Power Transformer leaking question for Bassman AB 165

[justsoldern]

I have an original Bassman head AB 165 on the bench. It used to shock people so the owner had the death cap removed and a 3 prong plug put on. Doing some maintenance - Seems to work ok, but I suspect that the power transformer might have some leaky AC still happening. I'm not sure if it's more than normal. It can still tweak you if not plugged into a grounded outlet.

So here are the symptoms. if I use a three to two ground lift adapter plug and defeat the ground to the house wiring I can read 60 volts AC from Chassis to the house ground. If I reverse the ac plug I get 4 volts AC to the house ground. I've tried measuring the amp draw from chassis to house ground in both plug configurations and I don't see any amperage using my Fluke 115. So no real dangerous amperage.

So my question: Is this normal? or do I have a transformer that is leaky on the primary side and should be replaced or is it on it's last legs?

If I measure resistance from either the hot or neutral of the power cord to chassis I get very high resistance (many megs)

[Chuck H]

First make sure it's power transformer (it is, but...). Disconnect the HV at the rectifier and do the same voltage test. Still there? Disconnect the filament secondary. Still there? Disconnect the primary, but leave the earth ground. Still there? I only mention it because the voltage drops out when you reverse the plug. A coil is a coil and it doesn't know what polarity is so it could be that there is something wrong with the primary connection??? The fuse socket??? Some wiring error? If it's definitely the transformer replacing it is the only responsible thing to do.

[Tom Phillips]

To give you additional perspective do the same test on a "known good" amplifier. Also keep in mind that capacitive coupling can play tricks on you when you make voltage measurements with a high input impedance meter.

...It can still tweak you if not plugged into a grounded outlet...

Is that when using an outlet that has been checked for correct Hot/Neutral wiring connections?

[Chuck H]

And... Has the AC end plug ever been replaced? Maybe there's earth/neutral contact in there.

[Tom Phillips]

And... Has the AC end plug ever been replaced? Maybe there's earth/neutral contact in there.

And maybe the cord itself is defective. I once received a batch of new power cords that had the hot lug of the plug connected to the green wire. Didn't find out until after it was wired to a chassis. Since then I always verify by doing my own buzz out before I install the cord. Ya can't trust anything!

[Justin Thomas]

Hey Tom,

"Buzz Out?"

I <DO> always check the isolation with my meter before I send an amp out the door with a new power cord...

Justin

[J M Fahey]

I´m not worried by the "floating" 60VAC measurement by itself, because a multimeter may have 10M input impedance, even a few hundred picofarads between primary and secondary or transformer core will show "something" and yet not be a problem at all.
In fact in many modern amps and supplies, important capacitance is added through the X or Y rated mandatory caps, on behalf of interference elimination.
But your "many megohms " comment is something else.
In fact anything above 100k or so will kill nobody, but I fear nonlinearity and those many megohms becoming WAY less under higher voltage than the puny 2 or 3V DC the multimeter applies, that´s why oooolllldddd plain resistance measuring between primay and secondary later required a "megger" , same thing but applying some 200 to 500V (the original generator was a modified hand cranked telephone ringer ) which evolved into today´s HiPot test, with chilling 1500 to 3000V applied between primary winding and chassis or transformer core.

This very simple one (there are some way more elaborate) applies 1000V DC to test probes and has meter with a full scale current of 200uA (since 1000V and 10M are half scale)


so with such data I guess I will design and build my own, for personal use of course.

There was an old British project in the 80´s, I guess it was in ETI magazine or a similar one, showing a Megger project, it used a then impossible to find (in Argentina) ferrite cup core to get some 500VDC from a PP9 battery, if I remember well.

If some British old hand happens to have a yellowing copy in his stash, it would be nice to see it; if not I´ll have to start with a blank piece of paper.

Of course, its calibration will hold no "official" value whatsoever,

[Tom Phillips]

Hey Tom, "Buzz Out?"...

Justin,
"Buzz Out" is an old term from a time when continuity checking was done with a device that made a buzzing sound when there was low resistance (I.e continuity) between the test leads. The device was used to verify connection cables and such. These days people use their DMM in the mode that "beeps" when there is continuity between the test probes.
Tom

[Chuck H]

So the modern slang would be "beep out"? Not workin' for me.

[Justin Thomas]

Chuck,

Well, I <do> swear every time I get a "bleep" instead of a reading because I forget to click my meter one notch past continuity... so I guess it does work out after all..

Tom, thanks for the explanation! I mighty have to look for one of those Buzzout-ometers... I'm going backwards in every other technology...

Justin

[J M Fahey]

So the modern slang would be "beep out"? Not workin' for me.

HEY !!!!!!!! , what´s YOUR problem with my

sound, huh?????????

[Chuck H]

Ok, that will actually be pretty cool. When more of our bench test gear is interfaced with modern information technologies and we can program personalized sounds and settings.

[g1]

Ok, that will actually be pretty cool. When more of our bench test gear is interfaced with modern information technologies and we can program personalized sounds and settings.

Whaddya mean modern? Plymouth had that tech back in '69

[TimmyP1955]

Such high voltages are common. At issue is whether it can deliver any current.

In the old days, Hammond (organ) had go/no go test for leakage:

Connect a 10K resistor and a .01uF cap in parallel.

Connect one end of the R/C to a known good earth ground, and the other end to the chassis of the device under test.

Measure the AC voltage across the R/C using a standard AC voltmeter.

If you get more than 4V, replace the power transformer.

[J M Fahey]

So max permissible leakage was 0.4mA under mains voltage???

Not a bad standard ... at all.