Re: 6K8 Sensitivity

On 4/26/06 1:31 PM, in article C07597319668681EB@xxxxxxxxxxxx, "Chris
Suslowicz" <chris+news@xxxxxxxxxxxxx> wrote:

In article <Pine.NEB.4.63.0604261440070.6083@xxxxxxxxxxxxxxxx>,
Straydog <asd@xxxxxxxxx> 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



.

Relevant Pages

  • Re: 6K8 Sensitivity
    ... replacing a bad tube. ... Audio first stage? ... A fair number of some of these tubes have, as the cap, the ... If the cap is the control grid of the mixer, ...
    (rec.radio.amateur.boatanchors)
  • Re: 6K8 Sensitivity
    ... When I put my finger on the top ... to the tube, which has a cap that runs to the tuning mechanism, the ... replacing a bad tube. ... Audio first stage? ...
    (rec.radio.amateur.boatanchors)
  • Re: 6K8 Sensitivity
    ... replacing a bad tube. ... 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. ...
    (rec.radio.amateur.boatanchors)
  • Re: Hissing TV.
    ... Anyway I cleaned up around the cap a bit untill I did not get ... So how do I make it work from cold, ie no blurred picture. ... tube, unbalancing it, I though maybe if I cleaned all of the ... It is when it has been switch on after a long period off that it ...
    (sci.electronics.repair)
  • Re: Tube Audio Question
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