[R-390] The Penultimate R-390* Ballast Replacement

[ToddRoberts2001 at aol.com]

Most of the 3TF7 substitutes I have read about involve buildng something 
small enough to plug directly into the 3TF7 socket or on a nearby bracket. This 
comes with problems of heat dissipation or else installing unsightly brackets 
near the I.F. subchassis. Some of the recent ideas got me thinking - Why not 
build a small separate power supply module with a 
well-regulated/filtered/bypassed 12.6 VDC output that could be placed next to the receiver and the only 
connection would be a small umbilical cable with a 9-pin plug that plugs directly 
into the 3TF7 socket? You could run the umbilical through the side of the R390A 
chassis thru one of the large holes and tuck the power supply and cord out of 
the way next to the receiver. This layout is similar to the way some 
audiophile preamps use a separate power-supply module with an umbilical. You could 
modify one or two pins of the 3TF7 socket with a jumper to ground to provide a 
ground return for the 12.6VDC so you wouldn't have to tie down a separate ground 
lead with a terminal and screw somewhere else on the chassis. When you plug 
in the umbilical it breaks the 25.2 VAC circuit and connects the 12.6VDC 
circuit and ground. The ground pins would have no effect on the original 3TF7 if you 
wanted to plug one back in.  This way you could build a nice little husky 
separate  regulated/filtered/bypassed 12.6 VDC power supply and not have to 
miniaturize it or compromize the performance. If you want to go back to a 3TF7 just 
unplug the DC supply 9 pin plug and put the 3TF7 back in. No unsightly 
permanent wires or brackets hanging off the I.F. subchassis.  The regulated DC 
supply should give the ultimate in stability and pure DC on the filaments of the 
BFO/PTO tubes. 73 Todd WD4NGG