strange and dangerous problem

[phsyconoodler]

I just fired up an amp on the bench and it has an unusual problem.The chassis is charged with electricity!
  With no rectifier tube in the chassis is not 'hot',but put in the rectifier tube and it gets charged up.Now I've built a LOT of amps and never had this issue.I checked any possible connection to the chassis but can't find anything sloppy of any stray wires.
  I lifted the board thinking there might have been something underneath touching,but no.
Then I though maybe a shorted 5Y3.Nope.Tried a few and tested them.Then I lifted the ends of the filter caps off ground and measured DC voltage the same as the input DC.???? at the negative side on all 4 filter caps.WTF!!!???
  This is as simple an amp as you can build and it's giving me a headache right now trying to figure this out.It's a 5F2A.
What the heck is happening here?The PT is a new Hammond 291AX and no frayed wires or anything like that.
  This is odd and obviously incredibly dangerous.I have never had this happen before.All 4 filter caps???
 

 I have a wall socket that has a ground screw on the outside that I tested with.When you turn the amp on and measure from that ground to the faraday shield on the PT I get DC voltage.I'm thinking the PT is shorting out.Never had this issue with a Hammond.
  

[jjasilli]

I just fired up an amp on the bench and it has an unusual problem.The chassis is charged with electricity!

Yes.  I too was surprised with this problem with a brown Princeton 6G2.  Fortunately only 94VAC on the chassis!

The Jack Darr book I think is available online.  Safety check for all amps (even if operating normally) Page 47:  connect a 1500R 10W resistor from chassis > ground.  Turn on amp and read voltage across the resistor.  Reading should be near -0-.  Reverse the plug in an old amp.  Also use the line reverse SW if any.  2-3V is dangerous.  Full AC line voltage = a dead short.  

Causes:  a) shorted dead man cap.  b) PT primary shorted to core.  If b) replace tranny.

Anhyway, do you have AC or DC on the chassis?

[phsyconoodler]

What bugs me is it doesn't short to the chassis until the rectifier warms up.There is nowhere I've made any wiring error.

[jjasilli]

Something is shorting to ground somewhere. 

1.  Measure for AC, chassis to ground.  If no AC then then everhything from power chord > fuse > on/ff SW > PT should be OK.  (Except maybe for heater supply)

2.  You say you are getting DC on the chassis. If the DC on the chassis is about the same as the B+ at the output of the rectifier, then there is a dead short: at the rectifier or downstream in the B+ supply. 

3.  Are your filter caps in a can? It seems not.  If you lift the ground lead of a filter cap from the chassis, and there is DC on the ground wire, then the cap is shorted internally. 

4.  When all filter cap ground leads are lifted, is there still DC on the chassis?  Then discionnect the B+ supply to the preamp.  There's got to be a bare wire or solder bridge somewhere.  A step-by-step process of elimination should help find it.  If not, at some point suspect the OT.

[phsyconoodler]

Yeah I lifted all 4 caps off the board and they ALL have full DC voltage at the neg.ends.
  I am wondering about a weber pilot light holder I used.It is very loose.
I'll go inch by inch and see what I come up with.
  It's pretty hard to screw up a 5F2A board.

Update:I disconnected everything except the rectifier socket and there is full DC voltage on the faraday shield on the PT.Definitely shorted out.This is a personal first for me.Lucky it wasn't 725v!!!A mere 376v is a light jolt for me

[JayB]

Yeah I lifted all 4 caps off the board and they ALL have full DC voltage at the neg.ends.
  I am wondering about a weber pilot light holder I used.It is very loose.
I'll go inch by inch and see what I come up with.
  It's pretty hard to screw up a 5F2A board.

Update:I disconnected everything except the rectifier socket and there is full DC voltage on the faraday shield on the PT.Definitely shorted out.This is a personal first for me.Lucky it wasn't 725v!!!A mere 376v is a light jolt for me

hmm.... so there shouldn't be any voltage of the faraday shield?

[phsyconoodler]

No,because it's on the outside of the transformer.It's an RF shield made of copper.Voltage =electric shock to chassis.
 :violent4:

[JayB]

No,because it's on the outside of the transformer.It's an RF shield made of copper.Voltage =electric shock to chassis.
 :violent4:

It's just a copper shield wrapping the outside of the primaries and secondaries right?

[JayB]

I have 82Vac on mine?

[jjasilli]

???I don't get how a shorted PT could put DC voltage anywhere???

[phsyconoodler]

I don't either but it's doing it.Everything is disconnected from the PT except the rectifier tube.I get DC voltage on the PT casing.No connection to the filter caps.

[eleventeen]

Are all shorts created equal? No. If you take a sharp needle and touch it with modest gusto to a metal plate, it will conduct better one way over the other, as tested via reverseing ohmmeter leads. Typically in the needle to plate direction, but let's not quibble. You're already talking about a supremely weird condition (shorted copper strap around the tranny) ....maybe there's a short which "grew" from a conductor getting oxidized...perhaps by a wayward piece of acidy flux caused oxide crystals to grow on something such that it passed factory QA but became cancerous on your watch!

Certainly a highly strange problem!

[phsyconoodler]

Quote:"I have 82Vac on mine? "

  Ok feel free to touch it then.What the heck are you talking about?If you have 82vAC on the faraday shield on the OUTSIDE of the PT you have trouble.

  That's what I have,only it doesn't have AC,it has DC after the rectifier tube warms up.Short,short,short.

[JayB]

Quote:"I have 82Vac on mine? "

  Ok feel free to touch it then.What the heck are you talking about?If you have 82vAC on the faraday shield on the OUTSIDE of the PT you have trouble.

  That's what I have,only it doesn't have AC,it has DC after the rectifier tube warms up.Short,short,short.

Just trying to understand is all.

I disconnected the shield wire and it has 82Vac, playing around with a PT here.  I don't have any voltage on the outside of the PT.

The way I understand how they are made is that the shield is wrapped around the primary and that it's self can pick up ac voltage. The next shield is wrapped around the secondaries and that is the copper shield we see on the outside.

DC voltage would be something shorted outside of the PT. I wonder if DC is shorting out to the heaters on the rect tube?

[RicharD]

If either side of the 5V winding is shorted to the chassis, you'll get DC but what puzzles me is why no earth shattering kaboom since the chassis is earthed?  That's not one of those funky transformers with a 5vCT winding?  Very odd indeed.

[phsyconoodler]

I stand corrected.This PT doesn't have a faraday shield.It just has the copper foil on the outside of the PT.
  Either way it sucks the hind tit on this one.Good news is Hammond is replacing it for me.Hope the new one isn't wired like a terrorist bomb.

[tubeswell]

If it shorts with the rectifier tube plugged in, then the short is either in-or-after the rectifier (i.e. maybe a shorted rectifier dumping AC onto the filter caps - which they wont handle, and/or a shorted filter cap as a result), but also check the chassis ground connection, because it doesn't sound like the chassis is properly grounded (otherwise if the chassis was at ground potential, then either the mains fuse should instantly blow when the HT winding/B+ got shorted to the chassis/ground - or the PT should fry), so check for a faulty death cap etc

[John]

If it shorts with the rectifier tube plugged in, then the short is either in-or-after the rectifier

Is it possible that the rectifier socket itself is faulty?

[duke of earl]

I would disconnect the rest of the circuit from the rectifier tube and see if the problem still exists. If not, hook up a little at a time and hopscotch through the circuits until the problem shows up.

[jjasilli]

Yes, it is possible that some other issue fried the PT.  If so, then the replacement PT is on a suicide mission.  The DC on the chassis is still bothering me. 

[phsyconoodler]

Read my posts.Everything is unhooked except the rectifier socket.No tube,no shocky.Tube in,shock.Tried two tubes,both new.New ceramic Fender tube socket.

  When it gets loaded by the rectifier tube,then the shocks start.Again,both tubes are new and tested as well.Something in those windings are shorting internally when a little load is on the PT.

[phsyconoodler]

My whole house has no ground.That is NOT the problem.I took the transformer to work and hooked it up to a power supply and a rectifier.The case gets charged right away.This PT is done.
  After a couple of jump starts the past couple of days I'm ready for anything.

[bnwitt]

I used to have a friend who said that every time he got electrocuted it made him want to chase momma around the house.

[phsyconoodler]

Actually it seems to increase circulation for a while
 
 Hammond was really quick to replace this transformer.They aired one in and it's here now.It seems the 'upgrade' version is what I had.The laminates are supposedly top grade vs medium grade for the regular one.But the laminates are thinner.That's the explanation they have for the 'upgraded' unit.
  It's almost unheard of to have an issue with Hammond stuff,which would explain the instant damage control.
Kind of like Rolls Royce.If one of their cars breaks down,anywhere in the world,they fly out a master mechanic to repair it on the spot.They say their cars NEVER break down.
  I guess some lesser individual may have electrocuted themselves on this one.I'm like Bnwitt.Too stubborn to die.

[RicharD]

>time he got electrocuted

He means "shocked."  Electrocuted means death by electricity.  Not trying to be a douche, I'm just an electrical contractor and misuse of that word is a pet peeve.

-Richard

[phsyconoodler]

Maybe he was dead before and the 'electrocution' brought him back to life.Hence the increased libido.

[Tone Junkie]

Thank goodness phsyconoodler your were not really electrocuted, its a scary thought you could grab the chassis and all of a sudden your ground. But its funny how getting a good shock will get the old blood pumpin  

[tubeswell]

Getting zapped is nothing to be proud of - in this case it is a salutary lesson in thoroughly testing new equipment. From now on I'm never taking new PTs for granted (and its all thanks to physconoodler getting shocked  )

[Fresh_Start]

[A] salutary lesson in thoroughly testing new equipment. From now on I'm never taking new PTs for granted (and its all thanks to physconoodler getting shocked  )

Crap, I never thought to test for voltage on the chassis in a new build!  Thanks for the "heads up" Physconoodler!

The Jack Darr book I think is available online.  Safety check for all amps (even if operating normally) Page 47:  connect a 1500R 10W resistor from chassis > ground.  Turn on amp and read voltage across the resistor.  Reading should be near -0-.  Reverse the plug in an old amp.  Also use the line reverse SW if any.  2-3V is dangerous.  Full AC line voltage = a dead short.

Sounds like a great first step from now on - especially since I expect to be working on a couple of vintage Fenders in the next month or so.  BTW the "References" sticky has a link to Jack Darr's book, minus all of the nifty schematics.

Cheers,

Chip

[JayB]

[A] salutary lesson in thoroughly testing new equipment. From now on I'm never taking new PTs for granted (and its all thanks to physconoodler getting shocked  )

That's why I got a G10 board with a bunch of turrets lined up. I always test them before hand. Then once built, flip the switch real quick with the amp on a light bulb. Then test.

I'm a pansy after being shocked a coupled times.

[phsyconoodler]

I'm not afraid of electricity ever since I made a couple of cool projects when I was a kid.In school we learned about making a telegraph set using a lantern battery.Well being creative,I thought it would be better with it plugged directly into the wall.Nobody told me about insulating the tapper piece

  And the time I tried to recharge my own D cell batteries with my Train set transformer.It actually worked.That is unless you left them on too long and the batteries became bombs.When one blew up in my face I learned fast. :bom:   I made lots of underground charges for my table-top train and war games setup.

 I guess I'm just a dumbass at heart.

 

[phsyconoodler]

New transformer is in and it works!

  Lesson learned to check everything even on a new build with new parts.

[jjasilli]

congrats!