Old Stereo Cabinet shocking people

[tech2014]

I got a very old cabinet style stereo. The customer says that they get shocked when using the radio. I took some pics of the unit and power cord. Would the shock come from a bad ground or possibly the power cord is bad. Any ideas?

[budm]

Re: Old Stereo Cabinet shocking people

Only 2-prong non-polarized cord so there is no safety GND on the chassis.
Can we see better pictures of the chassis?
There should be a paper sticker that gives you the tubes layout and full chassis model on the console some where. It has been since the early 70's that I worked on one of these consoles.

[tech2014]

Re: Old Stereo Cabinet shocking people

I don;t see any stickers on the inside or on the chassis. Here are some more pics of the inside. I did measure some AC on the metal pieces on the radio side, around 1.2VAC. Some kind of AC leakage??

[budm]

Re: Old Stereo Cabinet shocking people

It may have one of those paper cap connected between the AC and the chassis which they do fail badly.
You do have some kind of AC leakage current. You will also see difference reading if you reverse the plug when you plug it into the AC socket.

[tech2014]

Re: Old Stereo Cabinet shocking people

Would it be possible to build a 3 prong modern power cord for this cabinet, and would this fix the AC leakage and shocking problem.

[budm]

Re: Old Stereo Cabinet shocking people

Safety test.

[budm]

Re: Old Stereo Cabinet shocking people

Would it be possible to build a 3 prong modern power cord for this cabinet, and would this fix the AC leakage and shocking problem.

You need to check to see if the incoming AC wires has capacitor/s connected to one them and the chassis, or the power transformer may be bad.

[SteveNielsen]

Re: Old Stereo Cabinet shocking people

Yes and a 50/50 chance that the AC hot side is tied to the chassis depending on which way the 2 wire cord is plugged in.

[tech2014]

Re: Old Stereo Cabinet shocking people

thanks, i will do the safety check tomorrow and see if capacitors are connected.

[SteveNielsen]

Re: Old Stereo Cabinet shocking people

If you're reading 1.2vac from chassis to actual ground at an outlet then that's about what you'd see from neutral to ground due to the house wiring resistance (neutral and ground are bonded at the panel). That is of no concern and probably not causing the shocks. It doesn't indicate leakage or a bad transformer either.

That's a EZ81/6CA4 dual rectifier tube with a 6v heater next to the AC power transformer. That tells me that the the power transformer has a primary winding for 117v and at least two secondaries, one at 6.3v for tube heaters and one somewhere between probably 90v and 150v for the B+ voltage. But the primary winding is we want to deal with.

Each side of the transformer primary is tied to a side of the ac supply, one via the power switch on the supposed "hot" side. One is also connected to the metal chassis, that's "supposed" to be neutral when the 2 wire cord is plugged in the "right" way. To wire a three wire cord with safety ground cord to it, disconnect the side of the transformer primary from the chassis and wire that lead to the neutral wire (white) in the cord, wire the other primary wire to the hot wire (black) in the cord via the switch, then connect the green wire of the cord to the chassis. Of course you'll have to pull the chassis out of the cabinet to do this. You're also gonna want to replace those old power supply filter caps, they are well beyond their life expectancy.

[budm]

Re: Old Stereo Cabinet shocking people

I have never seen these consoles with power transformer has the neutral wired to the chassis especially when it is uses non polarized plug.
You can easily verify that by checking the resistance between the chassis and the AC plug.

[SteveNielsen]

Re: Old Stereo Cabinet shocking people

Yeah, you're right. It generally isn't tied directly to chassis but through a capacitor to chassis, which isn't much of a safety measure at all and in some circles called the "death cap". I don't think they've actually caused anyone's death but sure pack a wallop nonetheless.

I was mixing it in my older brain with the old ac/dc 5 tube radios with series strung heaters and no transformer and old 2 tube record players with with the heaters and turntable motor strung in series.

[Longbow]

Re: Old Stereo Cabinet shocking people

You probably do have a shorted or leaky paper cap on the input. But my question is where are you touching when you receive the shock? The radio tuning knobs and face plate all appear to be plastic. Maybe the metal chassis of the turntable is acting like a return.

[budm]

Re: Old Stereo Cabinet shocking people

The turntable is also attached to the chassis.