adjusting the doubler knob
I have been playing with B-610C & D models for about thirty years. The double knob tunes straight through on 75 meters. You need to use a 3.645 crystal for it to work on 40 meters and of course you would need the proper tuning unit in it to double. I just set my switch for PA grid and tune for the max grid current. I get over 100 mills of drive on 160 & 75 but it starts to drop off on 40 and above. Another way to get more grid drive is to replace the tube 5Z3 with a solid state full wave rectifiers on the RF deck. That's where the PS is located for the PA grid etc. You can do that with an old tube socket by merely installing the proper diodes in it.
Different tuning units will give different outputs as gained from my experience. Try it in the VFO position and see how much grid drive you can get to the PA. Maybe your crystal output is low or something? Also try moving the tuning unit to another slot. There could be a dirty position on the selector switch which can be cleaned up using a spray cleaner. Do this with the power off and let dry or you could risk a fire. The grid bias resistor may be open. I recall it's about 1000 ohms at 25 watts and is located under the chassis. with the rig on the grid bias voltage at the 250TH should be around -160 volts. You do not need to do anything other than have the transmitter on to make this measurement. Of course you want all power to be off when you hook up the meter leads. One last thing,
ALWAYS short out the tank circuit before you change coils. If the bleeder resistor from the HV supply becomes open, you get a 2000-2500 volt shock. It could kill you. That's why they call the 610 the Widow maker.
