Wurlitzer 120 Hot Ground When Using Line Out

[humphB]

Hello everybody!
I have a really annoying issue maybe someone can help diagnose.

I have an old 120 I found on the sidewalk in LA (seriously) and it had obviously been somebodies guinea pig for electronic experiments. The amp doesn't match the wiring schematic completely, so bear that in mind.

Any way on to the issue...

Whenever I use the headphone or speaker outputs I get a serious hum (50/60hz?) in both the onboard speaker and the amp I'm plugging into. The piano is very quiet otherwise. I can use the headphone jack for headphones alone and it sounds great. It's only when I plug into something that is also powered that the problem occurs.

Now the kicker is that when plugged into an amp, all of the components connected to the ground circuit on both the piano and amp become dangerously "hot", as in charged electrically. That is, the chassis, cable ends, screws, etc.
When plugged into a DA converter I get clip lights in ALL channels and the shock problem. Safe to say I didn't try that for long...

Thanks!

[4kinga]

I have an old 120 I found on the sidewalk in LA (seriously) and it had obviously been somebodies guinea pig for electronic experiments. The amp doesn't match the wiring schematic completely, so bear that in mind.

Be afraid.  Very afraid.

Check it out, or have it checked out.

Whenever I use the headphone or speaker outputs I get a serious hum (50/60hz?) in both the onboard speaker and the amp I'm plugging into. The piano is very quiet otherwise. I can use the headphone jack for headphones alone and it sounds great. It's only when I plug into something that is also powered that the problem occurs.

Sounds like a ground loop

Now the kicker is that when plugged into an amp, all of the components connected to the ground circuit on both the piano and amp become dangerously "hot", as in charged electrically. That is, the chassis, cable ends, screws, etc.
When plugged into a DA converter I get clip lights in ALL channels and the shock problem. Safe to say I didn't try that for long...

Does your power cord have keys on the legs (neutral is wide, hot is narrow)? On my 145a, if I reverse the power cord, I don't get any hum.  (Been meaning to change it to a keyed plug, but.....)

[humphB]

You got it! Thank you so much. It's always nice when it's a simple fix. My power cord was NOT keyed and turning it around fixed it like a charm. At least the shocking part that is. Now to fix the ground loop...

[Tim W]

Hi,

If you take a look at the schematic for the 120 (available out there on the web) you will notice that there is a .1uF cap between one leg of the AC mains and ground.  In the 120, the amp chassis is this ground.

This cap is probably shorted or going bad and needs to be removed for safety reasons.  To be frank, even if it isn't going bad, it should still be removed.  You can do it yourself if you can properly read the schematic, trace it, and identify it under the chassis.  You are working with very high voltage tube stuff here so if you do not know how to work on tube equipment safely (discharging HV caps, etc.), *don't do it.*  I would recommend having it professionally serviced if this is unfamiliar territory.

If the piano was used as someone's science fair project,  there could also be an unintended short somewhere else in the chassis causing this problem.

Here is why (if you are interested):

In the early days, it was common to put these caps on tube and other electronic equipment as it was there to help shunt RF interference.  Nowadays, this is considered unsafe (for the reasons you discovered), unless it is a certain type and value of cap rated for this safety application and the chassis is earth grounded (which it isn't with only a 2 prong plug).  In your 120, this cap is probably shorted or working improperly and inducing line voltage onto the chassis.  This is why you are getting zapped... especially when AC 'hot' is connected to that side of the transformer and cap. 

When you flip the plug, you are reversing hot and neutral (which 4kinga noted).  Since neutral is 'close' to earth ground you won't feel the shock (it is connected to earth ground in your breaker/fuse panel but floats a little above ground because it is the return path for AC line current to the panel).  Imposing line (or neutral) into your sensitive audio circuits through a shorted cap will induce the hum when the other equipment is properly grounded and expecting signals referenced to earth ground (as most modern day equipment does). 

It should also be noted here that '3rd prong' ground is earth ground and is NOT supposed to carry current in normal circumstances-- this way ensuring that it IS at earth ground potential at your wall outlets and the chassis of whatever devices are using the 3rd prong.

Again, a lot of older electronic gear has these caps between the line and the chassis (ground) that cause all sorts of shocking problems.  In your case, if removing the cap does not fix the problem, then I would look at ground loop issues.

All the best,
Tim
Retrolinear Inc (formerly Keyboard Cottage)
www.retrolinear.com

[4kinga]

/\
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What he said.
Much more eloquently spoken than I. :-)

Tim,

It should also be noted here that '3rd prong' ground is earth ground and is NOT supposed to carry current in normal circumstances-- this way ensuring that it IS at earth ground potential at your wall outlets and the chassis of whatever devices are using the 3rd prong.

Would cutting all "grounding" caps to chassis, then installing a standard 3prong (IEC 60320 C14), grounding the chassis to the earth be the proper procedure?

[humphB]

Thanks Tim, that was very informative!

Just so we're clear: Should I replace the cap in question (with one that has the right specs) or just plain remove it?

[pianotuner steveo]

If you are replacing the cord with a 3 prong, I would remove the cap in question and connect the third (green) wire to ground after being sure there is no other stray voltage going to ground.(the chassis)  I would not remove the OTHER caps that connect to ground as 4kinga asked, but be sure they are good. If in doubt, replace them, because they are old and it will not very expensive.

Be 100% sure that you know what you are doing if you are replacing the cord with a 3 prong so that the hot and neutral wires are not reversed again.

[Tim W]

All,

I am certainly not trying to be argumentative, but for safety reasons the capacitor should be removed (cut out) completely from the circuit.  No product designed today could be brought to market with a cap between AC line and an ungrounded (un-earthed) chassis.

Only the line-to-ground capacitor should be removed.  My Wurly 120 schematic shows a single such cap, but in some other old electronic equipment, you may find 2 'line-to-ground' caps (one from each side of the line to the chassis (ground)).  In this case, BOTH should be removed.

The original ceramic or film capacitors used between the AC line and chassis on older equipment are not rated for this kind of application.  If they should short, they can allow potentially lethal voltages to appear on the equipment chassis.

In modern day electronics, a cap that needs to go from line to earth ground must be "Y" safety rated.  This kind of cap cannot short out and will self heal in the case of an overvoltage transient.  However, even a safety rated capacitor can only be used when the appliance is earth grounded via the '3rd prong' .

You can read about this here:
http://www.kemet.com/kemet/web/homepage/kechome.nsf/file/KEMET%20Kollege%20Presentations/$file/EvoxRifaRFIandSMD.pdf

The first couple of slides have the useful information... the rest is marketing gibberish for engineers/designers.

If you wanted to go a step further towards improving the safety of your unit, you could replace the old 2-prong socket on the amp with an IEC connector (which I think is a good idea).  For the IEC connector to be effective, its earth ground pin must be connected to the chassis.  And, if you go through the trouble of installing a 3 prong plug, never defeat it by using a ground cheater adapter on the AC plug!

Again, the cap may not be the only or main problem.  Other possibilities are a short in the power transformer, AC wiring touching the chassis directly in the amp itself, a short within the wiring to the power switch (cracked insulation), a short in the power switch endblock, etc.

Just to check to be sure, after cutting out the old cap and with the piano unplugged from the AC mains, take a multimeter and make sure there is no continuity between either side of the AC line and chassis.  If the meter reads open for each 'prong' of the AC plug with respect to the chassis (test leads between the chassis and each prong of the 2-prong socket, one at a time), you are OK.  Do this test with the piano unplugged from the wall, and the power switch in both the on and off positions.  Make sure you have the multi-pin connector that goes to the power switch and volume controls plugged into the proper socket on the amp chassis while running these tests.

To sum it up:
1.  Cut out the old line-to-ground cap.  No exceptions for safety reasons.
2.  Check the continuity of each AC line terminal/prong to chassis with the power switch on and off (with piano unplugged from AC power).  It should always read open.  If it doesn't you will need to track down and eliminate the short.
3.  Leave out the cap if you leave it 2-prong.
4.  If you add an IEC connector and ground the chassis to earth ground, you can either:
    a) leave out the cap (recommended)
    b) replace it with a new "Y" rated safety cap of the same value.. but never defeat the earth ground.

Finally, as always, if any of this is unfamiliar or uncomfortable territory, and you don't understand 100% what you are doing, *don't do it.*

Tim
www.retrolinear.com

[adcurtin]

Tim is it common for ceramic caps to short out? is there anything specific that could cause this? I'm having some significant problems with my amp, and I've pretty much replaced everything but ceramic caps, resistors, and diodes.

[pianotuner steveo]

Good stuff to know,Tim.  In the past I replaced a few of those caps, I will now just remove them.

Someone misinformed me once and told me what I posted above about replacing the cap.

*I edited the previous post to remove any confusion*

[Tim W]

Glad I could help clear things up.

Generally speaking, ceramic caps are not unreliable.  They typically last a long time when operated within their ratings but they can sometimes crack due to mechanical stress, and even become microphonic.  Ceramic caps, depending on the type, can also have large temperature coeffcients (their value changes with temperature).  I would not expect them to fail due to electrical stress in a circuit like the 200A amplifier, but failures are always possible.

Ceramic caps across the AC line, however- that is a different story.  The power coming out of your outlets (in the US) is almost never a crisp clean 60Hz sine wave.  There are transient spikes, noise, surges, and a multitude of other things riding on the wave.  Line noise has gotten worse with the mainstreaming of switching power supplies and non-incandescent lighting.  These transients are what slowly degrade these ceramic and old film line caps.  They were not made with self healing properties, and eventually they can short out.  Caps across the AC line must survive some of the toughest conditions, and the old ones will eventually break down.  This is why there are new standards for these "X" and "Y" type safety caps... they build them as bullet-proof as possible.  Of course, as a result of this, they are large and expensive compared to a standard ceramic or film cap of the same value and voltage rating.

Tim
www.retrolinear.com