National NC-109 with problems
 

(previously posted to rec.antiques.radio-phono)

My dad recently gave me a National NC-109 bought at a surplus auction.
It works but has a problem. There is a "dead space" at the bottom of
each band. It's totally silent down there, no scratching, no
whistles, nothing. For example, I can't tune regular AM broadcast
below about 650. As the radio warms up the problem becomes worse,
after an hour I can't tune below about 750. The problem is similar in
all the bands, but less severe in the higher ones.

Otherwise the radio works fine, and stations come in where they should
on the dial when I can get them. Thanks to the folks at
rec.antiques.radio-phono I've got a schematic and a manual now, as
well as some pointers to look at the local oscillator. I've not done
anything with tube electronics for decades (since I was a kid), but
I'm looking on this as a project to maybe get back in. Would the
simple, dumb (and expensive) step of replacing all the tubes be at all
useful? Unlike when I was a kid at least I have money now... ;-)

Thanks in advance for any advice about how to go at fixing this
problem,

--arne

arne thormodsen wrote:

My dad recently gave me a National NC-109 bought at a surplus auction.
It works but has a problem. There is a "dead space" at the bottom of
each band. It's totally silent down there, no scratching, no
whistles, nothing. For example, I can't tune regular AM broadcast
below about 650. As the radio warms up the problem becomes worse,
after an hour I can't tune below about 750. The problem is similar in
all the bands, but less severe in the higher ones.

The tuning capacitor is damaged, probably by someone who tried to fiddle
with the plates to adjust the tuning. It shorts out at the bottom of the
dial. I bet a nickel. Not a dollar, but at least a nickel.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

My guess is probably some bent plates on the main tuning capacitor. I
wouldn't bet the farm on it, but at least the barn. It would be really
easy to check by just doing a continuity check between the stator and
the rotor. You might even feel some resistance in the tuning knob when
you hit the noisey section. With a little care you can bend the
offending plates back into position. Have an old RCA AA5 that still
works to this day after my Elmer showed me how to fix it 25 years ago.
Cheers. de W8IJG

Im going to have to second or third the two previous posters. Had
similar problems in the past and it turned out to be slightly bent
plates touching and shorting out at the bottom and can be bent back in
place carefully.

To confirm the diagnosis, you should probably hear a sudden decrease of =
the background noise at the point the capacitor plates touch each other.

Tony, I0JX

"arne thormodsen" wrote in message
...
(previously posted to rec.antiques.radio-phono)


Well...

I've set an ohmmeter across the plates of the tuning cap and found
nothing wrong, even after tapping the sucker hard a couple of time.
It seems that it isn't the problem, although this was suggested by
many folks and was a reasonable place to start looking.

The only other test I've done so far is to measure the B voltages,
which are correct. I'm now checking the values on resistors. If I
can borrow a scope from someone at work I might be able to figure out
more.

Any and all ideas about where to look are still appreciated.

--arne

Did you check each section of the tuning cap? In communications-style
receivers there are at least three separate sections all on a common shaft.

Yep. I checked each one. I was hoping this was the problem, as it
would have been easy to fix. Still digging around.

--arne

"Jerry McCarty" wrote in message
...
Did you check each section of the tuning cap? In
communications-style
receivers there are at least three separate sections all on a common
shaft.

"arne thormodsen" wrote in message
...
Yep. I checked each one. I was hoping this was the problem, as it
would have been easy to fix. Still digging around.

--arne


I found a link at:

http://www.gbronline.com/radioguy/convtr.htm

That contains some general hints for troubleshooting superhet radios.
Although the circuit is different, one thing caught my eye:

"no reception at low end of band" --- "weak convertor tube"

Sound reasonable?

Should I replace the 6BE6?

Which leads to another question, what's a good reputable site to by
tubes from? Also, is there only one sort of 6BE6, or do I need to
specify more? The tubes are the minature type, all glass.

Thanks in advance.

--arne


"Jerry McCarty" wrote in message
...
Did you check each section of the tuning cap? In
communications-style
receivers there are at least three separate sections all on a
common
shaft.