[R-390] R-390 Question

[Tim Shoppa]

David Hallam asks:
> Resistance measurements taken at the same points give
> values sufficiently close to manual values to be considered
> within component tolerance and ability to read values less
> than 10 ohms EXCEPT for reading taken at the pin
>of V504 and V505 (pin 4) that connect to the hot side of the filament
> transformer.  Here the manual calls out 7 ohm and I am reading
> 0.2 ohm on my DVM indicating an almost direct short.  To me this
> indicates a shorted bypass capacitor.  Checking with my

Is it possible that the 0.2 ohms is the DC winding resistance of the
filament transformer?

And if it really is 0.2 ohms from pin 4 to ground, how come that didn't
turn up on your other checks at the hot end of the filament transformer!

> capacitance meter I don't find any
> shorted bypass capacitors.  If the resistance is really
> 0.2 ohm, why aren't I blowing fuses?

0.2 ohms across 25V would give 125 Amps, which I do think
you might notice!

Do you make these measurements with all tubes in sockets? It's possible
that a tube has a cathode at ground and a cathode-to-heater short that
opens up when current is applied. Lord knows that there are plenty of
bad tubes with the OPPOSITTE problem!

Tim.