Diode used in Roberts RD50 & RD60 mains/battery switching circuit

[Old Sparky]

As far as I can tell, even with a magnifying glass! the type no. is 1N581, but I can find noreference to this device anywhere.  It looks like a 1N400x type, but the forward resistance, meaured with my DMM' diode test mode is only ca 200 ohms instead of the 5 -600ohms a silicon diode normally measures.   The usual type of c/o switch operated by inserting a plug into the DC socket, is employed, but the diode is part of the batt/mains circuit.  Should I replace it with a 4000 series device??

[Murphyv310]

Could be a schottky diode. Not familiar with the set though.

[Mark Hennessy]

A DMM in diode test mode should report the voltage drop in mV, not ohms. From the reported numbers, that does seem to be the case, and I'd agree with Trevor - it does sound like a Schottky type - probably a 1N5817, 8 or 9.

Aside from the low voltage drop, why do you think it's faulty?

I'm guessing it's in series with the power to provide reverse polarity protection, hence a Schottky to minimise losses.

[Old Sparky]

Sorry, Mark, I did mean to say 200mV and 5-600mV (must be getting old!!). You may be right about the function of the diode. It's mounted alongside the DC socket, and seems to be in series with the DC supply. I guess the low reading made me think the diode was faulty - I forgot about Schottky diodes when testing it.

[Old Sparky]

I've re-checked the diode, which, btw is a 1N5817, refitted it. Haven't yet checked the RD60 on mains operation, but it should be OK. Whether I disturbed a DS joint removing & refitting it I'll never know!

[Old Sparky]

I've since found that, after a few seconds on Battery operation the radio 'dies'. This is due to one faulty Duracell 'UltraPower' 'D' cell, which must have delveloped high internal resistance, as it reads very low on the Battery Test Function of an analogue meter.