I'm working on a Zenith model L727 "Empress" clock radio. It's a "hot" chassis. It has only one paper cap and it was replaced at some time in the distant past. Otherwise everything is ceramic. The cap that was replaced is C25. It's a .047 cap but no voltage is given. The replacement in there now is an old Aerovox rated at 1000v. There is obvious evidence that the previous cap went up in smoke.

The schematic shows the cap across two lines and grounded but the replacement is only connected from one line to ground, the line with the R2 resistor. The schematic shows the other end of the cap connected to the yellow wire returning from the clock. Ther wire from the clock goes directly to chassis ground. So, what recommendations do you guys have? What type of cap at what voltage and how should it be connected to the line, just like the Aerovox was? That's my thought but I want to be sure.

Thanks,
Larry

Larry

That cap is across the AC line. You could use a .047mf @ 630 volts or a more modern replacement is AC cap X or Y. Dave at Just Radios has an explanation on his site:

http://www.justradios.com/

I probably wasn't very clear on what I'm asking. Here's the way the radio is actually wired. The cap is from line B to ground. Line A is from the switch to ground. Here are my concerns. If line B is the hot line then the cap, and the rest of the chassis, is always hot. If line A is the hot line then nothing is hot until the switch is thrown but electrically it's running backwards (hum?) and the cap is now on the return line. Neither way seems ideal to me so I'm looking for some insight and/or suggestions. What am I missing?

Larry

Larry

Electrically there is no difference in the two schematics. Normally manufacturers connected a cap from the AC line to chassis. This gives the radio a ground and helps eliminate certain type of hum. Could reduce the value to .01 mf. A smaller cap reduces the amount of possible shock.

Some rewire the switch so it's in the hot side of the line. That way the chassis is only hot with a specific polarity of the line cord. Problem is hot side is near the volume control which can pick up hum. Also can't be sure of polarity of the line cord or even a house being wired correctly.

In your case switch is on a clock rather than volume control. You could switch the hot side rather than ground if the clock switch allows this change.

Norm it would be simply a matter of polarizing the plug. As it exists now the plug is non-polarized so whether the hot line is switched or not is a flip of the coin depending on how the cord is plugged in. I'm figuring a Y2 cap from line B to ground and a polarized plug to force line A (switched) to be the hot line. That sound about right? Not a perfect solution but acceptable?

Larry

Larry

It would take care of a big percentage of the outlets but some are wired in reverse.

There is no perfect solution. If you put the switch in the "hot" lead, that's fine, except in the many, many radios that have the power switch on the back of the volume control-- the audio will pick up a lot of hum from the nearby hot wires. That's why they usually put the switch in the ground line. Then the audio doesn't pick up any high AC voltage, except in the second or so after you turn the power off, and that's not so noticeable.

In a clock radio where the switch is usually in the clock, AC pickup is not going to be a major problem, so go for it.

Why not wire in a small isolation transformer. Would totally eliminate any chance of potential shock from touching the chassis.

Needs to be said weekly. Nobody knows if your wall outlets are wired correctly. Assume nothing.

In my opinion this is overemphasized. I have built 5 homes, as GC, for myself and 2 more for my sister and son over the years. Not a single outlet in all those houses was found to be wired backwards, and I tested them all. I have tested for numerous relatives and friends and again, have never found a single outlet wired backwards. I'm sure it happens but it certainly isn't epidemic.

Larry

Do you want to eliminate the chance of shock or reduce the chance of shock ?

The real protection against shock for a radio in household use isn't the radio wiring or house wiring.
It's the use of non conductive cabinet and knobs. It's ensuring no screws are exposed.
It's ensuring the chassis can't be touched while installed in the radio.

Glue the knobs to the shafts.
This will better reduce the chance of shock then rewiring the radio and/or not checking the outlet.

Larry, it sound like you're thinking "new" construction, and to be sure, such occurances would be purely accidental and not too common.

But there are a heck of a lot of old houses out there with really out-of-code and out-of-date wiring. I worked for many years for a heating/AC contractor. Much or our work was in the "old" part of town. These homes were originally furnished with knob-and-tube wiring where the hot and neutral wires did not travel together in neat pairs. The hot wire would snake up through the walls across the ceiling, down to switches, fixtures or outlets. Meanwhile the neutral circuit could travel by an entirely different route to the fixtures or outlets. The wires arriving at the outlets were identical in appearance, there was no color coding. But it didn't matter because the outlets were not polarized anyway. One side of the outlet was identical to the other. Wire junctions in these systems were soldered and taped (wire nuts not invented yet) and not enclosed in a box.

Over the years, a succession of home owners and handymen come along and make repairs, additions, and replacements to the system, resulting an incredible mixture of wiring styles. You find knob-and-tube mixed in with paper covered Romex, armored BX, ungrounded Etcoflex and lord only knows what else Old non-polarized outlets get replaced with polarized recepticals with complete disregard to polarity. Grounded recepticals get installed even though there is no ground wire, because those are the only ones the local hardware store carries.

I've found these kinds of Frankenstein systems are very common in old neighborhoods and may be the ones most people are mainly thinking of when they warn against assuming an outlet's polarity is correct.

Yeah, been remodeling houses in Texas for the last 35 years and have run across reverse polarity two times or so. But, a do-it-yourselfer without the motivation for research could leave that little surprise for you after he moved on down the road.

We're getting off track here. Niether you nor I can control what a future owner's wiring may or may not be. All we can do is to rewire or repair these radios according to code and best safety practice. That means if we replace a power cord with a polarized one the best we can do is assume the outlets are wired correctly and install the power cord accordingly. Constantly bringing up that an outlet may be wired backwards at some other location and/or at some other time in the future is irrelevant because we have no control over it. Our only control is how the radio itself is wired.

I have seen a number of people say "Never assume the outlet is wired correctly.", OK, what's the alternative? Assume it isn't and deliberately wire the cord backwards? There are only two wires so there are only two ways to connect it so I ask again, what's the alternative?

Larry