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 Post subject: Philco shadow meter, how much should it close?
PostPosted: Jan Wed 03, 2018 8:57 pm 
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Joined: Nov Sun 04, 2007 6:01 am
Posts: 2897
Location: Redding, CA
How much should the shadow meter on a Philco 650 close on a strong station? The shadow meter on my rebuilt chassis is closing from approximately 1/2-inch at no station down to 3/8-inch on a strong local.

Norman

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 Post subject: Re: Philco shadow meter, how much should it close?
PostPosted: Jan Thu 04, 2018 12:10 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 807
Location: Warren, Ohio, USA
Norman,
Shadow meter operation depends upon a good antenna, proper alignment, and good tubes.

What are you using for an antenna?

Steve Chambers


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 Post subject: Re: Philco shadow meter, how much should it close?
PostPosted: Jan Thu 04, 2018 1:33 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4292
Location: Near Brandon, Iowa
Steve Chambers wrote:
Norman,
Shadow meter operation depends upon a good antenna, proper alignment, and good tubes.

What are you using for an antenna?

Steve Chambers

Yep, and also add "meter mechanical condition" to the list as well. Dust/debris and damage resulting from past mechanical shock can interfere with movement of the shadow vane. Makes one long for a good, all-electronic Magic Eye tube (a technology fad which Philco never really embraced).

If your meter works at all, be happy.


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 Post subject: Re: Philco shadow meter, how much should it close?
PostPosted: Jan Thu 04, 2018 2:53 am 
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Joined: Nov Sun 04, 2007 6:01 am
Posts: 2897
Location: Redding, CA
The receiver is properly aligned with a distinct peak for each adjustment. The antenna is a short wire that I am holding, roughly equivalent in performance to a moderately long wire. I tuned into strong local stations. The meter is in excellent working condition with only a thin line visable before B+ comes up. I just want to know what normal operation (width on a strong sigmal) is for this indicator.

Norman

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 Post subject: Re: Philco shadow meter, how much should it close?
PostPosted: Jan Thu 04, 2018 3:38 am 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4292
Location: Near Brandon, Iowa
Braithwaite wrote:
The receiver is properly aligned with a distinct peak for each adjustment. The antenna is a short wire that I am holding, roughly equivalent in performance to a moderately long wire. I tuned into strong local stations. The meter is in excellent working condition with only a thin line visable before B+ comes up. I just want to know what normal operation (width on a strong sigmal) is for this indicator.

Norman

Your antenna seems to be more "hopeful thinking" than "practical reality". Just for grins, try using the set with the sort of antenna for which it was designed (an actual outside longwire) and see if that alters the action of the shadow meter.

The tuning indicator in the 650 works by sampling the DC current drawn by the converter tube plate circuit, which will be at a minimum when that circuit is tuned to resonance. It is not derived from the radio's AVC voltage, as is the case with most eye tube sets (and some meter-based sets- but these use a dedicated vacuum tube as a buffer/driver for the meter coil).

I seem to recall that the reason that the meter shunt resistor is there in the first place is to provide a "backup" plate current path in case the meter coil burns open. That would allow the radio to still be used even with a dead meter. However, a secondary reason for it may be that it limits the deflection of the meter. At least that was the case with the tuning meter mod that I installed in my National boat anchor. I used a multi-turn pot as a meter shunt and was able to set its resistance to obtain full meter deflection which didn't "wrap the needle around the stop post".

If the deflection is insufficient, you can try increasing the value of the meter shunt resistor. This will allow more current to flow though the meter coil and increase its degree of deflection. There's also a risk that you could burn out the meter coil if you are careless while fiddling around with the shunt resistor; but if you are experienced with working on tube radios that risk is probably small.


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