[R-390] R-390 Question

[David C. Hallam]

The low value of resistance does show up in the other 2 filament strings
also.  Pin 2 of the ballast tube is supposed to be 6 ohm and pin 5 of V507
is supposed to be 9.5 ohm.  Reading were taken with all tubes in their
sockets.  0.2 ohm might be the resistance of the filament transformer, but
when you take the reading, there are 4 series strings of tube filaments with
some series resistance in a couple of them in parallel with the transformer.
I think that's why the manual gives normal readings in the range of 6 to 9.5
ohm.  Could be an error in the manual though.  That's not unknown.

All 4 points in the IF module where the filament connection is directly to
the hot end of the transformer read 0.2 ohm on a DVM and 0 on a VTVM. As I
stated all reading should be in in the 6 to 9.5 ohm range depending upon
which tube the reading is taken.

Unfortunately my tube caddy is empty of 6BJ6's at the moment so I can't try
substituting tubes.  I just bought some more on line and am waiting for them
to arrive.  My tube tester indicates they are all OK but who trusts a tube
tester anyway?

The reason I got into all of this is that the receiver lost sensitivity and
had no carrier level meter movement.  What is an expected carrier level
meter reading for the calibrator on say 15MHz?  I could just barely hear 15
MHz WWV while on my 75S-3 WWV was coming in way over S9 and I could also
hear WWVH.

David
KC2JD

-----Original Message-----
From: Tim Shoppa [mailto:tshoppa at wmata.com]
Sent: Tuesday, September 05, 2006 1:28 PM
To: r-390 at mailman.qth.net; dhallam at rapidsys.com
Subject: Re: [R-390] R-390 Question


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.