Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems.

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Old November 3rd 18, 05:43 AM posted to sci.electronics.repair
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Default Variac question

Purchased used Variac. Good action, very smooth from 0 to 140V AC.
Question: Input and output are both 2 prong, no ground. Should I modify it to three prong with ground?
BTW, I do also own a functioning isolation transformer.
Should the IXformer be placed between the mains and the Variac? Please advise as to best procedure. Thanks. Main use of Variac will be to power up ancient tube equipment.
Thank you!

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Old November 3rd 18, 06:20 AM posted to sci.electronics.repair
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Default Variac question

On 2018/11/02 9:43 PM, Ivan Vegvary wrote:
Purchased used Variac. Good action, very smooth from 0 to 140V AC.
Question: Input and output are both 2 prong, no ground. Should I modify it to three prong with ground?
BTW, I do also own a functioning isolation transformer.
Should the IXformer be placed between the mains and the Variac? Please advise as to best procedure. Thanks. Main use of Variac will be to power up ancient tube equipment.
Thank you!


As long as the variac passes basic eletrical leakage test fine and the
powre cord and internal wiring are in good condition there is little
sense in trying to upgrade it to three conductor wiring.

In fact I recall reading somewhere (can't find the link) that it is
against the electrical safety regulations to convert two conductor
products to three conductor. This could be incorrect, until I can find
the reference take it with a grain of salt. However many items wired for
two conductor would need to be modified for three conductor beyond just
adding a ground - in pinball and jukeboxes the older two conductor items
correctly had the power switch on one side of the line and the line fuse
on the other - this is totally wrong on three wire setups!

So use the isolation Xformer to be safe.

Also, I trust you are aware of the series lamp (25 to 100W bulb
depending on the load) test for powering up gear that you aren't sure
hasn't a problem (shorted winding, cap, rectifier, etc.)...

John :-#)#
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Old November 3rd 18, 06:57 AM posted to sci.electronics.repair
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Default Variac question

John Robertson wrote:



As long as the variac passes basic eletrical leakage test fine and the
powre cord and internal wiring are in good condition there is little
sense in trying to upgrade it to three conductor wiring.


** To maintain the safety of equipment that normally uses a ground conductor with a 3 pin plug - earth continuity must remain intact.


In fact I recall reading somewhere (can't find the link) that it is
against the electrical safety regulations to convert two conductor
products to three conductor.


** If the product as originally sold was of Double Insulated (aka class2) design, this may be true. Earthing a Class 2 shaver is silly.

But I would not hesitate to add a 3 conductor cable to any guitar amp, powered desk or similar I found that did not have it.


This could be incorrect, until I can find
the reference take it with a grain of salt. However many items wired for
two conductor would need to be modified for three conductor beyond just
adding a ground - in pinball and jukeboxes the older two conductor items
correctly had the power switch on one side of the line and the line fuse
on the other - this is totally wrong on three wire setups!


** Sounds easy enough to correct, as it is to remove the infamous "death cap" fitted to many 2 wire guitar amps not so long ago.



..... Phil



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Old November 3rd 18, 06:57 AM posted to sci.electronics.repair
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Default Variac question

Ivan Vegvary wrote:


Purchased used Variac. Good action, very smooth from 0 to 140V AC.

Question: Input and output are both 2 prong, no ground.
Should I modify it to three prong with ground?


** Yes - all modern ones are made like that. It is not safe to de-earth an appliance just because you need to use a Variac with it.


BTW, I do also own a functioning isolation transformer.
Should the IXformer be placed between the mains and the Variac?
Please advise as to best procedure.


** Either way works, feeding 140VAC to the Iso may get it a bit hot though.


Main use of Variac will be to power up ancient tube equipment.



** I use mine for almost everything.

Cos I hate blowing fuses at switch-on.


..... Phil
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Old November 3rd 18, 11:13 AM posted to sci.electronics.repair
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Default Variac question

On Saturday, 3 November 2018 04:43:21 UTC, Ivan Vegvary wrote:

Purchased used Variac. Good action, very smooth from 0 to 140V AC.
Question: Input and output are both 2 prong, no ground. Should I modify it to three prong with ground?


it's safer to

BTW, I do also own a functioning isolation transformer.
Should the IXformer be placed between the mains and the Variac? Please advise as to best procedure. Thanks. Main use of Variac will be to power up ancient tube equipment.
Thank you!


that works, but don't assume you won't then get a shock from a live wire. Safer & easier now to use an RCD/GFCI.


NT


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Old November 3rd 18, 01:37 PM posted to sci.electronics.repair
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Default Variac question

On Fri, 02 Nov 2018 21:43:18 -0700, Ivan Vegvary wrote:

Purchased used Variac. Good action, very smooth from 0 to 140V AC.
Question: Input and output are both 2 prong, no ground. Should I modify
it to three prong with ground?


I certainly would.


BTW, I do also own a functioning isolation transformer.
Should the IXformer be placed between the mains and the Variac? Please
advise as to best procedure. Thanks. Main use of Variac will be to
power up ancient tube equipment.
Thank you!


The isolation transformer should always power the *device under test* to
remove the Earth reference from it.




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Old November 3rd 18, 04:05 PM posted to sci.electronics.repair
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Default Variac question

On 11/3/18 9:37 AM, Ralph Mowery wrote:
In just the last few years I have been looking at the older equipment
(some in the mid 1960s) and they were 2 wire devices. They had the
switch on one side of the transformer and the fuse on the other side.
I don't know about the code or why it was done that way. I would have
thought the fuse would come first on the hot wire and then the switch.
Much of the equipment is ham radio related and most users would have a
good connection of the chassis to the earth ground.


Because in the end, it's a series circuit. It doesn't make a damn bit of
difference.

Primarily, it was done from a "Is this the cheapest and easyist (also
cheapest) way to do it this way?"




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Old November 3rd 18, 06:38 PM posted to sci.electronics.repair
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Default Variac question

Thank you everybody. Great help! Will always use the isolation xformer
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Old November 3rd 18, 07:38 PM posted to sci.electronics.repair
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Default Variac question

On Sat, 03 Nov 2018 10:38:54 -0700, Ivan Vegvary wrote:

Thank you everybody. Great help! Will always use the isolation xformer


One last thing: don't ever get into the habit of using that variac of
yours as a convenient substitute for an extension lead.
Enjoy!




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