Hi all

I can't find anything out there on the net on this one. No schematic or nothing. I'd guess it's circa mid 70's based on band selection: AM, marine, SW 4 - 12 Mz, FM, Air/PSB (weather).

The question is: Inside on the specs sticker is this cryptic note:

KEEP BATTERIES INSERTED WHILE LISTENING ON AC

Care to hazard a guess where they are going with that one?

There is no manual battery/AC switch on the unit. Is this a matter of batteries completing a circuit, and if not in place performance suffers? Or is it something more serious, like lack of batteries somehow screws with the auto AC/battery switching.

There is no indication this model was designed for rechargeable batteries. Uses 4 "D" cells.

I just don't want to trash it and that note inside sounds rather stern.

This is interesting. With AC applied the set it putting 7.68 volts across the battery terminals. Is that pretty standard for a mid 70's radio as a charging circuit for carbon cell batteries? Not sure I would want to expose Alkalines to that.

Are you sure that the set doesn't take six C size batteries instead of 4 D's? If the set was designed to operate with six C-sized ni-cads, that would give a battery voltage of 7.2 V. The voltage that you measured (7.68 V) would be enough to tricklecharge the nicads without cooking them.

You might plug the set in, turn it OFF, and measure the voltage at the battery terminals. If it is zero, the set wasn't designed to operate with rechargeable cells. If you still have a voltage, the set was supposed to be able to tricklecharge the batteries anytime it was plugged in- so it was designed to be used with nicads.

The only dual-power item that I ever owned which bore a "do not operate from AC adapter without batteries installed" notice was a Texas Instruments pocket calculator (circa 1974) which had a nicad battery pack (in AA cell format). I think that the reason was that, without the battery in place to serve as a fixed load, the unregulated "wall wart" type power supply could float up to a voltage that could have been damaging to the integrated circuits.

:Are you sure that the set doesn't take six C size batteries instead of 4 D's? If the set was designed to operate with six C-sized ni-cads, that would give a battery voltage of 7.2 V. The voltage that you measured (7.68 V) would be enough to tricklecharge the nicads without cooking them.
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:You might plug the set in, turn it OFF, and measure the voltage at the battery terminals. If it is zero, the set wasn't designed to operate with rechargeable cells. If you still have a voltage, the set was supposed to be able to tricklecharge the batteries anytime it was plugged in- so it was designed to be used with nicads.
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:The only dual-power item that I ever owned which bore a "do not operate from AC adapter without batteries installed" notice was a Texas Instruments pocket calculator (circa 1974) which had a nicad battery pack (in AA cell format). I think that the reason was that, without the battery in place to serve as a fixed load, the unregulated "wall wart" type power supply could float up to a voltage that could have been damaging to the integrated circuits.
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Definitely four D cells - UM-1 to be specific according the the plate inside the radio. No mention of rechargeables. It's a standard battery box for standard cells. (In other words not designed for a proprietary battery pack.)

When the radio is off there is no voltage across the battery terminals. If I try to get an mA measurement across the terminals when power is applied the radio "shuts down" - volume goes to nothing.

Wish I could find a schematic on this one. Preliminary look-see shows it will be a real bear to get apart. The power supply is buried underneath the plastic AC cord & battery compartment.

You could install a zener diode across the battery pack terminals- that would clamp the power supply much more effectively than dry cells (dry cells will load, but not clamp, a higher voltage source). Measure how much current the radio is pulling and choose the zener's power rating based on that and the voltage differential of the battery pack and the AC supply (nominally 1.68V, I think, based on your previous comment).

Since a new battery pack will have a slightly higher voltage than 6V, you need to select a zener clamp voltage that is higher than the highest voltage a new battery set can put out. This probably differs between carbon-zinc and alkaline cells. Since the battery set can output a far higher current than the AC adapter, it is important to correctly "voltage size" the zener to avoid (1) burning it out and (2) unnecessarily draining a new battery pack. Even if you have to go up to 7V on the zener, that should still be low enough to prevent AC adapter voltage surges from damaging the set's electronics.

Or, you could take the "low road" and just keep a set of batteries installed for whenever you use the set on AC.