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 Post subject: NRI 34 signal tracer help needed! New problem!
PostPosted: Feb Fri 07, 2020 8:46 pm 
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NRI 34 signal tracer hum bucking coil help needed!

I’m restoring the above mentioned tracer and the hum bucking coil is bad. Does anyone know the value? It’s not mentioned on the schematic. Also, can I replace it with a resister? This is the first time I’ve run into this problem.

Terry

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Last edited by specops56 on Mar Wed 25, 2020 12:56 am, edited 1 time in total.

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Fri 07, 2020 8:59 pm 
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Location: Wayside, NJ Monmouth
Do you have a Model 33 or 34. The model 33 schematic shows a bucking (Field) Coil. The Model 34 Schematic dose not. You can use a resistor. You will need to experiment with the value to get the best results. I own a Model 34. Once recapped and aligned it works great. I some times hook a long wire antenna to it to listen the local AM and Shortwave stations.


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Fri 07, 2020 10:40 pm 
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Location: Lexington, KY USA
A resistor will do no good here. What happened to the coil? It's just a few turns of heavy wire, and will almost never fail if not vandalized.

Is the rest of the speaker OK? Some photos might help explain.

The speaker will work without the coil. Just connect the voice coil directly to the signal source.

How much hum are you getting with the volume turned down? You might be able to wind your own coil if the hum is excessive.

The model 33 schematic shows a bucking (Field) Coil.

Note that the hum bucking coil is not the field coil, although it is wound adjacent to the field coil. Practically all field coil speakers have a
hum bucking coil. Speakers without a field coil do not have a hum bucking coil. Schematics may or may not show a hum bucking coil when there is one. It's part of the speaker and requires no external connections, so it might not show up on a given schematic.

Ted


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Sat 08, 2020 1:22 am 
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I have some NOS Utah & Quam FC speakers should a replacement be needed...

Chas

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Sat 08, 2020 3:47 pm 
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Location: Blythewood S.C. USA
The nri34 does have humbucking coil.On obe of the filter caps the voltage is jumping up as high as 170 volts then goes erratic and down to nothing.Meanwhile eye tube flickers and goes out.I smell something over heated like a transformer.Am going to unplug eye tube to see if that is loading b+ down.Also 2 other connection are also tied to the filter.The other filter has 345 volts on it.

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Sat 08, 2020 5:04 pm 
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John is helping me with this restoration. Since he did the actual testing of the voltages I asked him to explain the problem.

Terry

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Sat 08, 2020 5:09 pm 
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Here is the best schematic for the model 34 that I have:

Image

Terry

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Sun 09, 2020 4:53 am 
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Location: Lexington, KY USA
Be very cautious powering the unit with something smelling hot. A short, or failing component on B+, can smoke the power transformer.

If the B+ voltage is lower than it should be, or the current higher, you want to turn it off right away.

Have you checked for B+ problems by visual inspection and by using an ohmmeter? You want to look for mis-wires, backwards electrolytic caps and for bits of solder or clipped wires lodged where they should not be. The ohmmeter can be used to verify that there is a high resistance from B+ to ground. These checks cannot assure that there is no B+ problem, but can reveal some of the things that might be wrong.

And the hum bucking coil should have nothing to do with B+, unless there is a mis-wire or insulation failure.

Ted


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Sun 09, 2020 3:09 pm 
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Ted, something is loading the B+ down that feeds the eye tube.The eye flickers and monitoring thevoltage to it is all over the place.starting at 170 volts and ends up 0.I will take the eye tube out of the circuit Two other lea ds also tied to that filter will be removed one at the time.Will ten trace te lie giving trouble.Will leteveryone know wat problem was.

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Sun 09, 2020 4:04 pm 
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Location: Lexington, KY USA
The eye tube runs off the same B+ as everything else. Unless you see arcing and sparking, or that puff of smoke, about the eye tube, it doesn't seem like a prime candidate to be the cause.

What is the B+ current? If you are applying power, you want to watch for excessive current.

Ted


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Sun 09, 2020 5:16 pm 
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I have one of these on my service bench. It was built through the NRI training course by my late father-in-law. I refurbished it some 30 ya. still works today.

Anything is possible that causes the intermittent B+. I would recommend pulling the rectifier, lift one side of each filament line. Power up and monitor the HV for a period of 30 minutes. If the transformer is ruined it will heat and fail. If the transformer is O.K. it will only slightly warm. Power down and take resistance reading of the B+ to B- should be at least 5k to 10k preferably higher. Be sure the field coil, output transformer are not leaking to the metal core or frame.. Do check the output tube and rectifier socket for shorts, burn traces. Since this may have been a student build there could be a miss wire, solder blob or wire clipping shorts.

GL

Chas

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 5:23 am 
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Does anyone have a high quality, sharp schematic for this? The one I'm working with is a copy of a copy and next to useless.

Terry

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 5:34 am 
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I presume the filter capacitors have been replaced with good modern units.

A hum bucking coil was an attempt to get better performance without better filtering. You don't need it if you don't mind some hum. You can also improve the filtering and obviate the need for it. A filter choke is one solution.


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 5:53 am 
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Location: Stafford, Texas USA
Look here for a high quality, sharp schematic

https://www.americanradiohistory.com/Ar ... -08-09.pdf

Page 15
Jimmie


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 7:28 am 
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Location: Lexington, KY USA
Unfortunately, the schematic in the NRI publication lacks component values. We have to use the list from the blurry scans.

I still wonder what happened to the speaker. Has anyone else ever encountered a problem with a hum-bucking coil? It's just not a usual failure item.

Ted


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 9:50 am 
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The part list at Bama is a little better for the 34.

http://bama.edebris.com/manuals/nri/33-34

jimmie


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 12:16 pm 
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Jimmie, yes to modern filter caps.I am going to unload that side with the problem.Power transformer runs just warm to the touch.choke was the way I was heading.It will be Friday before I get back to it.

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 2:58 pm 
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I went over it visually last night to see if I could see anything obvious. Nothing. We did replace all out of tolerance resistors. I'm going to test all the old ones we left in to see if any of them may have shorted.

Terry

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 5:19 pm 
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Usually Lurking wrote:
Unfortunately, the schematic in the NRI publication lacks component values. We have to use the list from the blurry scans. I still wonder what happened to the speaker. Has anyone else ever encountered a problem with a hum-bucking coil? It's just not a usual failure item. Ted
Still suspect it is a student build. Imagine the hum bucking coil across the filament supply, toast! Fixes problem, coil still "viable", corrects the error.... I have a couple of spare buck coils around but they are for the speakers that can be disassembled...

GL

Chas

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 7:48 pm 
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Thanks for the reply.Again it will be Friday before hands on again.

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John Moyer


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