Quote:
Originally Posted by dagskarlsen
The battery voltage should be 3V, but it may be ok with 4.5. If the current reaches to high values the granulated carbon my be damged, and you will need new transmitter capsules.
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I've never had problems with 9V (I've probably been very lucky here...), but I would keep the voltage down to 4.5V if you can get away with it. My Soviet TAI-43s were designed to run on a single 1.5V cell, but benefit from 4.5V.
The energising current of a British field-telephone mic (similar to, but not the same, as an Inset No:13 that sits with typically 6V across it in a GPO 300 series, say) is over the range 30 -200mA. I've only come across one mic insert with a resistance as low as 38 Ohms - in a field telephone!
Below are the results of the R. W. Barnes 'straw poll' mic insert tests:
Test-telephone: Siemens 366 'Neophone' wall-telephone.
Note that a direct multimeter resistance check is inaccurate! Current is needed to obtain a meaningful result.
- Ericsson round-back N7735A
22mA at 5.3V = 241 Ohms.
- Red-front BPO Inset No:13
20mA at 3.4V = 170 Ohms.
- Black round-back MOD field telephone No:9 (like yours, probably)
32mA at 1.1V = 34 Ohms.
- Red front Ericsson modern N7752
18mA at 3.4V = 189 Ohms.
- 'New' BPO type, as fitted to older 706s, stud on spring disc
20mA at 3V = 150 Ohms.