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#1
05-14-2009, 12:11 PM
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Testing B+, Resistors, IF Cans
Hello folks,
I am new to old radios but have recapped a Zenith X337 from 1968 ( a great sounding little radio) and a much smaller aa5, a Zenith M504 am only radio from the mid-1960's I acquired to experiment with and learn on. Both were quite simple jobs. My knowledge of electronics is limited to what I remember from high school (very little), what I have read online, and in books such as "Fixing Up Nice Old Radios". Since I am such a fresh newbie, my questions are pretty basic, but to me the answers will be very practical as I don't want to kill myself poking around the insides of 40-60 year old electrical equipment! Thanks for indulging me... 1. How does one test B+ voltage? I know one lead should be clipped to negative, ground I presume, and the other to various spots in the power supply circuit. Is ground a common ground such as a water pipe or is ground the chassis? In order to test B+, the radio should be plugged in and turned on? Also, if I want to test the voltages of the various tube filaments in series, I'd clip the red lead to the appropriate tune pin and black to ground (common or chassis?)? 2. On an ac-dc set, to test the chassis to see if it is hot, clip the red lead to the chassis and black to common ground such as a water pipe, turn on the set, then see if there is voltage? I realize there can be voltage with the set off as well and I have read the importance of using an isolation transformer. 3. On Photofacts for the 2 radios, I see charts for resistance measurements for various pins of tubes. These measurements should be taken with the radio off? Also, can accurate measurements of this type be made with typical Radio Shack analog multimeter (I've read where you need a VTVM for this)? Unless otherwise noted, the red lead of the meter is clipped to the indicated pin and black clipped to ground (again is it common ground such as a water pipe or to the chassis)? 4. On my little Zenith M504, I think there is silver migration problem with an IF can. Is there a measurement I can take to test the can before I try to pry it off the chassis and take it apart? Many thanks in advance. I am learning and really appreciate any feedback you can offer. 
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#2
05-14-2009, 12:42 PM
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For a direct line powered radio like one of these, the return for resistance and voltage measurements is the B- connection (minus of filter cap), not chassis ground. For transformer powered sets, chassis is ground. Resistance is always measured cold (unplugged). It's a good idea to check for voltage on the filter caps if there's any doubt...
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#3
05-14-2009, 01:00 PM
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Good afternoon! and welcome to AK! You're going to have a lot of fun here, you'll learn a bunch of great information, and, hopefully, we'll keep you alive in the process!
__________________
I may be growing older, but I refuse to grow up. 
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#4
05-14-2009, 07:17 PM
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Tom, thanks, follow ups below please?
 "For a direct line powered radio like one of these, the return for resistance and voltage measurements is the B- connection (minus of filter cap), not chassis ground. For transformer powered sets, chassis is ground. Resistance is always measured cold (unplugged)." Makes sense as I am measuring voltages at various points along the B+ power circuit. To clarify, the set should be powered on before I CAREFULLY measure voltages, but measuring resistance on tube pins is done cold, again with negative lead clipped to B- unless otherwise stated? Also is it real important to use VTVM for measuring resistance in this instance? "It's a good idea to check for voltage on the filter caps if there's any doubt..." Um, forgive me but what does that mean, if there's any doubt about what  Fisherdude, thanks for the welcome. I noticed you posted a long reply earlier that I was excited to read when I was not at work but now it is gone. I hope, if there was good info., that you will share? Many thanks.
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#5
05-14-2009, 07:38 PM
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Hello again!
I'll be happy to redo the post you're talking about. I wasn't comfortable that my choice of phrases or examples was as clear as it could be. Although I'm a long way from being an expert I do have some background in this area, and I try to make sure I'm explaining things in a way that is easy to understand from a beginner's perspective. Got to finish up dinner and some chores, but I'll be back!
__________________
I may be growing older, but I refuse to grow up.
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#6
05-14-2009, 07:46 PM
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Yes, voltages are measured with unit powered. Clip on the meter negative lead BEFORE you plug it in. Measure voltages one handed - a shock arm-to-arm (through the heart) can be deadly. Old-timers say to measure voltages with one hand in your back pocket (that's how they got to BE old-timers...).
A $25 digital voltmeter has a 10 megohm input impedance like a VTVM, so it will read the same. A $5 DVM may have a 1 megohm input, and MIGHT read lower in some spots, where there's a resistor in series with the voltage being measured. A analog V-O-M will work as well. The filter caps are what stores energy and can have considerable voltage after it's turned off. The circuit will drain this energy quickly IF the tubes have had a chance to warm up. Turn it on and off immediately and the caps can store that energy for a LONG time. Clip your meter across the main filter cap and try it. Whenever you're about to reach inside a radio, it only takes a few seconds to check for voltage first... that's what I meant - if there's any doubt DON'T until you've checked for voltage.
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#7
05-15-2009, 07:25 AM
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Quote:
Tom, you've helped connect the dots some for me, thanks a bunch  Fisherdude, thank you so much in advance for your time!
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#8
05-15-2009, 09:17 AM
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Quote:
In any radio, almost all the current is drawn by the tubes. They won't draw any current if the heaters are cold. So the power switch won't make any difference.
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#9
05-21-2009, 09:30 AM
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Quote:
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Linear Mode
