[R-390] R390 troubleshooting - I'm stuck, 3/8A fuse pops

[Roger Ruszkowski]

Frank,
 
I vote for a cap that leaks at B+.
 
Likely a tube may have developed an internal short.
Back when the receivers were young it was almost always a tube.
Now its equal chance for a leaky cap.

R390's are bit more troublesome to trouble shoot by just unplugging
tubes and modules. The filaments come in 24 volt strings. and some of the
strings cross between modules. 

But unplug all the tubes and run them through a tester for shorts if you can.

Plug in the 26Z5 if not solid state, the 6082's and V607 DC amp and the voltage regulator 
tubes. Then you have to plug in the other tubes in the V607 filament string V604, V603, andV509.

Then you can turn on the receiver and see if the B+ fuse holds. If not go looking for a leaky cap.

Then you can plug the tubes in one filament string set at a time.
Until you find the string that blows the fuse.

You hope to find a shorted tube with the tube tester and resolve this problem easily.

Eye balling caps in the modules may work. But this is an R390 not an R390/A
so you do not expect to be looking at black / brown beauties known to fail.

There are two nice large oil caps in the R390 that may need checking.

My R390 schematic does not show the B+ fuse so I do not know if it comes before or after the caps.

My schematic show a couple resistors across the filaments of V507 and V510 to imply that 1/2 the 12AU7's
are running at less than full power. This could be true. 

There can be frayed wires in harness and plugs. You may have to open some of the harness back
shells to look for a broken wire.

Down load Tm 11-5820-357-35 from the R390.net site and read through the resistance checks for the
wire harness if tube testing does not solve the problem before just opening up wire harness back shells.

Roger AI4NI