UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Field Telephone Type F Mk II

russell_w_b · 6th Sep 2009, 3:09 pm

Interesting when 9V applied directly across the mic! Currents and derived resistances below in italic.

Ericsson round-back N7735A
22mA at 5.3V = 241 Ohms.
200mA at 9V = 45 Ohms.
Red-front BPO Inset No:13
20mA at 3.4V = 170 Ohms.
100mA at 9V = 90 Ohms.
Black round-back MOD field telephone No:9 (like yours, probably)
32mA at 1.1V = 34 Ohms.
210mA at 9V = 42 Ohms.
Red front Ericsson modern N7752
18mA at 3.4V = 189 Ohms.
60mA at 9V = 150 Ohms.
'New' BPO type, as fitted to older 706s, stud on spring disc
20mA at 3V = 150 Ohms.
70mA at 9V = 129 Ohms.

Depends on position, I suppose, as the resistance changes with the granule movement. The book ('The Telephone Handbook', Poole) says '... between 30 and 80 Ohms...'

6th Sep 2009, 3:09 pm	#6
russell_w_b Dekatron Join Date: Jul 2007 Location: Penrith, Cumbria, UK. Posts: 3,687	Re: Field Telephone Type F Mk II - Help needed Interesting when 9V applied directly across the mic! Currents and derived resistances below in italic. Ericsson round-back N7735A 22mA at 5.3V = 241 Ohms. 200mA at 9V = 45 Ohms. Red-front BPO Inset No:13 20mA at 3.4V = 170 Ohms. 100mA at 9V = 90 Ohms. Black round-back MOD field telephone No:9 (like yours, probably) 32mA at 1.1V = 34 Ohms. 210mA at 9V = 42 Ohms. Red front Ericsson modern N7752 18mA at 3.4V = 189 Ohms. 60mA at 9V = 150 Ohms. 'New' BPO type, as fitted to older 706s, stud on spring disc 20mA at 3V = 150 Ohms. 70mA at 9V = 129 Ohms. Depends on position, I suppose, as the resistance changes with the granule movement. The book ('The Telephone Handbook', Poole) says '... between 30 and 80 Ohms...' __________________ Regds, Russell W. B. G4YLI.