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Old 4th Aug 2004, 8:45 pm   #1
st.mellion
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Default Alignment AC/DC Chassis

Hi,
I want to align a Bush DAC10. I have an Advance sig. gen. The Bush has a live chasis and the generator is earthed. How do I get round this ??

Last edited by Darren-UK; 26th May 2007 at 12:52 pm. Reason: Subject clarified.
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Old 4th Aug 2004, 8:53 pm   #2
paulsherwin
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Default Re: Alignment

The correct way is to use an isolation transformer and earth the chassis. The quick and dirty way is to use a suitably rated capacitor in the earth connection.

Make sure the chassis *isn't* live, i.e. is connected to mains neutral.

HTH, Paul
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Old 4th Aug 2004, 8:55 pm   #3
st.mellion
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Default Re: Alignment

Thanks Paul. I'll give it a go.
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Old 4th Aug 2004, 9:01 pm   #4
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Default Re: Alignment

Hi st.mellion.

The correct way is to use an isolating transformer. this is the safest way to work on AC/DC chassis. However if you make sure that the chassis of the radio is definately connected to neutral (as it should be) then at a pinch you can disconnect the mains earth of the generator. This will at least ensure that when you connect the generator to the chassis of the set there will be no nasty bangs!!

I suppose you could also use an isolating capacitor in the earth lead of the generator and retain the mains earth....something like a .01uF at 1000volts. I've never tried this though.

Still emphasise that an isolating transformer is the only proper way.

What do other forum members think?


Rich.
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Old 4th Aug 2004, 9:45 pm   #5
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Default Re: Alignment

If you just want to do a simple RF/IF alignment you probably don't need to connect the generator to the radio chassis anyway. There'll be enough signal with just the signal probe connected.

As Richard says, an isolation transformer is always safer though.

Best regards, Paul
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Old 5th Aug 2004, 2:01 pm   #6
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Default Re: Alignment

If you do connect the sig.gen. to the chassis- not recommended anyway - but if you do, and you've carefully checked that the chassis is at neutral, then as a last check before actually making the connection just stick a meter between the generator earth and the chassis. It should read 2v- 3v ac.

With the best will in the world one can always make a mistake; if the meter reads 240v you can correct things before making it.

Joe
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Old 5th Aug 2004, 10:15 pm   #7
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Default Re: Alignment

Hi all

If your house has an earth leakage trip, when you connect neautral to earth the 3 or 4 volt differance will be enough to trip the trip.

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Old 6th Aug 2004, 12:02 pm   #8
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Default Re: Alignment

In which case it's probably better to use Pauls idea of 'loose coupling'. You should find that there is enough pick-up by just looping the generator output cable around the frequency changer valve making sure that the earth lead is insulated and well out of the way. For RF alignment you can place the output cable near the aerial rod (if it has one) or via a capacitor to the aerial socket (1000pF should be ok). If you connect the generator direct to the aerial socket it might cause too much damping and give poor results.


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Old 6th Aug 2004, 2:10 pm   #9
st.mellion
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Default Re: Alignment

Thanks to evryone for their advice. I feel much better informed and more confidant of approaching this now .....and more safely !!
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