RC-M70C voltage on chassis when plugged in

[Transistorized]

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?

Oh yes. That was my Quasar GX-3612. It has a fixed non polarized power cord and a manual AC/DC selector. A while ago I noticed this by pure coincidence on a quiet evening and that box started the polarity awareness war in my home. My C100s seem to hum equally....which actually annoys me because I don't know if I have them connected right without any indications. Might also be the condition of the electrolytic filtering caps in the Quasar. Not sure. Probably not...as my guess is failing electrolytic filtering caps would probably hum badly in either direction.

Is the resistor in question on the M70 the 2.2M R681?

 

[Superduper]

Yup. Remove it and let us know what you discover.

 

[dan]

Composition - Composition resistors are probably the most common resistors used and are made by combining a resistive material, such as carbon, with a binder. The binder is used to hold the carbon together so that it can be molded or formed into various desirable shapes.

Resistor failures are considered to be electrical opens, shorts or a radical variation from the resistor specifications. The failure modes experienced vary with the type of construction. A fixed composition resistor normally fails in an open configuration when overheated or overly stressed due to shock or vibration. Excessive humidity may cause an increase in resistance.

from some info i found from 1978 : https://www.fda.gov/iceci/inspections/inspectionguides/inspectiontechnicalguides/ucm072904.htm


is that why they made sure to specify that R681 is a 2.2M "comp" resistor? even though "a fixed composition resistor normally fails in an open configuration", i don't think normally is good enough. sometimes things that are not normal happen.

 

[dan]

i disconnected a black wire that comes from the main circuit board and connects near the transformer. no matter which way you plug it in now, there's only around 12vac from the chassis to earth ground and the leakage current is 0.00uA. i checked continuity between each pin of the plug and the chassis and the resistance is infinity, there is no continuity. it seems like this disconnecting of the wire has essentially taken R681 out of circuit. i think the 12vac from the chassis to earth ground is some kind of magnetic or capacitive coupling somewhere. there is no current or continuity .

 

[Transistorized]

Hey Dan. Is there any change in performance or noticeable drawbacks such as humming etc.?

I guess it would help more to understand why it is they did this...and if this was/is a common practice.

 

[dan]

Hey Dan. Is there any change in performance or noticeable drawbacks such as humming etc.?

I guess it would help more to understand why it is they did this...and if this was/is a common practice.

don't notice anything bad so far. also, my left speaker doesn't pop anymore when i shut the unit down. i was gonna replace the main filter cap because of that issue but now i don't think i will. C520 had hardly any ESR when i tested it anyway, unless the ESR is hidden by other parallel caps.


also, to update, with batteries installed and the unit plugged in to the wall, chassis voltage to earth ground is around 15 volts ac / 0uA. if i connected a bluetooth adapter to line in, chassis to earth ground is around 18 volts ac with about 20uA of current.

 

[Superduper]

Probably better to disconnect the other end (see image) because it appears that R681 is still "live" as it terminates at the screw, which is connected to the tin wire wrap, which apparently also happens to wrap around a bundle of wire. Why guess or take a chance for any capacitive or inductive coupling or EMI etc.

Even better yet, just remove the R681 pigtail entirely. A pigtail, connected to 120vac for no reason, would probably behave like a 60hz mini antenna.

Don't think a simple wire like that operates as an antenna? Let's just say that whenever I need to inject a RF signal to the tuner for AM or FM tracking adjustment, a simple wire or loop placed near the tuner is all that's necessary for the tuner to receive the signal.

Or put that pigtail back and note that the circuit will eventually be connected to the front panel trim piece. Transistorized's handheld no-touch tester is visual proof that it is sensing some transmission or radiation, probably 60hz. I'm presuming that the screw wire-wrap terminal in the photo is the same as the one in the diagram.

 

[Sonic]

My guess: it's antistatic resistor. Can be of any help in areas with dry air.

 

[dan]

i've noticed that with or without that cable connected, i get a humming sound on the speakers when turning the unit on and if something is connected to the line in. even if the volume is down and the "line in" is not selected. doesn't happen all the time, like one out of every five turn ons. sometimes it happens right away, sometimes it develops after a few minutes. a power cycle usually clears it up.

 

[Superduper]

Wow, you sure got a lot of questions about this M70. When this is all over and done with, you should be an "expert."

As for your humming, just do a thorough cleaning of all the switches and controls, and the record bar. You might be pleasantly surprised. This is especially true if the issue you are having is sporadic.

 

[dan]

got the 12VDC adapter today and would you believe with that hooked up, there's about 50VAC at 70uA from the earth screw on the chassis to earth ground!

how this is even possible? i'm assuming there's galvanic isolation between the AC/DC 120VAC to 12VDC transformer so even if the DC12V backfed to the transformer in the M70, current shouldn't be flowing between the m70 and earth ground because of isolation.

 

[Fatdog]

 

[Transistorized]

So far mine has been behaving...and that's the way I like it.

Hopefully you will get it all sorted.

 

[Superduper]

Don’t presume anything, check. Just because it’s a DC adapter doesn’t mean anything, it’s still plugged into AC.

 

[dan]

Don’t presume anything, check. Just because it’s a DC adapter doesn’t mean anything, it’s still plugged into AC.

i checked the specs of the ac/dc adapter and it states it has a 0.250mA max leakage current. so much for that experiment. when i get my belts and open the m70 up again, i'll disconnect that black wire again and see if that 50v disappears.