Far from all resort of mirth, / Save the cricket on the hearth!

August 8, 2007 2:46 AM

Obsolete Technology Filter: Please help me figure out what the problem is with my Roberts longwave radio!

The radio is a circa late 50s/early 60s Roberts long- and medium-wave portable receiver. It runs off two of the rather large, old-style 9-volt batteries ("PP9s" apparently). These were new last time England won the Ashes.

Last week I couldn't get Radio 4 on longwave anymore, though Five Live came in fine. Medium wave was fine. I thought maybe it was the weather and forgot about it. Last night there was nothing on longwave at all, no Five Live, and medium wave still okay. This morning, I switched back to longwave and found that strong medium wave stations were bleeding through to longwave, but still no Radio 4 and no Five Live. The third test vs India is starting tomorrow, so I consider this a matter of urgency :)

I'm tempted to think it's not the battery, but could it be? The radio hasn't been dropped and while portable is rarely moved. Any ideas? I've checked out some online tips but they all seem to require a power meter, which I don't have, and hands-on knowledge of transistors, which I also don't have. Someone who can hand-hold through the process of fixing my baby would be very much appreciated...

posted by methylsalicylate to technology (5 comments total)

I don't know this design, but I do know that the first step in troubleshooting is verifying proper power. Get new batteries, if you can.

Most significant piece of evidence is that the stations are gradually disappearing, meaning that almost all of the radio was working properly. (This includes RF amplifier, local oscillator, mixer, filters, detectors, audio amp, speaker(s)). The fact that these are working means that there is nothing major wrong.

Power, power, power! Get replacement batteries and retest. Almost certain that is all you need to do.
posted by FauxScot at 5:04 AM on August 8, 2007

iḿ bored, so here are some stab in the dark comments...

i imagine the signal received from the aerial is going to be weaker for longwave (longer wavelength means poorer coupling with the aerial). in which case it would need more amplification, have a bigger power drain, and so fail before medium wave when the batteries are dying. but that's only a wild guess.

medium wave leaking through to longwave could be related to poor power too, but that might also be some component failing (something that should have a linear response starting to have a non-linear response, and so introduce modulation with other frequencies).

it might be worth finding a dc power supply and wiring that up to the battery connection, since those big old batteries (you mean the really big ones with springs on top, right?) are probably expensive. however, i suspect that would not work and/or be very noisy, because any cheap dc power supply won't be smoothed (so you'll hear a lot of mains hum). might work well enough to convince you that it is the batteries, though (if you could hear something).
posted by andrew cooke at 11:26 AM on August 8, 2007

I tried replacing the batteries, and the problem persists so that's not it :( o well.

6-volts are the batteries with springs on top, 9-volts have little things that look like snaps.
posted by methylsalicylate at 2:55 AM on August 9, 2007

Radio is fixed now... how I did it here.
posted by methylsalicylate at 3:27 AM on August 10, 2007

ha! just a broken bit of plastic. glad it's working again.
posted by andrew cooke at 2:59 PM on August 13, 2007