60VAC on Chassis of B3

[bcmatt]

After about 10 months of an emotional roller coaster, a friend/colleague/bandmate of mine acquired a B3 and 122. Up till now he has been using my M3 and 142 (this is relevant because they may be helpful for troubleshooting).

Anyways, the B3 only has the 2-prong power cable. With vintage guitar amps we just measure the chassis ground of the amp to actual ground to see if it carries a charge. If so, then it becomes vital to solve that (usually with the grounded power cable). I measure the difference on ground from this B3 amp and the actual mains ground and got about 60VAC. Also, we do have an annoying buzz coming out of the Leslie that gets gets louder with the Leslie volume being turned up.

We have recapped the B3 already. We haven't gotten to the 122 yet, but will soon. However, his 122 works just fine with no buzz when used with my M3, so I assume it is coming from his B3. We were hoping the cap job would have resolved this, but it hasn't...

So, I guess my first question is regarding this 60V on the chassis. Should I be trying to do a modern-style grounding? Is there a Death Cap typically in the B3 that perhaps should be replaced? I'm going to try and re-dig up the B3 schematic... but a little direction on this matter would be very helpful. I don't want to assume this is exactly the same as working with guitar amps, because the player is not necessarily directly touching anything conducting directly to this chassis...

Thanks guys!
Matt

[JoeyB3]

Should I be trying to do a modern-style grounding? Is there a Death Cap typically in the B3 that perhaps should be replaced? I'm going to try and re-dig up the B3 schematic... but a little direction on this matter would be very helpful. I don't want to assume this is exactly the same as working with guitar amps, because the player is not necessarily directly touching anything conducting directly to this chassis...

This question pops up quite often. Most people say, "NO" to modern grounding. The best response I found came from forum member, sfp1954:

"The AC and DC sections of this equipment were designed to be independent.

Once you get past the rectifier everything is DC with a floating ground, ie there are no connections/pathways between the DC chassis/ground and any AC components.
Therefore there is no reason for the DC ground/chassis to be at the same potential as the AC ground in your house.
DC voltage is a measure of potential difference between two points (B-A). There is no rule that says A must be at zero volts relative to anything else.
By grounding the chassis to your earth ground you are changing the design of the equipment."

[enor]

With vintage guitar amps we just measure the chassis ground of the amp to actual ground to see if it carries a charge. If so, then it becomes vital to solve that (usually with the grounded power cable).

Wouldn't that be more like "masking" the problem than "solving" it?

[muckelroy]

Does the issue persist without the Leslie connected to the B-3?

[Wes]

Have you performed the first diagnostic test in the service manual?
Have you tried flipping the power cord around?

[UncleClark]

I measure the difference on ground from this B3 amp and the actual mains ground and got about 60VAC.

It is important to measure the voltage to ground with the setup shown in the service manual which includes a resistor and capacitor. Modern voltmeters have an extremely high input impedance, which makes the resistor and capacitor even more important to get an accurate leakage assessment.

Did you make the measurement with the setup shown in the service manual?

Some Fluke voltmeters (and probably other) have a "Lo Z" setting on AC voltage, you can use that setting if you want. But the standard AC setting has too high an impedance for this measurement.

[muckelroy]

Above is all true and important.

If it only occurs when the Leslie is connected, check the Leslie amp for "death caps". Some searching around the forum will show you what those are and what they may look like. If your Leslie has these still, clip them out. They could be replaced with X1Y2 safety-rated caps, if radio / AC interference is an issue in that location.

[David Anderson]

I measure the difference on ground from this B3 amp and the actual mains ground and got about 60VAC.

If you're measuring this from an ungrounded chassis to earth ground with a standard AC voltmeter, this measurement tells you absolutely nothing about whether or not there's a safety issue -- on Hammonds or vintage guitar amps. It's perfectly normal for there to be a floating AC voltage on the chassis of a two-wire amp or preamp that will usually change if you flip the plug polarity; it's an effect caused partly by asymmetries in the secondary windings. The critical question is how much current can flow from chassis to local earth ground from power transformer leakage.

There's a test at the front of the Hammond Service Manual for measuring current leakage. This is the test that should be used for Hammonds and vintage guitar amps prior to earth-grounding because earth-grounding will simply mask a leaky power transformer. It will not fix it. There are cases, particularly with older Ampeg amps, where earth-grounding will cause the fuse to blow due to high power transformer leakage.

Hammond did not use death caps in the AO28 preamp. Some Leslie amps have them, but some don't. It depends on the year the amp was made.

[indianajo]

The service manual so often quoted is on archive.org. It is called Hammond_service_manual_A_A-100_BA_BC-BV etc etc etc
archive has a search function to find it.
If you don't have a pdf viewer, you can download one from irs.gov that is where I got mine.

[enor]

earth-grounding will simply mask a leaky power transformer. It will not fix it.

"The hull of my boat has severe rust damage, and large amounts of water is getting in. But instead of welding up the holes, I fixed it by installing a large bilge pump."

Sort of. :)

[UncleClark]

"The hull of my boat has severe rust damage, and large amounts of water is getting in. But instead of welding up the holes, I fixed it by installing a large bilge pump."

To me the analogy would be :
"My boat's never going to leak, so why bother installing a bilge pump?"

Earth grounding is a safety precaution to keep you from being killed if the console ever does develop a current leak. There is no reason you can't disconnect the ground once in a while, perform the leakage test, and then reconnect the ground.

[enor]

To me the analogy would be :
"My boat's never going to leak, so why bother installing a bilge pump?"

Earth grounding is a safety precaution to keep you from being killed if the console ever does develop a current leak. There is no reason you can't disconnect the ground once in a while, perform the leakage test, and then reconnect the ground.

The question wasn't whether an earth ground is good protection or not, it is whether it is to be considered a "leakage fix" or not.