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Old 28th July 2006, 06:18 PM   #1
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Question Why this resistor?

I have a Yamaha "Natural Sound" CT-610 tuner. It is an analog unit from the 70s. I was doing a cleanup on it when I noticed that there is a resistor (measures about 2.2 meg) going from a line voltage wire to the chassis. The power plug is non-polarized 2 wire, typical from that era. The power switch switches the transformer secondary... The transformer is always energized.
Being an Electrician (but not an electronics technician), I chopped off the cord cap and replaced it with a 2-wire polarized plug, making the wire with the resistor attached always plugged into the building neutral (which is always grounded).
My questions.. Why bother putting in this resistor at all?
Why switch the xfrmr secondary and not the primary?
The Yamaha engineers must have had a reason.. but what? Maybe because this was high end gear, it was more to do with niggly stuff.
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Old 29th July 2006, 10:33 AM   #2
cpemma is offline cpemma  United Kingdom
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So the 'resistor' is in parallel with the transformer primary? You sure it's an ordinary resistor and not a varistor for suppressing mains spikes?
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Old 29th July 2006, 11:40 AM   #3
SY is offline SY  United States
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Back in the '70s, three wire plugs were not very common. Today they are and they're MUCH safer- and unless the tuner has a double-insulated case, they're required (as an electrician, you're probably more familiar with this than most).

I'd update it to current standards. That means three wire cord, green wire FIRMLY attached to chassis, and the transformer primary totally isolated from ground.

And keep the 2.2M in a box somewhere as a souvenir. BTW, what did it look like? It's possible that it was an MOV or something like that.
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Old 29th July 2006, 10:35 PM   #4
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Hi cpemma and SY.
Here is a pic. There are more pics, including inside shots of newer models at
http://audiokarma.org/forums/showthread.php?t=75738
I'm not sure if non-members can see the pics though...?
The newer models have a resistor and an mov (or a cap- I can't tell) connected to the chassis, the older "non II" models just have a resistor. I was worried that in a catastrophic situation, the chassis could become highly en-live-enated and en-jigglefy anyone that touches it.
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File Type: jpg yamaha-ct610-inside01-closeup.jpg (46.5 KB, 348 views)
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Old 30th July 2006, 01:44 AM   #5
SY is offline SY  United States
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That's an excellent worry. Three wire! Green one to the chassis with a star washer, well torqued down. Resistor chopped out and mashed to bits under repeated hammer blows.
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Old 30th July 2006, 05:03 AM   #6
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I'm not an electronics tech, so my worry with adding a ground wire would be ground loops. Not even new equipment has 3 wire cords... they just use standard polarized 2 wire cords like incandescant lamps. (In the old days, you could get a shock just changing a lamp's bulb if the screw shell was hot. You could plug it in any old way. There was no upside-down). I guess the new equipment is electrically isolated from the mains. To me, the wonderment comes from the fact that tuners aren't like tape players... They need antennas. That involves grounding. My head is spinning. javascript:smilie('')
xeye
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Old 30th July 2006, 05:45 AM   #7
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my understanding of that 2.2 MegOhm resistor is to supply a discharge path if the chassis voltage floats above the line voltage due to static buildup and what-not. If there is no other leakage path from your perhaps floating chassis, there is a danger that static electricity could arc through the varnish on your transformer windings. The 2.2 Meg resistor bleeds these currents off to the AC grid. If you decide to wear rubber shoes and comb your hair, then touch the volume knob your generated static will be bled through the resistor and not through a hole punctured in the transformer winding varnish. ...at least that is my understanding.
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Old 30th July 2006, 12:14 PM   #8
SY is offline SY  United States
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New equipment with two wire power cords will be double insulated. Your chassis is not! Metal chasses will always be grounded and used with three wire cords.

Ground loops are a separate issue; the three wire system just provides a safety ground. Your signal ground can be tied to the chassis ground at one point. If ground loops develop, the signal ground can be lifted from the safety ground via a small (20 ohm) resistor to break up the loop. Do NOT lift the safety ground under any circumstances.
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Old 30th July 2006, 12:50 PM   #9
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"...why switch the secondary and not the primary?". That's a quizzler. . I don't think it would have been a question of money. A difference in switch contact rating for current and flashover shouldn't be an issue. They were already switching the primary's on their receivers. Maybe they couldn't get in under the wire for UL or CSA certification?? Perhaps it reduced certification headache to just hardwire all of the line side and switch the isolated side? I think putting on the polarized plug with the resistor always on the neutral side is not a bad idea. You'll measure less chassis to earth voltage than you would get if the plug was flipped (assuming the resistor is the dominant leakage path). I don't think there would be any impact on any "ground loop" related issues, but your fingertips would know the difference if you were grounded and lightly touching the chassis.
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Old 30th July 2006, 06:24 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
I am real glad I live in the UK.
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