Home » Audio » Group Build » Ground isolated from chassis - now it's an AM radio
Ground isolated from chassis - now it's an AM radio [message #30842] Sat, 05 March 2005 16:35 Go to next message
colinhester is currently offline  colinhester
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Registered: May 2009
Location: NE Arkansas
Illuminati (3rd Degree)
I got some teflon tape wrapped around the volume pot to isolate it from the chassis. That seemed to be the point of contact between the signal ground and chassis. Now when I touch the metal volume shaft, I get WLW clear as a bell. Pretty cool, Doug!! How did you manage to design that? The noise at full attenuation without signal has gone down quite a bit just by isolating the chassis, so I'm pretty happy

I'm still chasing down the hum. I did the star ground off the volume pot's ground node, so this might be the problem. I'm going to move the ground point over to the inputs and use heavier gauge bus wire. I was using wire that was thinner than the in B+, except in the connection going to B-.

Hopefully all will be sorted out this evening. Now if I can just figure out how to tune in something other than WLW......Colin


un-documented feature.... [message #30847 is a reply to message #30842] Sat, 05 March 2005 17:39 Go to previous messageGo to next message
PakProtector is currently offline  PakProtector
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wish I could take "credit" for it...
good luck, I've never had one do *THAT* yet.
regards,
Douglas


700 WLW - Voice of America [message #30849 is a reply to message #30847] Sat, 05 March 2005 18:31 Go to previous messageGo to next message
colinhester is currently offline  colinhester
Messages: 1349
Registered: May 2009
Location: NE Arkansas
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700 WLW is the old Voice of America and is a powerful AM station out of Cincinnati. I live 1/2 mile from its tower. I'm really surprised we don't get more interference from it. I need to get down there and tour the VoA park and see what it's like. I could also look through their dumpsters and see if any old, worthless tubes have been tossed.....Colin

1170AM [message #30854 is a reply to message #30849] Sat, 05 March 2005 23:12 Go to previous messageGo to next message
Wayne Parham is currently offline  Wayne Parham
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Registered: January 2001
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Here in Tulsa, the one you'll always get is 1170AM. It's a 50kW boomer, and for 75 years was all country. I can remember for years, just about everything you wired up with a speaker would play country music. Bad grounds, country music. Open input, country music. Hang a wire on a diode, country music. Now it's talk radio and it's still a 50kW boomer. But I can remember many, many times geting that faint twang of bluegrass in some audio or radio circuit that I was working on.


Re: 1170AM [message #30858 is a reply to message #30854] Sun, 06 March 2005 05:48 Go to previous messageGo to next message
Manualblock is currently offline  Manualblock
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Registered: May 2009
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From country to talk radio; sad how things tend to go downhill.


Re: Ground isolated from chassis - now it's an AM radio [message #30862 is a reply to message #30842] Sun, 06 March 2005 08:56 Go to previous messageGo to next message
Forty2wo is currently offline  Forty2wo
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Registered: May 2009
Master
Hi Colin,
If you are going in there, I would replace that pot. Whatever is shorted may be the source of your noise, or acting as a diode to pick up the radio.
Then again I used to live near a 50k station and it would play from my toster...John

Re: Ground isolated from chassis - now it's an AM radio [message #30863 is a reply to message #30862] Sun, 06 March 2005 09:02 Go to previous messageGo to next message
colinhester is currently offline  colinhester
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Registered: May 2009
Location: NE Arkansas
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I pulled the signal path apart last night and will redo it soon. I did notice that the insulation in the shielded cable I was using in the signal path had stretched in some places and was making contact with the shiled drain in more than one place. I'm going back and replace with either non-shielded solid-core Cu or stranded Ag wire......Colin

Re: Ground isolated from chassis - now it's an AM radio [message #30865 is a reply to message #30863] Sun, 06 March 2005 09:07 Go to previous messageGo to next message
Manualblock is currently offline  Manualblock
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Colin; I too examined that shielded wire last night as I am starting the signal section. It is very iffy in terms of possible shorts. How did you deal with the shield wrap? Maybe pull it back an inch or so; I mean you did not ground it to anything did you? Just asking for my own edification.

Re: Ground isolated from chassis - now it's an AM radio [message #30866 is a reply to message #30865] Sun, 06 March 2005 09:34 Go to previous messageGo to next message
colinhester is currently offline  colinhester
Messages: 1349
Registered: May 2009
Location: NE Arkansas
Illuminati (3rd Degree)
This is the same wire that is used in Bottlehead's SEX kit, so I followed thier directions on use. The red wire is used as the signal wire and is hooked up as nornal. The black wire is the the ground and is hooked at one end to the ground. The drain bare drain wire is wrapped around the black wire at the ground end and soldered. The shield is pulled back far enough to accomplish this.

Essentialy at one end the black and bare wire at arrached to ground and the red signal wire is to the siganl's path. At the other end everything is trimmed back except the red wire, which is hooked to the other end of the siganl's path.

I noticed on several connections that I had stretched the red wire's insulation enough that it was making contact with the foil shield. I just got in a hurry and used linesman's strippers to strip the black outer casing. The teeth cut a little too deep and caught the inner casings. I did not notice this until I started to examine my solder connections under an 8x loupe. I then checked for continuity between the siganl and ground and had full contact, damn....Colin

Re: Ground isolated from chassis - now it's an AM radio [message #30868 is a reply to message #30866] Sun, 06 March 2005 11:56 Go to previous messageGo to previous message
Manualblock is currently offline  Manualblock
Messages: 4973
Registered: May 2009
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Always something. Got my heaters working and if I can get a little more time the signal wiring should be done today.
BTW I too get 13v on the heaters. And to be truthful it looks like the tube is running a little hot.

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