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 Help, I don't understand this crazy transformer!

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Bob D.



Posts : 13
Join date : 2010-10-13

PostSubject: Help, I don't understand this crazy transformer!   Sat Oct 16, 2010 10:35 pm

I recently posted about my shorted AMI jukebox transformer, asking if it was make of "unobtainium". The short appeared to be in the lower half of the HV winding. I removed the transformer from the chassis, and the short magically disappeared. I re-installed the transformer and the short reappeared. (This is with all the tubes removed, and the wiring carefully inspected.)
Now here's the weird part, I'm measuring 2.5 ohms from the transformer lead marked "BL" to the lower HV lead marked "R". The "BL" lead goes to ground. So the real short appears to be from the "BL" lead to the lower "R" lead. What the heck does the "BL" go to inside the transformer, and what does it do? Can I just leave the "BL" lead disconnected? I have proper voltage on all leads with the "BL" lead disconnected.



Bob D.
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Johnnysan



Posts : 64
Join date : 2010-10-12

PostSubject: Re: Help, I don't understand this crazy transformer!   Sat Oct 16, 2010 10:55 pm

The short may be occuring just under the outer wrapping. Sometimes you can cut it to get to the area where the wires are attached.
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Alan Douglas



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PostSubject: Re: Help, I don't understand this crazy transformer!   Sun Oct 17, 2010 10:21 am

"BL" could be just "blue". Maybe a shield between primary and secondaries?
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Bob D.



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PostSubject: Re: Help, I don't understand this crazy transformer!   Sun Oct 17, 2010 8:00 pm

Alan Douglas wrote:
"BL" could be just "blue". Maybe a shield between primary and secondaries?

Thanks Alan, I did some googling and found out it's an "electrostatic ground shield". Evidently, this transformer has a shield between the primary and secondary windings that reduces the capacitance between the two. This keeps transients in the primary from capacitively coupling into the secondary. A lead is provided to ground the shield to the chassis. I must have seen hundreds of radio and amplifier schematics in my lifetime, and this is the first time I've run into an "electrostatic ground shield". Maybe this is an "optional" feature since I never heard of it before. (Or do other transformers have this with the shield internally grounded to the transformer frame? confused )

Bob D.
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Alan Douglas



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PostSubject: Re: Help, I don't understand this crazy transformer!   Sun Oct 17, 2010 8:16 pm

It's common in high-quality transformers, or in any equipment that's sensitive to line transients.
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battradio@

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Join date : 2010-10-18
Age : 64
Location : Near ST. Louis

PostSubject: Re: Help, I don't understand this crazy transformer!   Mon Oct 18, 2010 2:41 pm

Has it ever been wet .
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Bob D.



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PostSubject: Re: Help, I don't understand this crazy transformer!   Thu Oct 21, 2010 5:19 pm

battradio@ wrote:
Has it ever been wet .

Boy has it ever been wet! Exclamation The jukebox it came out of was ruined in a flood. The owner gave me the amplifier for free. I have it running now (with the "electrostatic ground shield" lead disconnected). It sounds great. After a couple days of burn-in on the bench, I plan to replace my SS HiFi amp with it.

Bob D.
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tubes4life



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Join date : 2010-10-12
Location : northern Florida

PostSubject: Re: Help, I don't understand this crazy transformer!   Thu Oct 21, 2010 8:29 pm

Hmmmm....I'm wondering if one of the windings has shorted to that ground shield; that would explain why everytime you connect it, bad things happen.
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Bob D.



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PostSubject: Re: Help, I don't understand this crazy transformer!   Thu Oct 21, 2010 11:23 pm

tubes4life wrote:
Hmmmm....I'm wondering if one of the windings has shorted to that ground shield; that would explain why everytime you connect it, bad things happen.

Yes, when I had the transformer out of the chassis, I measured 2 ohms from one of the outside HV leads to the "electrostatic shield" lead. So now, back in the chassis, the open "shield" lead has 250Vac on it.

There doesn't appear to be any ill efects runnning it this way. Sounds great.

Bob D.
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gadget73



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PostSubject: Re: Help, I don't understand this crazy transformer!   Fri Oct 22, 2010 3:25 pm

Is the transformer case hot as well? If it is, thats kind of dangerous.
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