ICOM IC-M21 Mystery part - UK Vintage Radio Repair and Restoration Discussion Forum

newlite4 · 24th Nov 2022, 9:31 pm

Just starting to rebuild the Ni-Cad pack on this one. Under the batteries is this component which is in series with the batteries. Difficult to measure the resistance, it looks as though it has overheated, but it reads around 1Mohm. It is marked 32102 with BBS75B2N-03 on the second line. I wondered if it was some sort of protection device and if it was strictly necessary, there is already a 1NK14 diode between the positive pack terminal and the first battery.
Neil

SiriusHardware · 24th Nov 2022, 10:21 pm

It's almost certainly a one-shot thermal fuse which, if it is, did exactly what it was designed to do (went open circuit / high resistance when the batteries got too hot).

newlite4 · 25th Nov 2022, 10:23 am

Thanks for your input. That makes a lot of sense. Possibly blew open as a result of an accidental short circuit of the battery terminals.
Neil

Herald1360 · 25th Nov 2022, 10:41 am

Wouldn't any fuse be downstream of the battery terminals and unaffected by a direct terminal short? (Unless it were of sufficient duration to heat the battery up significantly perhaps?)

SiriusHardware · 25th Nov 2022, 12:14 pm

If it's a thermal fuse as I think it is then it would go open circuit as a result of the battery temperature going above a certain threshold, howsoever caused.

24th Nov 2022, 9:31 pm	#1
newlite4 Octode Join Date: Nov 2003 Location: Bath, Somerset, UK. Posts: 1,805	ICOM IC-M21 Mystery part Just starting to rebuild the Ni-Cad pack on this one. Under the batteries is this component which is in series with the batteries. Difficult to measure the resistance, it looks as though it has overheated, but it reads around 1Mohm. It is marked 32102 with BBS75B2N-03 on the second line. I wondered if it was some sort of protection device and if it was strictly necessary, there is already a 1NK14 diode between the positive pack terminal and the first battery. Neil Attached Thumbnails __________________ preserving the recent past, for the distant future.

24th Nov 2022, 10:21 pm	#2
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,554	Re: ICOM IC-M21 Mystery part It's almost certainly a one-shot thermal fuse which, if it is, did exactly what it was designed to do (went open circuit / high resistance when the batteries got too hot).

25th Nov 2022, 10:23 am	#3
newlite4 Octode Join Date: Nov 2003 Location: Bath, Somerset, UK. Posts: 1,805	Re: ICOM IC-M21 Mystery part Thanks for your input. That makes a lot of sense. Possibly blew open as a result of an accidental short circuit of the battery terminals. Neil __________________ preserving the recent past, for the distant future.

25th Nov 2022, 10:41 am	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,535	Re: ICOM IC-M21 Mystery part Wouldn't any fuse be downstream of the battery terminals and unaffected by a direct terminal short? (Unless it were of sufficient duration to heat the battery up significantly perhaps?) __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

25th Nov 2022, 12:14 pm	#5
SiriusHardware Dekatron Join Date: Aug 2011 Location: Newcastle, Tyne and Wear, UK. Posts: 11,554	Re: ICOM IC-M21 Mystery part If it's a thermal fuse as I think it is then it would go open circuit as a result of the battery temperature going above a certain threshold, howsoever caused.