MR 67 w/ ac voltage on chassis

[Russellc]

While fiddling around the other day, I accidentally touched my knee against one metal component while attaching an interconnect to the preamp. I could feel a slight "tickling" of electricity at the knee and fingers. I traced it to the MR 67 having a nice 120 volts a/c on its chassis, which apparently went along the ground side of the interconnect to the chassis of the preamp, through my body, out the knee, to the chassis of the power supply my knee was touching. What the hell is causing this? Any answers are welcome. Disconnecting the MR 67 tuner removed the problem, and when plugged in but not connected to anything else, the 120 v A/C is on the tuner's chassis.
Thanks,

Russellc

[ron-c]

You may have a power supply capacitor which is leaking to the outer case of the can. I would check the unit under the bottom cover and look for any obvious AC paths.

Ron-C

[dewickt]

MR67 has a pair of .01uF ceramic capacitors across each leg of the line cord to the chassis, can cause about 60V to apear on the chassis at a very low (safe) current, if one has been hit by a big surge it may be shorted or have some resistence, pull plug and check each side of line cord to chassis with ohm meter, should read open, if not start with these two caps as suspects.

[Wardsweb]

Terry, as always you are a wealth of information. Thanks !

[Russellc]

Thanks for the input, I have heard from others about these capacitors on the line cord....one question, if both are out, would it produce 120 v. on the chassis insted of 60 v ? I definately measure 120 on the chassis! You're right about the puch of it, just a slight ticking could be felt.

Thanks, Russellc

[dewickt]

Yes it could go up to 120V, if the capacitor values are un balanced, they are only 20% caps and can have a wide mismatch (not critical). I would not worry about this, they are good at cleaning up the noise from the AC line and afford some protection from spikes.

[Russellc]

i,m not sure what you mean by not to worry about it, do you mean to go ahead and change out and don.t worry about what voltage is on the chassis, or not to worry about the voltage on the chassis? The reason I ask is that it was causing problems with the preamp,as it (the 120v) was traveling through the ground side of the interconnect, placing the 120v on the chassis of my AVA super pas iii preamp, which in turn was making its phono stage do some occasional strange things like an occasional howl sound added to the sound a couple of times. Removing the tuner cures this problem. The other problem with the MR 67 is that during warmup, volume occasionally jumps up or down a little as the unit warms up. Once hot, not a problem.
I,ve been meaning to measure the b+ when this happens, but haven't got around to it. Should replacing the line cord caps cure this voltage on the chassis problem?

Thanks again,
Russellc

[dewickt]

I would do some resistence measurements before changing the caps, no harm in changing them with known good ones.
Time for some troubleshooting, does your preamp have a 3 wire cord, have you tried a ground lifter on it, sounds like you have some ground loop problems, much of the older equipment with 2 wire line cords do not like to be tied into a system with 3 wire cords, this type of internal line filter can cause a ground loop with lots of Hummm.

[Russellc]

Hello again, no none of my equipment has three prong plugs, dyna pas pre amp w/ AVA super pas III rebuild, Dyna Stereo 70 with the welbourne rebuild, another st70 w/ triode electronics board ( apart right now) a mcintosh mc 225, and a large push pull KT88 amp I built, other tuner is dyna FM3, all two prong, including CD (rotel) and turntable (AR ES1). For the most part, pretty much restored/upgraded vintage. Wouldn't a ground loop[ cause a hum? I just thought that since the chassis of the tuner is connected to the ground, as is the ground portion of the female rca socket, so of coarse could travel around via the negative leg of the interconnect, which would also be in contact with the chassis and the 120 vac. With all components up and running, I could also measure this voltage on the chassis of the preamp, until I narrowed it down to the MR 67. Until I removed it from the system, then all is normal. All components being used go into a single 10 outlet power strip/surge protecter. There is no unusual hum, just this voltage. I believe it is your idea about the line cord caps. seems like it would be easy enough to test, wouldn't just temporarely removing them rid the problem? (assuming that is the cause)

I appreciate the help,
Russellc

[EchoWars]

If the cap changing becomes necessary, be sure to use mains-rated caps (safety caps).

If it becomes a battle, you could always just add a three-wire power cord and be done with it. :D