RC-M70C voltage on chassis when plugged in

greetings, i recently purchased a RC-M70C off ebay and noticed something with it. depending which way the wall plug is plugged in (because there is no polarization on the plug), if you plug it in one way vs. the other way, there's about 100 volts on the metal chassis and metal strip under the tuner dial window, referenced to earth ground. there's also about 59uA (micro amperes) of current.

is this a normal operating condition?

No.

any idea what could be causing this?


if it's useful, there's about 3.2 mega ohms of resistance between any blade of the plug and the chassis (including ground screw). i think this suggests it's not capacitive coupling that is going on here.

can't be C681 to C684, can they? because those are on the secondary.

in the service manual, R681 is listed in the diode section as QRC121K-225 but in the diagram it is shown as a resistor labeled "2.2M (comp)" . this is connected to the primary of the transformer and looks like it goes back to the negative of the main circuit board. can this be the issue? if it is, this looks like part of the way it is designed.

take a look at this M70 from an ebay ad:

https://i.ebayimg.com/images/g/sOcAAOSwCV5cgncg/s-l1600.jpg

it has a safety certification sticker that claims max allowable leakage current is 500uA and it was tested and showed 10uA.

The label was merely affixed by whoever "serviced" that set, and as a matter of store policy, probably tests the insulation of the devices they work on before releasing back to customer for liabiity reasons. That is not a factory label.

As for the resistor, I didn't notice that before, as most boomboxes have their primary windings completely isolated from the secondary. However, with the presence of that resistor, then you WILL have some leakage if the cord was installed a certain way. Typically speaking, on systems like this, the cord will be polarized to ensure that it is not plugged in the wrong way. After all, if the system ground is attached to one of the AC leads, why in the world would a set be designed in a manner where the system ground can be tethered to the hot lead, as opposed to AC earth? 59ua is not a lot, but I'd much rather not have that going through my body. Depending on who you ask, only 1mA is necessary to feel electrical sensation and 10mA is strong enough for muscle contractions to prevent release of the source of current if you were grasping it. If you are concerned about that current leakage, you can probably remove that resistor. I'd note that the JVC RC-M71jw schematic shows that it doesn't even have that resistor and the primary is completely isolated from the secondary, so I suppose one might surmise that it's not essential. Aside from some cosmetic differences between the JVC RC-M70jw and RC-M71jw, it would seem they are otherwise identical anyhow.

I would just put a piece of tape on the plug to indicate the right side up. Or, just buy a polarized plug and throw out the generic one he sent you. I really doubt it's an original.

Hi Dan. I have noticed this with my box as well. Seems like when I would slide my finger down the metal front trim it almost had a vibrating feel to it as if voltage was present (similar to a CRT when turned on). With Neutral on the top pin of the AC input on the box, the metal trim felt smooth again however, no noticeable change in performance was detected.

I do have one box with a fixed non polarized power cord (my Quasar) that has an audible hum when plugged in one way versus the other. I have simply marked all of these situations and plugged them in accordingly. Only issue there is you never know if someone wired the outlet correctly from one location to another.

Another interesting fact about power plugs whether they are polarized or not is that the neutral side usually has a raised ridge or texture on the power wire jacket that dictates the neutral assignment of the power cord. This has always helped me with "fixed" power cord component systems to eliminate noise and ensure everyone is earthed together on the same circuit. But with the JVC M70 (and many other boxes) you can plug them in either way. Some seem to have interesting characteristics while others do not and it is almost always due to design in most cases

Reli, a polarized plug wouldn't make a difference because the end that goes into the radio has no keying to ensure it is polarized.


i wonder if that resistor was put there for EMI purposes? maybe if it's removed, there would be lots of static and hum?



did a search, this is the cable that came with the M70 i got:



https://www.amazon.ca/Prong-Power-KKDK-KKS-18-KKP-10/dp/B00UR8UG0Y

dan said:

i wonder if that resistor was put there for EMI purposes? maybe if it's removed, there would be lots of static and hum?

Click to expand...

If that were true, then the M71 which doesn't have that resistor would perform horribly but it does not. Operationally speaking, doubtful anyone has ever noticed anything different between those 2 models, and the M71 is a more highly coveted boombox. Personally speaking, I think I'll be removing that resistor (if I can find it) on any sets that I'll be using regularly. I'm not particularly liking the thought of any part of a boombox's grounded metal trim being tethered to 120V mains, whether through a resistor or not. Remember, that voltage is only really present when the cord is plugged in one direction. It is not on the other direction, or is so small as to be virtually nothing.

Another way to think of this is: If I handed you a new 2.2mΩ resistor and ask you to hold one end with your fingers, and insert the other end into the hot connection of the AC outlet, how would you feel about this? Yes, current will go through your body, and you might not even feel the sensation, but would you want that for any reason?

nope, because if that resistor fails or the power lines get struck by lightning, things would be bad.


another thought, would polarization even make a difference? either side of the line is at +120v or -120v (+170v, - 170v peak) depending on what part of the ac cycle is happening.

dan said:

Reli, a polarized plug wouldn't make a difference because the end that goes into the radio has no keying to ensure it is polarized.

Click to expand...

I think what Reli was saying was to mark each end on a non polarized plug so it can be inserted the same way on both ends and on one end if polarized. But even if it were keyed, if an outlet is wired backwards, you'd have the same situation.

dan said:

another thought, would polarization even make a difference? either side of the line is at +120v or -120v (+170v, - 170v peak) depending on what part of the ac cycle is happening.

Click to expand...

Of course, it absolutely would make a difference. First, you need to understand electricity and how AC is delivered to your home. If your equipment was connected to 240V, and the neutral was not connected, your unit would be connected to TWO hot leads. So regardless of which lead you touch, you are fried extra crispy. But your unit is NOT connected to 240v (is it?), but 120 and on 120V, your set is only connected to ONE hot lead. The other lead is a neutral wire, which is actually bonded to ground (earth) at the main panel. In other words, the neutral wire and ground wire will be on almost equal potential. I say "almost" equal because there is some miniscule resistance at every connection, and therefore, the potentials might vary by some miniscule amount, but if everything is properly connected, then the difference is negligible for all practical purposes. Effectively, this means that if you were to accidentally touch the hot lead of a 2-wire AC cord, you are fried and likely kaput. But if you were to accidentally touch the other neutral lead of a 2-wire AC cord, then you are actually touching ground. While you are into testing, why don't you do this: Insert one meter probe into the round ground hole of your AC outlet. Then insert the other meter probe into one of the flat blade slots, then the other slot. You will notice 120v potential from one of the slots, and 0v or some other small number from the other slot. Remember, that the secondary of your boombox is tied to "one" of those AC receptacle slots through that 2.2 mΩ resistor. So does it make a difference which way the plug is plugged into the outlet? YES. You make it sound like when the AC swings from the +/- parts of the cycle, that the "hot" leads will reverse. It does not. While the electricity does indeed flow the opposite direction, the neutral lead will always be neutral and not "hot".

dan said:

nope, because if that resistor fails or the power lines get struck by lightning, things would be bad.

Click to expand...

Exactly! Which is precisely how the boombox is connected. A 2.2 mΩ resistor is tethering the system ground to one lead of the AC line. If it's tethered onto the earth lead, then no bigge. In fact, on every appliance or metal encased equipment, the cabinet is actually tied to the ground (earth) lug when a 3-pronged cord is used. But if the polarized plug is reversed, then the system ground is tethered to the live "hot" AC lead, then that would be equivalent to you holding one end of the resistor while the other end is connected to 120V. BTW, at the very least, the metal trim below the tuner dial is actually tied to the system ground. You'll see this inside as that trim has a leg below the right LED which protrudes inside the boombox, and that leg is connected to system ground. That's why you saw voltage from that trim measured against the AC.

Just did this test this evening. A little surprised to see how pronounced the leakage is.

https://www.youtube.com/watch?v=qigbI3alGMk&feature=youtu.be

So..... you've discovered that, YES, polarity does make a difference? Haha.

Superduper said:

So..... you've discovered that, YES, polarity does make a difference? Haha.

Click to expand...

Indeed! I enjoyed this experiment. What I was surprised most was seeing how far away the voltage was being detected It was amplified in detection from the source at the outlet. Must be due to the overall surface area. The voltage whizzing off of the box like nuclear fusion particles

So this raises another question. With this resistor tied to the chassis, in Countries where the box is being supplied 220/240 it stands to reason that no matter what...the chassis would/should be hot. Am I correct on this?

Update: On my Conion C100s they show voltage no matter which way they're plugged in.

yup, i messed up here. current will flow from the neutral to the hot in the negative part of the cycle but because it is bonded to ground at the panel and connected to the center tap of the transformer on the street which is also grounded, the voltage potential will ideally be zero. if that neutral should happen to be be disconnected, then there is problems.

Superduper said:

another thought, would polarization even make a difference? either side of the line is at +120v or -120v (+170v, - 170v peak) depending on what part of the ac cycle is happening.

Click to expand...

Of course, it absolutely would make a difference. First, you need to understand electricity and how AC is delivered to your home. If your equipment was connected to 240V, and the neutral was not connected, your unit would be connected to TWO hot leads. So regardless of which lead you touch, you are fried extra crispy. But your unit is NOT connected to 240v (is it?), but 120 and on 120V, your set is only connected to ONE hot lead. The other lead is a neutral wire, which is actually bonded to ground (earth) at the main panel. In other words, the neutral wire and ground wire will be on almost equal potential. I say "almost" equal because there is some miniscule resistance at every connection, and therefore, the potentials might vary by some miniscule amount, but if everything is properly connected, then the difference is negligible for all practical purposes. Effectively, this means that if you were to accidentally touch the hot lead of a 2-wire AC cord, you are fried and likely kaput. But if you were to accidentally touch the other neutral lead of a 2-wire AC cord, then you are actually touching ground. While you are into testing, why don't you do this: Insert one meter probe into the round ground hole of your AC outlet. Then insert the other meter probe into one of the flat blade slots, then the other slot. You will notice 120v potential from one of the slots, and 0v or some other small number from the other slot. Remember, that the secondary of your boombox is tied to "one" of those AC receptacle slots through that 2.2 mΩ resistor. So does it make a difference which way the plug is plugged into the outlet? YES. You make it sound like when the AC swings from the +/- parts of the cycle, that the "hot" leads will reverse. It does not. While the electricity does indeed flow the opposite direction, the neutral lead will always be neutral and not "hot".

Click to expand...

Transistorized said:

So this raises another question. With this resistor tied to the chassis, in Countries where the box is being supplied 220 then it stands to reason that no matter what...the chassis would/should be hot. Am I correct on this?

Update: On my Conion C100s they show voltage no matter which way they're plugged in.

Click to expand...

Well, if it was set to 240V in USA, it would definitely be hot on either AC leg if you have the proper adapter (240v outlets are different than 120v outlets). I know this because that's how our USA electricity is delivered. Our overhead electric drops (supply cables to our homes) have 2 hot conductor and 1 neutral. The ground and neutral will be bonded at the main panel where electricity enters the home. Water and Gas lines are supposed to be bonded together too. Each hot conductor delivers 120V and they are not the same conductor. If you take a meter, you will find hot conductor (either one) to neutral will yield 120V. Hot conductor to hot conductor will yield 240V. The 2 hot conductors are in different phases. You'll even find that for some circuits like in the kitchen, that the neutral wire may be shared between 2 circuits. This is allowed as long as the 2 circuits are not powered by the same hot conductor.

However, I can't be positive with regards to the electric grid in other countries. For example, IF their system basically provides 220/240v through a single hot, and a single neutral, then it will still behave the same (polarized) except that the voltage is 220/240.

Transistorized, it all depends if it's 220v between the the start and end of the windings of the street transformer. then either side would have voltage. if there's a center tap, and you're getting 220v between neutral and hot, then it would be 440v between hot and hot.



i marveled at the complexity of the M70 but i gotta wonder why the heck they put that resistor in there? is it a mistake? industrial sabotage? for crying out loud, look at the complexity that went into this boombox! hey, lets connect the chassis and the ground of the circuit board powered by the secondary of the transformer to one leg of the primary through a resistor. heck, lets make it without polarization either!

this is something we need to get to the bottom of.

All of this makes perfect sense.

Now what I am interested to see next is the results of removing that resistor since it's not in the 71 from the 70...and if no change is detected...why the heck did they put it in there?

If you remove that resistor, the cord polarity will no longer make any difference to the system. The secondaries will be completely isolated and your tester should no longer detect any AC voltage regardless of how the plug is connected. Now, can you even find that resistor?

As for the C100, for the test that you are performing, a non-contact tester isn't going to be that accurate since there isn't 100% isolation between system circuit and the primary (in this case, the system ground is connected to mains through a 3.3 mΩ resistor, 50% higher resistance than the M70). The higher resistance provides greater isolation and if your cord plug end has some oxidation on the blades (or even in the outlet contacts themselves), very common, it could be enough to trigger your tester. From what I can see from the schematic, the polarity for the C100 DOES make a difference. In fact, wasn't it you that one time, that detected noise or interference on the C100 depending upon which way the cord was plugged in?