I have an old multiband transistor radio I got at a thrift store a few years ago. It is a Unisonic brand, model #6222. Fourteen transistors, wood case, made in Japan, AM MB SW PSB FM.

It has some thermal noise in the audio circuit which it's had since I got the radio (I think it probably needs new electrolytics, although it looks like it has germanium transistors in the audio stage, perhaps they're causing the noise), but that's not what my question is about.

I just noticed that the radio has a sticker on the inside of the back cover that says 'Do Not remove Batteries, as this is special circuit floating AC DC."

What are they talking about? What is 'floating AC DC?' Why would it make a difference if I ran it on AC only with no batteries in it?

Most likely is a radio designed for a rechargeable battery. When plugged into AC, it floats a charge on the batteries. The batteries also serve as a filter (just like an electrolytic cap). If you run it on AC with no batteries, you'd have no filter. On DC it runs purely off the batteries.

We would need to see a schematic to be sure, but the explanation above is one plausible explanation. I think I have also seen devices that need (primary) batteries installed even with AC power.

Also, keep in mind that --- for many early devices made in Asia---the instructions often involved some "creative" usage of English.

not sure what floating AC is but floating DC is when you chuck your old batteries in the river. if they float they were still good

Obviously the set needs to be filled with water, to "float the batteries"...

Unfortunately, I have no schematic. But from looking at the radio with the back off, it looks similar to other Japanese multibands with a battery case. The case that came with it is a C cell holder that holds 4 batteries, and the plastic on it looks old enough to be original.

There is an AC-DC switch on the front of the radio that is a three pole slide switch. The plus side from the battery is directly connected underneath the PCB somewhere (I haven't taken the radio apart yet). The AC-DC switch seems to switch the ground side of the battery pack in and out of the radio's circuits, i.e., when the switch is on AC, the battery pack's ground wire isn't connected to the circuitry at all. The plus wire of the battery, of course, is still connected because it's soldered to the board somewhere.

No, no, NO! I think the idea is to throw it in the lake. If it floats, it's a witch. Or something like that...

Brett

I recall that back in the '70's chargers were being sold to "freshen up" regular carbon-zinc batteries. Maybe the radio puts a very slight trickle-charge on the batteries when using AC.

Dave

I have an AC/DC multi band radio with the same message about the "floating AC DC" label. From what I've been able to trace out, when the AC is on, the DC voltage is also across the batteries. With the batteries out and the AC is used, I read ~7 volts across the battery terminals. So , it looks like it had a "charging " aspect when in AC. Of course after I found this out, I want the batteries to be isolated from the voltage when used on AC. I don't use rechargeable batteries and regular carbon or alkaline can be damaged from charging. So, anyway, I'm putting a schottky diode in series with the battery to isolate the batteries.

One would think that any trickle charge the circuit employed would not harm whatever type of battery was used, unless they state, "use rechargeable batteries only".

Have you measured the voltage across the terminals both with the batteries in and the AC unplugged, and then batteries still in and AC plugged in? I would imagine the difference to be small.

Overmore, most probably the power source AC , are not regulate but only some kind of battery charguer.
If you take of the battery , unregulated high voltage go to receiver.

Of course they are good if they float down the river.

Seriously I have always heard of a floating charge on batteries, NiCads and Car batteries.