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FDP Forum / Amp Mods, Repairs, and Projects / Sunn Sentura I blowing fuses after replacing two conductor power cable with a three conductor.

Stratmanx
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Memphis, TN

Oct 1st, 2017 10:20 AM   Edit   Profile   Print Topic   Search Topic

I went ahead and snipped out the death cap and polarity switch and rewired for the three conductor plug.

However I keep blowing fuses when turning on the power switch.

I've blown them with all the tubes in place, as well as removing all of them, including the rectifier tube, so it tells me I'm missing something simple in the 1st stage of the power delivery.

(Hoping my transformer isn't shot !)

I would really appreciate if someone could check out if I've done this properly.

This is how I've got it presently wired:

Stratmanx
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Memphis, TN

Oct 1st, 2017 10:21 AM   Edit   Profile   Print Topic   Search Topic






I also wired it like this to hopefully light the power lamp and it did illuminate briefly before blowing the fuse:

Stratmanx
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Memphis, TN

Oct 1st, 2017 10:22 AM   Edit   Profile   Print Topic   Search Topic


This is the power section of the Sentura I schematic:

Stratmanx
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Memphis, TN

Oct 1st, 2017 10:23 AM   Edit   Profile   Print Topic   Search Topic

.

Full Sentura I schematic

willie
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Too Near Atlanta GA

Amp Tech Emeritus
Oct 1st, 2017 10:49 AM   Edit   Profile   Print Topic   Search Topic

Hard to tell based on your drawing...

Why not use a 3 amp slow blo fuse?

Can you take digital photo of the AC mains wiring that you you replaced? Might help...

Have you tested the transformer primary and secondary windings?

w

(This message was last edited by willie at 12:52 PM, Oct 1st, 2017)

Stratmanx
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Memphis, TN

Oct 1st, 2017 11:35 AM   Edit   Profile   Print Topic   Search Topic

I've enclosed this shot, not the best, but coupled with the drawing should give you an idea.

So the way I've got it wired is the Neutral (White) goes directly to the 2nd connection on the power switch.

The black (Hot) goes to the fuse, then to the black wire on the transformer, then the other black wire on the transformer goes to the 1st connection on the power switch.

The Ground (Green) goes directly to the chassis.

I also ran a hot wire (The red one with the capped off end) to the number three connector on the switch to light the power lamp

What's the best way to test the primary / secondary windings ?

Thanks !


Wiring.

ECS-3
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USA / Virginia

Oct 1st, 2017 01:03 PM   Edit   Profile   Print Topic   Search Topic

If you have both the black(hot) and white(neutral) connected to the power switch, then you have a dead short.

The white (neutral) should go directly to one of the transformer leads.

The black (hot) should go to the fuse, then the switch, then from the switch to the second transformer lead.



Stratmanx
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Memphis, TN

Oct 1st, 2017 01:38 PM   Edit   Profile   Print Topic   Search Topic

I'll give that a shot.

Thanks !


Stratmanx
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Memphis, TN

Oct 1st, 2017 02:16 PM   Edit   Profile   Print Topic   Search Topic

Tried that as well.

Still blowing the fuse.

Transformer fried ?


Stratmanx
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Memphis, TN

Oct 1st, 2017 02:27 PM   Edit   Profile   Print Topic   Search Topic

measuring both black leads of the primary gives me 3.2 ohms.

Measuring both leads individual to ground shows no short.


ECS-3
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USA / Virginia

Oct 1st, 2017 02:46 PM   Edit   Profile   Print Topic   Search Topic

With all the tubes removed, you still have those 2 diodes (1N2070s) connected to the secondary.

If either one of those diodes are shorted that would put a heavy load on the transformer. I would check those two diodes, or disconnect them and see if the transformer will idle without blowing a fuse.

ECS-3
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Oct 1st, 2017 02:49 PM   Edit   Profile   Print Topic   Search Topic

The more I think about it, if those diodes are shorted you could have some other failed components downstream, like filter capacitors being exposed to AC instead of DC.

I'd probably disconnect the diodes and see what happens, and test the two diodes.

Stratmanx
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Memphis, TN

Oct 1st, 2017 04:19 PM   Edit   Profile   Print Topic   Search Topic

I unsoldered the red / black wire going from the transformer to the diodes and capped that off and still blowing the fuse.

All tubes still removed as well.



Stratmanx
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Memphis, TN

Oct 1st, 2017 05:40 PM   Edit   Profile   Print Topic   Search Topic

So with the Red / Black wires from the secondary capped off and then removing the Red / Yellow that goes to Ground from the secondary can capping that off, the fuse stays intact.

But either of them connected and the fuse blows.

I can see how the Red / Black from the secondary feeds the circuit and could understand why something down stream of that might blow the fuse, but why when the Red / Yellow is connected to the Ground and its supposed to, with the Red/ Black still disconnected it blows the fuse ?

Roly
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Whitehorse Canada

I don't get out much
Oct 1st, 2017 08:24 PM   Edit   Profile   Print Topic   Search Topic

Did the amp work before you changed the AC cord?

What does your meter say (ohms) when measuring from each side of the primary to centre tap (red/yellow).


ECS-3
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USA / Virginia

Oct 3rd, 2017 04:49 AM   Edit   Profile   Print Topic   Search Topic

I think you just need to disconnect ALL the transformer leads, including the two ground leads. Then start ohming out everything.

With all the leads disconnected each winding should have an ohms reading but no reading to the frame of the transformer and no reading to any other winding.

It should ohm out just like the schematic, the yellow winding should have no reading to either of those ground leads or to the frame. The primary should have no reading to either ground lead or the frame.

Roly
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Whitehorse Canada

I don't get out much
Oct 3rd, 2017 11:29 AM   Edit   Profile   Print Topic   Search Topic

" The primary should have no reading to either ground lead or the frame."

That's what I was getting at.

FDP Forum / Amp Mods, Repairs, and Projects / Sunn Sentura I blowing fuses after replacing two conductor power cable with a three conductor.




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