02-11-2014, 09:40 PM
If you are replacing with a PM,
1. It not fully correct: 20, 70 and 90 all have chokes in the filters that are separate from the field coil.
2. Simply putting a resistor instead of field coil is just fine.
3. Yes you also could find an additional choke to put instead of the field coil. Try to match the resistance to what it is (in all three radios it is about 3.2kOhm). If using choke, add the resistor equal to the difference of 3.2kOhm and the choke's DC resistance.
4. Transformer should match your impedance to about 7kOhm. The voice coil is 0.7 Ohm and the ratio of the transformer in all three radios is 100, so the impedance is transfered as 10,000 times the voice coil impedance.
A 125C Hammond transformer is a universal taht works fine.
Between 6.5K and 8K is just fine.
5. You need a SE transformer as this is what 70 (and some 90s) use.
1. It not fully correct: 20, 70 and 90 all have chokes in the filters that are separate from the field coil.
2. Simply putting a resistor instead of field coil is just fine.
3. Yes you also could find an additional choke to put instead of the field coil. Try to match the resistance to what it is (in all three radios it is about 3.2kOhm). If using choke, add the resistor equal to the difference of 3.2kOhm and the choke's DC resistance.
4. Transformer should match your impedance to about 7kOhm. The voice coil is 0.7 Ohm and the ratio of the transformer in all three radios is 100, so the impedance is transfered as 10,000 times the voice coil impedance.
A 125C Hammond transformer is a universal taht works fine.
Between 6.5K and 8K is just fine.
5. You need a SE transformer as this is what 70 (and some 90s) use.
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