|[AMRadio] Valiant Problems|
w5jo at brightok.net
Thu Jun 10 21:40:00 EDT 2004
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
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.
> 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, ab0zz at arrl.net
More information about the AMRadio mailing list
This page last updated 17 Oct 2017.