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Author Topic: Hallicrafters S-120 Bandspread calibration?  (Read 1545 times)
KX4QP
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Posts: 369




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« Reply #30 on: March 10, 2019, 01:22:50 PM »

The cap is between the floating chassis potential and the case, not the plate supply.

Okay, I looked again -- I see C29, .047 microFarad, 600 V rated with a 470k bleeder resistor, between chassis (the GND contact beside the ANT connector) and B-, (the neutral cord wire).  Seems to me my ohmmeter should read that 470k as other than "open", even if the capacitor (as normal) reads open on the meter's DC.  I'll specifically check resistance (when powered off and unplugged) and voltage (when operating) between that GND screw and the case.  Looks like grounding that (which also should improve antenna performance) ought to either blow the breaker on the power circuit (if the cap is shorted), or make the chassis and case safe to touch even if that cap leaks to much lower than 470k resistance.  It also gives me a hint where to look for that capacitor to check if it's a modern replacement or needs to be replaced.

If nothing else, that finally identifies for me which wire goes where if/when I install a polarized plug.
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AC2EU
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« Reply #31 on: March 12, 2019, 07:12:33 AM »

Quote
If nothing else, that finally identifies for me which wire goes where if/when I install a polarized plug.

I found  a schematic for the s-120. If you have one, look at the bottom where you will see c29 (.0047)in parallel with a 470K resistor with a connection between chassis ground ( triangle) and case (fork) . This pair should be removed and replaced with a safety type cap.

I hope you can also see that if that cap shorted and chassis happened to be the line (hot) leg, you would have wall plug AC on the case!

I will say this one last time; A polarized cord is an improvement, but does not protect you if the wall outlet is mis wired with line and neutral reversed. I have seen this a lot in old houses with "upgraded" electric.
 
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KX4QP
Member

Posts: 369




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« Reply #32 on: March 12, 2019, 04:44:51 PM »

Quote
If nothing else, that finally identifies for me which wire goes where if/when I install a polarized plug.

I found  a schematic for the s-120. If you have one, look at the bottom where you will see c29 (.0047)in parallel with a 470K resistor with a connection between chassis ground ( triangle) and case (fork) . This pair should be removed and replaced with a safety type cap.

Exactly the cap and resistor I cited in my last reply.

Quote
I hope you can also see that if that cap shorted and chassis happened to be the line (hot) leg, you would have wall plug AC on the case!

I certainly can -- or if the 470k resistor got bypassed in some way.  A little quick research suggests a Y rated (or X/Y) safety cap is what I want (Y, for electric shock potential, are essentially designed to never fail and/or self-heal in the event of an arc-through).  Looks like they're cheap, well under a buck each; since I'm likely going to have more tube sets before I have fewer, I'll order a number of them in assorted values around that .047 uF.

Quote
I will say this one last time; A polarized cord is an improvement, but does not protect you if the wall outlet is mis wired with line and neutral reversed. I have seen this a lot in old houses with "upgraded" electric.

No argument on that.  I know this house is wired correctly; it's a manufactured home new enough to have had GFCI in the bathrooms (but not the kitchen), all outlets grounded with 12 AWG solid copper green wire ground, and I've checked several outlets in the past and found them with correct polarity.  None the less, there's no telling about other outlets I might find in the future, or someone else who has this radio after me might find.  Thanks for leading me through this -- my shack is pretty safe in terms of nothing to ground to within reach of the radio, but it's good to know what to look for and what to do about it.  After eleven years of repairing 120V (and rarely 240V) power tools (which are brought in because of often-unknown malfunctions) I have great respect for 120V 60Hz power.
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