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   Home   Help Search  
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Author Topic: Resistance of plate transofrmer windings  (Read 2563 times)
N3AJB
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Posts: 38




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« Reply #15 on: June 12, 2017, 09:08:25 PM »

I got up the courage to manually close the plate relay with the secondary of the plate transformer disconnected from the bridge circuit.  No smoke or explosions, fuses didn't blow and VAC was 2830v.  So the transformer is good!   At least I know the problem lies on the secondary side.

Not sure what to do next to find the short.  I could manually close the plate relay with the secondary reconnected but not the cable from the power supply to the rf amp.  That would eliminate the rf deck as a possible culprit. That would apply 240 VAC to the plate transformer but the low voltage (bias, screen and filament) transformer would not receive voltage.

Thanks to all for suggestions and comments.

JON
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VR2AX
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Posts: 799




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« Reply #16 on: June 12, 2017, 11:02:05 PM »

Have you checked the bleeder resistor(s) across the HV output?
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N3AJB
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Posts: 38




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« Reply #17 on: June 13, 2017, 10:00:13 AM »

I did check the HV bleeder resistor, the screen dropping resistor, and everything else I could think of for a short or low resistance to grown. 

There is a high current cutoff relay which has an adjustable current limit  consisting of a variable resistor and fixed resistor to ground. It's factory set and the resistances are close to specs.  It's in the B- side of the supply. I'll lift the ground and see what happens.

JON
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KC4ZGP
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Posts: 1300




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« Reply #18 on: June 13, 2017, 10:50:17 AM »


See! If you bought and assembled the EB-104 amplifier, you wouldn't have such problems.

I think 'E' stands for evil like Dr. Evil.

_ _ ... ... _ _

Kraus

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KM3F
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Posts: 788




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« Reply #19 on: June 13, 2017, 04:16:17 PM »

In a case like this you should be able to find the component that has the low resistance.
If not, put a 240 volt 50 watt light bulb or 2 120 volt bulbs in series in series with the AC line to show the issue without causing a bigger problem.
Then you can open parts until the light goes Dim.
None of this is rocket science if you use you head about it.
Good luck.
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VR2AX
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Posts: 799




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« Reply #20 on: June 13, 2017, 05:34:55 PM »

I did check the HV bleeder resistor, the screen dropping resistor, and everything else I could think of for a short or low resistance to grown. 

There is a high current cutoff relay which has an adjustable current limit  consisting of a variable resistor and fixed resistor to ground. It's factory set and the resistances are close to specs.  It's in the B- side of the supply. I'll lift the ground and see what happens.

JON

If possible, I would isolate the bleeder resistor, and measure its actual resistance. If it has gone high it may be failing to perform its bleeding function, and HV direct into un-bled filter capacitors could be producing high surge current.
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N3AJB
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Posts: 38




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« Reply #21 on: June 13, 2017, 09:44:14 PM »

Believe me I have checked every component.  

The HV circuit is a bridge rectifier (all new diodes), choke input with an oil filled 10UF cap. It has a 100k, 100w bleeder (it measures 96.4k).  There is a 50k, 100w dropping resistor to a string of VR tubes.  In addition there is over current relay in the B- circuit the opens with excessive current (> 1000ma) with a 4.5 ohm to ground in series with the coil winding.

A second transformer supplied bias, filament, and 48 v for the plate relay.  All those voltages are as per specs.

I've checked every component and replaced electrolytic caps and the the screen dropping resistor.  

I still can't find the short.

Murphy rules!!!!

JON
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VR2AX
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Posts: 799




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« Reply #22 on: June 13, 2017, 10:26:09 PM »

Fair comment!!

Maybe one of the components is failing (shorting to ground) when voltage is applied, but giving an "apparently ok" reading when tested without voltage. Can you try bringing the voltage up on a variac, which at least may point to where the problem lies?
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N3QE
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Posts: 4755




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« Reply #23 on: June 15, 2017, 02:44:24 PM »

Believe me I have checked every component.  

The HV circuit is a bridge rectifier (all new diodes), choke input with an oil filled 10UF cap. It has a 100k, 100w bleeder (it measures 96.4k).  There is a 50k, 100w dropping resistor to a string of VR tubes.  In addition there is over current relay in the B- circuit the opens with excessive current (> 1000ma) with a 4.5 ohm to ground in series with the coil winding.

A second transformer supplied bias, filament, and 48 v for the plate relay.  All those voltages are as per specs.

I've checked every component and replaced electrolytic caps and the the screen dropping resistor.  

I still can't find the short.

Murphy rules!!!!

You have to back off a little bit and take this step by step, with something above DVM ohmmeter voltages (which is probably topping off at a few voltes).

Obviously below you are not sticking fingers or probes into anything on the secondary side because there are lethal voltages there. Your DVM can't measure high voltages without a HV probe anyway.

Check the transformer with nothing hooked up to the secondary. I think you said you've already done this, and no fuses blew, so the transformer is not the problem.

Now hook up the rectifier but no filter capacitors and no bleeders. Apply power and make sure this doesn't blow anything. If no fuses blow, them presumably your rectifier is good.

The next step is to connect the negative side of the rectifier to ground (or to ground through the plate metering resistor). Turn power on and make sure this doesn't blow any fuses.

Now hook up the bleeders as a series string, but no electrolytics. Apply power and make sure this doesn't blow anything. Remove power and check that the bleeder resistors actually got warm and about by equal amounts.

Now the big test, the electrolytics with bleeders. For this you probably need to re-engage the step start feature which I think you had earlier mentioned bypassing. If hooking up the electrolytics and applying power blows the fuses, then it's time to start checking the individual electrolytics at their rated voltage. Having a bench 350V supply would help.
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VR2AX
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Posts: 799




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« Reply #24 on: June 16, 2017, 06:17:30 AM »

Respectfully that's what good safety advice is. Just double check the caps question as I thought you said it was an oil filled cap. Just one?
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KM3F
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Posts: 788




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« Reply #25 on: June 17, 2017, 05:52:54 PM »

Maybe the point is being missed!
The point of using a lamp in series with the AC line is in place of blowing fuses and risking further damage.
A common method of finding these kinds of power faults safely and not do more damage.
If not ... fool away.
Good luck.
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