Bill: Increase the size of the fuse to 1.8 amp. or so. Johnson had problems with that in some transmitters, even the Valiant II. There was a factory notice about it and they first changed it to a slo-blo fuse which fixed most problems. That rating is right on the ragged edge and should have been 1.5 times the actual current draw. You might want to measure it to see what the supply is drawing to be sure it is proper, meaning less than about 1.5 amps or so. Put an ammeter in the variac setup to measure it.
The manual specs the modulator at 50-70 mils. Use the meter on the transmitter and set the current in that range with the meter set in the MOD position. The later manual specs the RF amp. grid bias at -69 volts with respect to ground. That setting will do what you want. Also be sure to set the clamp pot after you get it right. Be absolutely sure the meter shunts are tight. They used nichrome wire for the shunts with which will not take solder. They are just crimped tightly then solder is flowed over them to hold them in place. Chances are, over the years heat and current flow has loosened the solder. You can replace them with wire wound resistors of the proper value and wattage rating. The meter has an internal resistance of 20 ohms, as I recall. 73 Jim de W5JO > Could really use some help here from those of you who have had experience with the Johnson Valiant xmtr. Three questions... > > 1) Why does the 1.6 amp slo-blo low voltage transformer primary fuse blow after an hour or so of just sitting there with the filament switch on with no plate voltage applied? The 300 vdc and -250 vdc bias circuits are reading as they should when measured. Transformer and filter choke case temperature are warm to the touch but not "HOT". Line voltage is @ 110vac thru a Variac. > > 2) What is the correct setting for the voltage at the modulator grids (pin 5)? Page 9 of the ops manual says "-55 volts", page 31, socket voltage chart says "-46 volts", and the assembly manual eratta sheet says "-35 volts". > > 3) What is the correct voltage for the RF amp bias? Page 8 of the ops manual & the socket voltage chart says "-70 volts", service bulletin #1 says "-52 volts" and the assembly manual errata sheet says "-85 volts". > > Any help here would be greatly appreciated. Thanks, Bill, [EMAIL PROTECTED]