Regards
Arran
Thanks, I get what the line caps are doing now, but not needing a polarized plug? What happens if the 80 year old transformer in this radio begins to fail? For example, the primary winding overheated sometime in its life and the insulation is breaking down shorting itself and/or to the iron core. Doesn't a polarized plug with the hot side wired to the switch and a grounded chassis offer some protection?
No you don't need a polarized plug what you need is a fuse if you are concerned abt the transformer shorting and catching fire. Fuse size = watts used by radio x10= millampes of require fuse. 50w set x10=500ma=.5amp fuse.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 07-08-2017, 01:52 AM by Radioroslyn.)
Yes, a fuse would take care of that problem. I see a knockout for a 1/2" hole on the back of the chassis right next to the hole for the power cord. Boy that's convenient. I'll add a fuse there.
As for the power cord, the original was cut off probably when the radio malfunctioned. So a polarized cord will be installed since that's what I have at hand. I will digress from the schematic and wire up the safety cap after the switch. I know it shouldn't matter but I'm comfortable with it.
At first I thought this radio would be a lot less work than a AK 545 I restored for my boss/Elmer. since the Philco is a more modern radio with one side of the filament grounded leaving only the other side to re-wire. But like the AK, all the wiring had crumbling rubber insulation and so all that was replaced. Now I can complete the re-cap and replace some out of tolerance resistors. I hope to fire this thing up in a week or two.
Philco chassis from 1939-42 model yrs have rubber wire. The exception is sets made in Canada .Most of the '37 and '38 models have a rf subchassis that makes components replacement difficult without it's removal. '28-'36 are pretty standard to repair but sometimes you'll find a open coil or two along w/ the cap and resistor replacement. Two other major players come to mind AK and Zenith lots of rubber wire.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
