6K8 Sensitivity
 

In article ,
Straydog wrote:


On Tue, 25 Apr 2006, Rod wrote:

Hello, I'm restoring a Hallicrafters S-20R. I've managed to bring it
back from the grave by installing new caps, a few new resistors and
replacing a bad tube. Although its back to life, it still has some
issues, one I hope someone here can help with. When I touch the cap of
the 6K8 tube with my finger, the audio becomes much louder. Or the
sensitivity increases.

I don't recognize what the 6K8 is. Audio first stage (diode-triode)? or
Mixer? A fair number of some of these tubes have, as the cap, the
connection to the control grid. If its an audio tube, then you should be
introducing more 60 cycle hum than anything else. When you say "the audio
becomes much louder" are you talking about the audio on the signal you are
listening to or background static or hum or what. I'm not at all familiar
with Hallicrafters before about S-38, S-40 vintage. Give us all some more
clues, and tube listing. I've read all the other responses up to the
moment of writing this posting; most are worth checking out.

If the cap is the control grid of the mixer, then just maybe you are
injecting--by capacitance pickup with your body (as antenna)--more
broadcast RF into the circuit than is coming in normally through whatever
else the input RF goes into (RF amplifier, coils, caps, whatever).

It is, indeed, the common grid connection for the triode-hexode mixer,
and I would agree that it could be RF pickup that's giving him the
extra audio output. Definitely points the finger at the RF stages
being out of alignment or otherwise faulty.

In my
experience, lots of times you can touch the antenna terminal of a receiver
and pick up a lot of signal that would not be there if that terminal were
not connected to anything.

Time for a signal generator, I suspect, and to check backwards from the
6K8 to the aerial terminal. If he's getting audio out then the IF,
detector, and audio stages are probably OK....

....oh, and check the RF stage tuning capacitor for bent/shorted plates.

Chris.

--

... ... File not found, I'll load something *I* think is interesting.

6K8 Sensitivity
 

On 4/26/06 1:31 PM, in article , "Chris
Suslowicz" wrote:

In article ,
Straydog wrote:


On Tue, 25 Apr 2006, Rod wrote:

Hello, I'm restoring a Hallicrafters S-20R. I've managed to bring it
back from the grave by installing new caps, a few new resistors and
replacing a bad tube. Although its back to life, it still has some
issues, one I hope someone here can help with. When I touch the cap of
the 6K8 tube with my finger, the audio becomes much louder. Or the
sensitivity increases.

I don't recognize what the 6K8 is. Audio first stage (diode-triode)? or
Mixer? A fair number of some of these tubes have, as the cap, the
connection to the control grid. If its an audio tube, then you should be
introducing more 60 cycle hum than anything else. When you say "the audio
becomes much louder" are you talking about the audio on the signal you are
listening to or background static or hum or what. I'm not at all familiar
with Hallicrafters before about S-38, S-40 vintage. Give us all some more
clues, and tube listing. I've read all the other responses up to the
moment of writing this posting; most are worth checking out.

If the cap is the control grid of the mixer, then just maybe you are
injecting--by capacitance pickup with your body (as antenna)--more
broadcast RF into the circuit than is coming in normally through whatever
else the input RF goes into (RF amplifier, coils, caps, whatever).

It is, indeed, the common grid connection for the triode-hexode mixer,
and I would agree that it could be RF pickup that's giving him the
extra audio output. Definitely points the finger at the RF stages
being out of alignment or otherwise faulty.

In my
experience, lots of times you can touch the antenna terminal of a receiver
and pick up a lot of signal that would not be there if that terminal were
not connected to anything.

Time for a signal generator, I suspect, and to check backwards from the
6K8 to the aerial terminal. If he's getting audio out then the IF,
detector, and audio stages are probably OK....

...oh, and check the RF stage tuning capacitor for bent/shorted plates.

Chris.

I see that rcvr has an RF amp (6sk7). What I would do is do to the plate of
the RF amp, BUT THROUGH A CAPACITOR, what you did to the 6k8 grid, and don't
be touching the chassis while you do it. This will give you a clue about
whether there is an interstage problem. Maybe the problem is at the RF amp.
Also, measure the AGC voltage on the 6sk7 control grid. If you see a
voltage it will mean the coil(s) aren't open.

The schematic I downloaded is so bad I can't read the part numbers.
However, there is a capacitor from the chassis to all the coils at the 6sk7
input (secondarys of the antenna transformers). You might replace it or
just temporarily tack another one across it.

Tell us test equipment you have.

Don

6K8 Sensitivity
 

Well, I gave everything a very close visual check and didn't see any
problems. I cleaned up some suspect solder connections, made sure
there were no shorts or even just some things barely touching each
other and plugged it in. Better, but not there yet. I tapped a few
things here and there and found that the 6SK7 RF amp tube was just not
right. Lots of static when I tapped it with a long piece of plastic.
I replaced it with a tube from my other S-20R and bingo, better than
ever. So I guess visual checks and a bit of detective work solved this
mystery and prevented major surgery. Thanks to all who offered their
suggestions.

6K8 Sensitivity
 

(quoted material below)

I read a couple other guy's comments as well. If there is an RF amplifier
ahead of the converter (converter = mixer+ oscillator), then something is
dead in that stage (coils, variable cap shorted/shorting, dead tube, open
interstage capacitor, and other possibilities). Dead tubes can have blown
filaments (if the receiver is on for a while, an open filament tube will
still be cold compared to the other tubes), or internal short, or gassy
tube, or weak cathode emission, lots of things. Whatever the RF stage is,
if that tube is identical to the IF stages, then swap the tube (RF into IF
socket) and if the whole receiver goes dead, then its that tube that is
dead. Get a new tube. Preferably you should try to locate someone with a
tube tester (this may not be practical in an age where everything is: i)
throw away, and ii) all solid state). All the other things you need to do,
need lots of checking and a schematic. If you can't get one, then you have
to trace it out by hand and use a tube manual to get pinouts on the tubes.
I'll keep reading from time to time; will post if I have any bright ideas.

===== no change to below, included for reference and context =====

On Wed, 26 Apr 2006, Rod wrote:

The 6K8 is the converter oscillator. When I put my finger on the top
to the tube, which has a cap that runs to the tuning mechanism, the
received signal becomes louder. It seems to happen on all bands. The
lower ends of each band are affected the most. I do have about a ten
foot wire connected to the antenna terminal. I also have another S-20R
which functions normally with respect to the received signal.
I hope this is the info you're asking for. Your help very much
appreciated.


Straydog wrote:
On Tue, 25 Apr 2006, Rod wrote:

Hello, I'm restoring a Hallicrafters S-20R. I've managed to bring it
back from the grave by installing new caps, a few new resistors and
replacing a bad tube. Although its back to life, it still has some
issues, one I hope someone here can help with. When I touch the cap of
the 6K8 tube with my finger, the audio becomes much louder. Or the
sensitivity increases.

I don't recognize what the 6K8 is. Audio first stage (diode-triode)? or
Mixer? A fair number of some of these tubes have, as the cap, the
connection to the control grid. If its an audio tube, then you should be
introducing more 60 cycle hum than anything else. When you say "the audio
becomes much louder" are you talking about the audio on the signal you are
listening to or background static or hum or what. I'm not at all familiar
with Hallicrafters before about S-38, S-40 vintage. Give us all some more
clues, and tube listing. I've read all the other responses up to the
moment of writing this posting; most are worth checking out.

If the cap is the control grid of the mixer, then just maybe you are
injecting--by capacitance pickup with your body (as antenna)--more
broadcast RF into the circuit than is coming in normally through whatever
else the input RF goes into (RF amplifier, coils, caps, whatever). In my
experience, lots of times you can touch the antenna terminal of a receiver
and pick up a lot of signal that would not be there if that terminal were
not connected to anything.

Not sure which. But my question is why? What
should I look for in order to have the radio play at the volume without
holding my finger there. It gets very boring. Any help would be
appreciated.