Wiring up an old radiogram

August 14, 2009 10:25 AM

I found a radiogram on the street. There are two short wires out the back, both stripped about 1/4" bare, and no plug. With regard to polarity, does it matter how I attach a new lead and plug? I'm in the US and will be plugging it in to 110V.

I did search through previous AskMe posts, but such is the parlous state of my electrical knowledge, I could not come to any firm conclusion.

It's probably a Magnavox Astrosonic, and it's solid state. Here's a couple of photos: 1 2

posted by carter to home & garden (12 comments total)

Polarity shouldn't be a concern - it looks plenty old to have not made a difference. Make sure what you're plugging in IS the electrical cord, and not an antenna lead or RCA-line out.
posted by AzraelBrown at 10:31 AM on August 14

Thanks, AzraelBrown. I think the wires are the power supply. (I should have taken a photo of them too.) There are no antenna leads, although there is a place to plug these in in the back. Also there is no RCA line out, although from googling I think that there are RCA sockets inside on the chassis, accessible by removing the back. - So I can run my iPod on it, provided it works - yay!
posted by carter at 10:38 AM on August 14

I found with numerous pieces of old electronics over the years that there is usually a good reason why the plug has been cut off. Be very wary of sparks, smoke, and deadly shocks when you power it back up. Plug it into a power bar (off), then turn on the power switch on the radiogram, and then turn on the power bar. This will allow you to kill the power if needed from a somewhat safer place.
posted by Paid In Full at 10:51 AM on August 14

A lot of these old consoles actually had quite good amplifiers in them. My guess is that it probably needs some work unless it has been in continuous use and even then.... The capacitors tend to go bad over the years, especially when not used for years. It may also need new tubes. Restoring old tube amplifiers and radios is a hobby practiced by a few people. If you are handy with a soldering iron switching out the caps in one of these is not too difficult. Having the plug cut off is not necessarily a good sign. Usually when I fire up an old amp that has not been used for years I run it through a variac which lets me slowly ramp up the voltage from say 30 to full power over hours. It is said that this can reform some ailing capacitors and if there is any smoking to be seen it occurs at lower voltages and thus causes less damage. If you plug it in and it works great. If you are going to work on it you will want to read up on safety issues like testing anything you are going to touch even if it has been turned off for days. Some designs omitted bleeder resistors and the caps can hold a charge long after the power is off, and in a tube amplifier there are regularly voltages of 300 to 500 volts - that is a lethal level if one is not careful.
posted by caddis at 11:42 AM on August 14

hmmm. Just reread your post and notice that this is solid state. When you say two wires coming out of the back what do they look like? Anything from the solid state era is going to have wires easily recognizable as a power cord, even if the plug is removed. As for how to wire the plug, it would be best to wire it with the neutral in the right place. Otherwise you may be putting live voltage on the parts of the amp you can touch. That is why modern plugs are polarized. Usually the neutral side of the power cord will have a ridge or some other type of embossment to indicate that it is neutral. Connect this to the larger blade of a two prong plug. Even better would be to see if either of the wires references to ground in the amp. That should be wired to the neutral plug.
posted by caddis at 12:03 PM on August 14

Seconding the notion that polarity is very important. A lot of old electronics had keyed plugs (one blade wider than the other) and used the neutral from the mains as a local ground, connected directly to chassis metal. Check with an ohmmeter or continuity tester if one of the wires connect to any chassis metal or the turntable center post.
Also you should be able to tell if these wires are for AC power by taking the back off the box and seeing where they go. If it's directly to a transformer (probably quite a large one, too) then that's your AC in.
posted by rocket88 at 12:33 PM on August 14

Almost definitely the wires you have are the power cord. The only other possibility I can think of would be wires for an external set of speakers. But, that sort of defeats the whole all-in-one intent of these old consoles. I vote "power cord."

I suspect the original plug was an old-school, two-blade, non-grounded, non-polarized plug. Both blades were the same size. That's just the way it was back then. Heck, my late-70's Pioneer receiver has just such a plug. So, polarity really isn't an issue with old equipment like this.
posted by Thorzdad at 12:51 PM on August 14

Great, thanks everyone! As far as I recall, the wires are identical, and cruddy. No ridges, etc. I estimate that this is a mid-1960s unit - i.e. about 45 years old? Unfortunately the radiogram is near where I found it, and not where I am at the moment, and so I can't check further.

I think Plan A (so far) is to take the back off, and try and trace the wiring, and see if anything goes to ground ... followed by Plan B (Paid In Full, thanks!) which is to set it up on an extension with a trip, switch everything on, and see what happens.

Hopefully the polarity will not be an issue ... it was free anyway so nothing to lose I reckon.

Thanks again everyone.
posted by carter at 2:15 PM on August 14

carter writes "As far as I recall, the wires are identical, and cruddy. No ridges, etc."

Sometimes where the polarized wires are stuck together one wire will have a square edge and the other will be round. Usually not as pronounced in the drawing though. Other times the conductors are different colours; silver or tin and copper or brass.

Polarization has been around since at least the early 60s when the NEMA 5-15 standard was mandated for outlets. It wasn't on many devices initially because houses had incompatible sockets though.
posted by Mitheral at 3:12 PM on August 14

Even if the device was not initially designed for a polarized plug you can often get benefit from wiring it as such in the afterfact. When I refurbish an old tube amp I always put on a three prong plug and make sure the amp is properly grounded. I learned the hard way with a guitar amp that had something like a forty volt bias on the knobs. It might have been plugged into an outlet with a floating neutral, but that was still dangerous. With the rewiring and proper ground there have been no future problems. It can be as simple as wiring the ground to the chassis, but sometimes you need cap or a snubber to silence some noise. Just wiring the neutral prong correctly gets you most of the way there.
posted by caddis at 8:03 PM on August 14

Update: I went back today and took the back off. On closer inspection, one of the wires does appear to have a small ridge on it. I'm assuming that this is the neutral, and the other is the live. Following the wires inside, it looks like the wire with the ridge (neutral) is connected to a black wire, that is then connected directly to the circuit board. The plain wire (live) is connected to a blue wire, that is connected to a rotary volume switch that functions as the on-off switch (i.e. turn it all the way anticlockwise and it clicks 'off'). I'm assuming that this is the live.

I'll go back tomorrow and attach a new power cord.

FYI - the inside is pretty empty (photo). The cabinet is vertically divided into two unequal halves, each of which has a woofer and a tweeter. There's a canted circuit board mounted in the center. I found a model number - 1P3303 - and a manufacturing date - October 1967.

No indication of how an aux line might be attached, though. I'll have to think about that more.
posted by carter at 8:11 PM on August 15

It works! Now I guess there's a lot more issues to deal with ... but it turns on and makes noise, so that's a good start. Thanks very much everyone!
posted by carter at 9:09 AM on August 16