Johnson Viking II PTT relay

[W7SOE]

I am interested in using a 24V relay for the PTT circuit in this rig.  I have the schematic (attached) for the factory PTT relay circuit.   Hmm, it looks like it draws off the segment of the transformer that provides the -75V supply.  It appears that the current is limited by a 3300 ohm resistor feeding a "10k ohm" relay coil. 

That 10k ohm rating seems like a high value, the coil on my 24V relay ohms out at 75.  Could it be as simple as uping the value of the series resistor in order to use this relay?

What is the purpose of C2, C3, and R2 at the mic switch, perhaps to prevent arcing?

73

Rich

[Rob K2CU]

The 10K Ohm coil resistance relay is of the type intended to be in the plate circuit of a vacuum tube realy driver. They were nominally a 48 VDC relay. A four pole double throw 48 VDC relay is used in the Drake T4X and TR4 radios. That part is still available at a number of outlets (Newark, Mouser) and is Potter/Tyco Part Number R10-E1Y4-V2.5K. AS the part number suggests, it is a 2500 Ohm coil. You could try to use on here, but you will need to drop the 3300 Ohm resistor down to 750 Ohms @ 1.2 W. There are some other  relays you could look for, but try for 48 VDC coil types with over 2K ohms.  Will draw on the order of 200 mA.

The two caps and resistor are probably for arc supression as these high winding coils have enough inductance to give quite a kick.

You could also consider using a low power 12 VDC type and power it from a voltage doubling type rectifier circuit on the 6.3 VAC line.  At  1/4 the voltage figure roughly four times the current.