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Author Topic: Scratchy Mohawk?  (Read 6493 times)
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AB3FL
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« on: February 27, 2009, 07:46:18 AM »
I seem to be having an issue with my Mohack.  Something in the IF is causing static.  I can remove the jumper from the RF deck to the IF deck or even turn the RF gain down all the way with no antenna and it is still there.  There is a scratchy static coming from the IF which is casuing hell with the AVC.  I have not been able to track it down.  This was even there before I did the K6AD mods.   Any ideas?

thanks

Tom - AB3FL
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N3DRB The Derb
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« Reply #1 on: February 27, 2009, 07:50:19 AM »
bad mica caps inside ur if cans? a guess, I've never worked on one.
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The Slab Bacon
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« Reply #2 on: February 27, 2009, 08:02:38 AM »
I had a similar problem with my NC-300, it wound up being an intermittant screen dropping resistor. Short the grid of each stage to ground, when the noise stops, carefully look over the preceeding stage.

                                                           the Slab Bacon
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WQ9E
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« Reply #3 on: February 27, 2009, 08:04:11 AM »
Tom,

Does mechanical shock have any effect?  If so try gently tapping tubes and transformers to localize the source.

If it isn't affected mechanically, then pull tubes in a logical fashion to narrow down the stage where it originates.  I would start with the 6BA6 1682 IF amp stage and work my way through the 2'nd mixer and low IF towards the detector stage.

Is the noise still present with AGC defeated?

Rodger WQ9E
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Rodger WQ9E
W3SLK
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« Reply #4 on: February 27, 2009, 09:05:11 AM »
Frank said:
Quote
I had a similar problem with my NC-300, it wound up being an intermittant screen dropping resistor.

That must have been a bitch to find! Cool
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Mike(y)/W3SLK
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W2VW
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« Reply #5 on: February 27, 2009, 09:53:33 AM »
FWIW mine did the same thing when the dropping resistor to the VR toob went west.
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The Slab Bacon
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« Reply #6 on: February 27, 2009, 10:15:34 AM »
Quote from: W3SLK on February 27, 2009, 09:05:11 AM
Frank said:
Quote
I had a similar problem with my NC-300, it wound up being an intermittant screen dropping resistor.

That must have been a bitch to find! Cool

It was!! It was the one for the first mixer tube, hidden UNDER the coil catacombs!! It was no fun to replace either.
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w3jn
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« Reply #7 on: February 27, 2009, 02:04:26 PM »
Start yoinking toobs from the front end towards the detector, one by one, until it quits.  Then yuo've at least locallized the problem.

Could be a bad resistor, bad capacitor in the IF cans, intermittent winding in the IF cans, bad toob, bad solder joint on a toob socket, arcing bypass cap, etc.  In other words could be anything  Tongue
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AB3FL
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« Reply #8 on: February 27, 2009, 03:06:07 PM »
You get the prize.....It was a bad 470pf mica cap on the grid of the 1st 50KC IF toob (pin 1 to be exact)

I have also finished the K6AD mods (except for the temp comp) and it makes a huge improvment.  I have no plans on doing the temp comp mods.

Tom - AB3FL



Quote from: N3DRB The Derb on February 27, 2009, 07:50:19 AM
bad mica caps inside ur if cans? a guess, I've never worked on one.
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N3DRB The Derb
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« Reply #9 on: February 27, 2009, 05:45:07 PM »
I'm shocked.  Roll Eyes Tongue Seriously just a guess. my second bet would have been cracked rx somewhere.
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K7NCR
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« Reply #10 on: February 27, 2009, 07:24:00 PM »
I didn't do the temp mods to mine either. I just turn it on Friday night, and turn it off Sunday when I'm done for the weekend. The Apache VFO fil. is on all the time, so no excessive drift problems there.  Grin
73! Norm K7NCR
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w4bfs
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« Reply #11 on: February 27, 2009, 10:05:44 PM »
gottum scratchy mohawk Huh?  ...   useum selsyn blue  ....beefus
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Beefus

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