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 Post subject: Zenith 12H689
PostPosted: Nov Thu 30, 2017 5:22 pm 
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Joined: Nov Thu 30, 2017 4:58 pm
Posts: 6
Hi. I'm brand new to this forum. I'm restoring a Zenith 12H689. So far, it's going well. The main problem is that it came to me with the tuning meter missing. For what it's worth, the part number is 122-16. I asked around at Kutztown and nobody I spoke to was even aware that Zenith ever used a meter. The schematic gives no technical specifications for the meter. I temporarily wired in a modern meter with a DC resistance of about 30 ohms. It gives me a weak indication of about 1/4 scale on the strongest station and barely any movement on most others.
Is anyone aware of this meter or where I can get one?
Thanks in advance for any help.


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 Post subject: Re: Zenith 12H689
PostPosted: Dec Fri 01, 2017 3:29 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 3613
Location: Near Brandon, Iowa
You need a milliammeter, not a voltmeter. Temporarily replace the meter with a 1K resistor, tune in the strongest station that you can find, and measure the voltage differential across the resistor, and compute the current. Use a high-impedance voltmeter (VTVM) to make the measurement; a low-impedance meter will corrupt the reading. This will be the maximum current range needed for the milliammeter movement. It's OK to use a more sensitive meter and shunt it with a resistor to limit its response. For example, suppose that you determined 3 mA provided maximum deflection: you could use a 0-10 mA meter, but it would only register partial deflection. A 0-1 mA meter would give full deflection if shunted with a "calibrating resistor" to allow bypass of some of the current around the meter.

The 3 mA figure is a hypothetical example. I have no idea what the actual current is... it may even be in the microamp range. But whatever it is, you should be able to find a meter movement for it.

Finding a "naked movement" (caseless) that will fit in the allocated chassis space may be challenging. You may be able to adapt an analog tuning meter from a discarded 1970s stereo receiver, although it may be difficult to determine in advance what its electrical characteristics are.

Another approach might be to adapt a 1950s "ribbon style" magic eye tube to the radio. Something like an EM84 looks like it might be a good fit in the meter opening. A suitable tube holder and mask would have to be fabbed up, of course. The control grid of the EM84 could be fed off the AVC bus (not the grid of the meter amp tube, which appears to have some "normalization" parts associated with it).


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 Post subject: Re: Zenith 12H689
PostPosted: Dec Fri 01, 2017 6:31 pm 
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Joined: Nov Thu 30, 2017 4:58 pm
Posts: 6
Thanks. I'll give that a try. I had considered using a tube for the display, but I'm trying to keep it as original as possible. My grandparents had the same radio. I may still opt to go that way, though.


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