Replacing power cord with new polarized cord, what is best orientation?

little-al

Active Member
Well, during the rehab of my FM-50 tuner and 30-A amplifier, I found the original cord on the 30-A has a very tempermental and intermittent short.... very intermittent ....... :dammit: :D


I spent an hour trying to track down why I had 50+ VAC on the chassis that would come and go only when the amplifier was oriented a certain direction :scratch2:

......... anyway, it turns out the cord, which is in great shape externally and cosmetically, clearly has an internal fault that has only showed up now after months of tinkering. So it needs to be replaced. No big deal, but a shame since it had the original cord.



I've read several threads here at AK and elsewhere about the options of changing to a grounded connection or replacing the old cord with a new polarized zip-cord. I plan to do the latter.

What hasn't been clear, at least to me, is what orientation the new polarized cord should present to the circuit.

I've also read quite a bit about the original non-polarized plugs and how they were meant to be "flexible" in the orientation one could plug them into outlets of the time period so as to allow one to find the orientation that produced the least hum.

What I wonder is, although this flexibility was inherent in the non-polarized plug design, if in fact the "mystery" of which orientation produced the least hum wasn't actually a mystery to the component builders. I wondered if the design of the component itself always assumed that the correct orientation would present the hot leg and neutral leg to the circuit one correct way, and the real "mystery" was always the time period non-polarized sockets which may or may not have the hot and neutral legs clearly identified.... so flipping the plug to find the orientation with the least hum (or putting a meter on the outlet) was the only way to find that correct orientation.

So.... this assumption leads me to believe that when I install the new polarized cord that the hot and neutral legs of that cord should be connected to a very specific part/orientation of the power circuit in the 30-A.

I have my theory on what that would be, but I thought I'd just as the experts.


With that synopsis, my question: Which part/orientation to the power circuitry should I correctly connect the hot and neutral legs of this new polarized cord when replacing the old non-polarized cord?

Thanks!
Al
 
how about a picture of the termination, does one wire go to the power switch? as you look at a standard 15/20 amp 120 volt plug with the grounding conductor facing down the neutral conductor is on the left. or at least should be on the left. not sure if this answers your question.
 
I can't post a photo now, but I can later this evening.... being a 30-A there isn't a power switch. I'll post a photo of the FM-50 circuit as well for reference.

Thanks,
Al
 
I would try it both ways. Measure the leakage current to earth ground for each orientation, then go with the one with the least leakage current.
 
When I replaced the cord on my 30A in the 560 companion, I also did the "flip and check. Which gave me the hot on the FUSED side. (with a polarized plug hot being the narrow blade). Your readings may or may not be the same. However this also resumes that your outlets are wired correctly, Get one of those outlet testers. It wll tell you if your outlet is wired correctly or not. In any case, change out the "death" caps with an X-Y rated cap. The X-Y cap fails OPEN instead of closed.

Larry
 
Photos 30-A

Here are some photos of the guts of the 30-A and closeup of the AC connection.
 

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Photos FM-50

And here are some photos of the FM-50 insides including a closeup of the AC connection.
 

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When I replaced the cord on my 30A in the 560 companion, I also did the "flip and check. Which gave me the hot on the FUSED side. (with a polarized plug hot being the narrow blade). Your readings may or may not be the same. However this also resumes that your outlets are wired correctly, Get one of those outlet testers. It wll tell you if your outlet is wired correctly or not. In any case, change out the "death" caps with an X-Y rated cap. The X-Y cap fails OPEN instead of closed.

Larry

I've started doing a hat-trick with X-Y caps in tube gear: line to ground, neutral to ground and line to neutral. A little more expensive, but a hell of a lot safer, especially if flipping the AC plug around (I've seriously considered fusing the neutral side as well, but that may be overkill).

-D
 
Derekva.....The Hat trick is damn good compensation for the "correctly wired" (note sarcasm)house. I'd make the outlet correct, and do the hat-trick on a polarized plug. OVERKILL???? HELL NO!!! Anything that keeps you VERTICAL and awake is NEVER Overkill.

Larry
 
Can you detail this "hat trick". I think I see what you are saying, but I'd be curious about the value of caps used for example.
 
Can you detail this "hat trick". I think I see what you are saying, but I'd be curious about the value of caps used for example.

Just buy a trio of X1/Y2 rated capacitors like these from Mouser / Digikey / Justradios.com. Install one connecting the hot and neutral wires (where the AC comes into the unit), one connecting hot and (chassis) ground, and one connecting neutral and (chassis) ground. I like the Vishay's I linked to as they aren't too bulky and Vishay seems to have good quality. Likewise, you can also add a second fuse to the neutral (I like the internal-mount variety that doesn't require a hole in the chassis) and if possible, connect the neutral to the switch as well. That way, however you orient the plug, you'll be as protected as reasonably possible with a two-prong device.

Hope this helps, and remember to be very careful when rooting around inside your amp (and always add a 300k / 2W drain resistor between the first cap and ground in your power supply).

-D
 
Thanks a few questions though:

1) What benefit does having both the cross (X) directly between the hot & neutral provide if one already has a bypass (Y) on one or both legs to chassis?

2) Referencing question 1, does having now three capacitors in play across the mains and tied to chassis introduce any oddball risk of hum or other peformance oddities? It just seems like a lot of paths to neutral/ground now exist.

3) If you look at my photos, you will also see a resistor in parallel to the "death" cap on one leg of my FM-50 power connection. Should I replicate that same resistor in parallel with an X or Y that you recommend installing across the mains and to chassis ground on the other wire?

4) For all three capacitors now in play, would they all be the same capactance value as the original single "death" cap as spec'd? Or would they have to be some other summed or decreased value?

Thanks!
Al
 
Just buy a trio of X1/Y2 rated capacitors like these from Mouser / Digikey / Justradios.com. Install one connecting the hot and neutral wires (where the AC comes into the unit), one connecting hot and (chassis) ground, and one connecting neutral and (chassis) ground. I like the Vishay's I linked to as they aren't too bulky and Vishay seems to have good quality. Likewise, you can also add a second fuse to the neutral (I like the internal-mount variety that doesn't require a hole in the chassis) and if possible, connect the neutral to the switch as well. That way, however you orient the plug, you'll be as protected as reasonably possible with a two-prong device.

Hope this helps, and remember to be very careful when rooting around inside your amp (and always add a 300k / 2W drain resistor between the first cap and ground in your power supply).

-D

We were schooled on these DEATH :eek: caps by a friend of Daddy's
We bought these http://www.mouser.com/ProductDetail...=sGAEpiMZZMt1mVBmZSXTPFACafpHt/4LwS60X5dGAGw=

There's some even cheaper ones from the Toshiba company that we found too late, and are rated for a lot more juice! :yes:

I think you might be paying way to much money for a cap that's never going to see 440 volts?

We're buying them by the sack now and putting them on everything in the house! :)

P
 
We were schooled on these DEATH :eek: caps by a friend of Daddy's
We bought these http://www.mouser.com/ProductDetail...=sGAEpiMZZMt1mVBmZSXTPFACafpHt/4LwS60X5dGAGw=

There's some even cheaper ones from the Toshiba company that we found too late, and are rated for a lot more juice! :yes:

I think you might be paying way to much money for a cap that's never going to see 440 volts?

We're buying them by the sack now and putting them on everything in the house! :)

P

Good find on the Murata.

-D
 
Hey derekva,
How about a picture of these death caps installed please?
I'm having trouble visualizing them in place.
 
Ask and ye shall receive. This is a revision in response to a few inquiries.

attachment.php
 
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So, in my case the original line-to-chassis capacitor is a .01uf capacitor. Would the new capacitors all be of the same .01uf value?

Also, if you look at the attached photo of the FM-50 for example (back in posts 6 and 7 of this thread), you'll see a resistor also in parallel to the capacitor. What function does the resistor play, and for the recommended new across-the-line and neutral-to-chassis capacitor, would one use a resistor in parallel with those as well, or just the new neutral-to-chassis cap?

Thanks!
Al
 
Replace the caps with the same value as current. I think the resistor is there to bleed off residual voltage after the unit is turned off or if the cap fails open based on the old way(someone correct me here.) The resistance is high enough that it doesn't show as a short across the resistor. You may not need the resistor with the "triple hat".

Larry
 
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