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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Mon 10, 2020 10:55 pm 
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I have a good clear scan of the 34 sch with parts list if anyone needs one PM me as it's too big for here. I remember when I rebuilt mine I had to move the output transformer as it was picking up hum from the power transformer!


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Tue 11, 2020 7:55 am 
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Location: Lexington, KY USA
I can't imagine how the hum bucking coil could possibly become connected to the heater supply. The connections to it are not really external to the speaker. And if you connect the speaker terminals to the heater winding on the transformer, it's probably the voice coil that will expire first. But why would anyone...

Ted


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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Tue 11, 2020 3:03 pm 
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I may have found the problem:

Image

That appears to be R21 on the schematic. When we get back on it Friday we'll disconnect one end and double check it. I find it hard to believe the circuit could pull the reading down that far. If it is truly shorted then the question becomes, what caused it? The saga contunues.

Terry

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Tue 11, 2020 5:36 pm 
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specops56 wrote:
That appears to be R21 on the schematic. When we get back on it Friday we'll disconnect one end and double check it. I find it hard to believe the circuit could pull the reading down that far.

R21 is in parallel with the band A coil on that gang of the band switch. You're essentially reading the resistance of the coil, since the resistance of R21 is so high in comparison to the resistance of the coil.

A basic resistance test from B+ to B- is the place to start, once you've replaced the electrolytic and paper capacitors. It'll be easiest to interpret the results if you disconnect B+ from the filter cap, otherwise you have to wait until the filter cap is charged.

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Tue 11, 2020 7:35 pm 
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Yep, I found that out. That 220K resistor reads 280K when cut. It's way over tolerance anyway and needs replacing so at least it wasn’t a wasted effort. I’ll save anymore troubleshooting till Friday when John is here to help me. He’s much better at the troubleshooting side of things than I am.

Terry

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Feb Sat 15, 2020 3:49 am 
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Location: Blythewood S.C. USA
The hum bucking coil is bad.I wired a 300 ohm choke across the filters .No more flickering eye tube.Works fine now.I now pick up a radio station but have to run the volume control all way up.Now installing bnc connectors on the front panel

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 Post subject: Re: NRI 34 signal tracer hum bucking coil help needed!
PostPosted: Mar Tue 24, 2020 7:15 pm 
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Back on the nri34.on pin 6 of thr6sq7 I have 115 volts.On pin 5 of the 6k6 I had 97 volts.it looks like r15 a 470 ohm resister abd r17 a 1 meg are in series to ground.R17 was missing.I installed it and the97 volts on pin 5 went to negative .385 but unsteady.Only picking up one local radio station at 840am and the volume is very low.Where to go to next.

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Mar Wed 25, 2020 3:19 am 
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Location: Lexington, KY USA
The hum bucking coil is bad.

How was the hum bucking coil bad? Did you replace it? Did you just connect the voice coil directly, without going through this coil?

I wired a 300 ohm choke across the filters.

Why? How exactly? From B+ to ground might not work so well. Could you have connected it across the field coil? In place of the field coil? What did this do to the B+ voltage?

No more flickering eye tube. Works fine now. The whole unit works fine, or just the eye tube? Does the eye tube respond correctly to RF signals?

I now pick up a radio station but have to run the volume control all way up. Does the eye tube only respond on this one station?

What is the speaker situation? If the hum bucking coil was disconnected, you won't have much sound out of the speaker. If the field coil is disconnected or bridged-across, you might have weak sound. Does the eye tube respond to stations you cannot hear in the speaker?

On pin 5 of the 6k6 I had 97 volts. Pin 5 is G1. 97V here may have damaged the 6K6 or its cathode resistors. The output transformer and B+ supply might also be damaged by this, as it would lead to lots of excess B+ current.

With the power off, check the resistance from pin 8 (cathode) of the 6K6 to ground, and also the voltage on the cathode when power is on.

There must by a typo on the schematic about the 6K6 grid resistors, R15 & R17. It might be interesting to hear how these actually should be connected. I guess it would be helpful to measure from pin 5 (G1) of the 6K6 to ground, as well.

Sorry about so many questions, but some things don't seem to make any sense. Trying to do this at a distance can be frustrating to all concerned. Given enough patience, we usually succeed, though.

Ted


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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Mar Wed 25, 2020 11:58 am 
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I wire a choke across the filter caps.cathode pin 8 is positive 26 volts.Thats way too high.I dont remember if this had a pm speaker or not.Will check

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Mar Wed 25, 2020 12:00 pm 
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Pin 5 was +46 volts until I installed 1 meg resister in series with r16 a 470 ohm resister.

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Mar Wed 25, 2020 2:00 pm 
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Usually Lurking wrote:
The hum bucking coil is bad.

How was the hum bucking coil bad? Did you replace it? Did you just connect the voice coil directly, without going through this coil?

Near as I can tell, the OP is calling the field coil a "hum bucking coil", because there's a label on the schematic for the hum bucking coil but no label on the field coil. So he's replaced the field coil with a choke. Consequently he needs to replace the electrodynamic speaker with a permanent magnet speaker.

Usually Lurking wrote:
There must by a typo on the schematic about the 6K6 grid resistors, R15 & R17. It might be interesting to hear how these actually should be connected.

Yeah, the schematic seems to show R17 (1 Meg) in parallel with R23 (1500Ω). That doesn't make any sense. And the pair of them seem to be connected to the junction of R15, R16, and C26. (I need to get Lou's scan, I can barely make out the component designators on the BAMA scan.)

John Moyer wrote:
On pin 5 of the 6k6 I had 97 volts.

John Moyer wrote:
Pin 5 was +46 volts until I installed 1 meg resister in series with r16 a 470 ohm resister.

Which was it, 97 volts or 46 volts?

It appears that you replaced C24 with a modern film capacitor, but high positive voltages on pin 5 of V4 indicate either C24 is leaky or there's a leakage path on the V4 socket (a carbon track?) or leakage in V4 itself. Has C24 been damaged by accidentally being touched with a soldering iron?

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Mar Wed 25, 2020 7:48 pm 
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Pin 5 of 6k6 was 97 til I added the 1 meg resister now it is 46 Volts

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Mar Wed 25, 2020 7:49 pm 
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Going to replace speaker on friday with a pm.Let you kniw how it goes.

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Mar Wed 25, 2020 8:00 pm 
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John Moyer wrote:
Pin 5 of 6k6 was 97 til I added the 1 meg resister now it is 46 Volts

Earlier you had said pin 5 was -0.385 volts once you added the 1 meg resistor. What changed?

With a grid voltage of 46 volts, it seems like there's a significant problem in the grid circuit of V4. What's the voltage on pin 8 of V4?

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Apr Sat 04, 2020 11:29 am 
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G1 of 6k6 reads .018 volts positive.The cathode pin 8 of 6k6 reads +30 volts The b+voltages off the filter xaps reads 260.cap c24 wasnt melted any.replaced 6k6 again no change.resistabce on pin 8 cathode is 2.2k to ground.The schematic isnt clear on how to connect the cathode resisters on the 6k6.

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Apr Sat 04, 2020 11:32 am 
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Location: Blythewood S.C. USA
I also tried a pm speaker off the audio transformer.Low audio.Tried a used audio xfmr and pm speaker audio came up much more.Can only pick up one radio station at 840 on dial.

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Apr Sat 04, 2020 2:12 pm 
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You need a signal tracer to fix your signal tracer, sort of like fixing a scope.

Seriously, you can troubleshoot like you would a radio. Start with the output stage and work forward by injecting signals.

RRM


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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Apr Sat 04, 2020 3:04 pm 
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Retired Radio Man wrote:
You need a signal tracer to fix your signal tracer, sort of like fixing a scope.

Seriously, you can troubleshoot like you would a radio. Start with the output stage and work forward by injecting signals.

RRM

That's our plan. I have a Heathkit IT-12. We'll be using it to trace the NRI next week. Gotta plant my veggie garden right now.

Terry

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Apr Sat 04, 2020 4:21 pm 
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John Moyer wrote:
Pin 5 of 6k6 was 97 til I added the 1 meg resister now it is 46 Volts

John Moyer wrote:
G1 of 6k6 reads .018 volts positive.

John Moyer wrote:
cathode pin 8 is positive 26 volts.

John Moyer wrote:
The cathode pin 8 of 6k6 reads +30 volts

We're not going to get far in debugging the output stage if we can't get consistent voltage measurements. Are they really varying this much? That would indicate some intermittent connection.

Assuming the cathode of the 6K6 is somewhere between +26V and +30V:
* If the grid of the 6K6 is really +46V, that means a positive grid bias of +16V to +20V. That'll be killing the 6K6.
* If the grid of the 6K6 is really +0.18V, that means a negative grid bias of -26V to -30V. The 6K6 must be nearly cut off.

In either case, something is seriously wrong with the grid bias on V4, the 6K6 output tube.

Retired Radio Man wrote:
Start with the output stage and work forward by injecting signals.

We already know the output stage is not working correctly, from the OP's voltage measurements.

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 Post subject: Re: NRI 34 signal tracer help needed! New problem!
PostPosted: Apr Sat 04, 2020 4:34 pm 
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John Moyer wrote:
resistance on pin 8 cathode is 2.2k to ground.

That's within 20% tolerance. R16 (470Ω) and R23 (1500Ω) are in series between pin 8, the 6K6 cathode, and ground. So the nominal resistance is 1970Ω, plus or minus 20%.

John Moyer wrote:
The schematic isnt clear on how to connect the cathode resisters on the 6k6.

Yeah, I think in the grid circuit, the connection between R15 and R17 is a missing jump over the wire between C26 and the junction of R16 and R23.

The resistance from pin 5, the grid of the 6K6, to ground should be about 1.22 Megohms. R15 and R17 are in series, and connect pin 5 to ground. The junction of R15 and R17 should not connect to the cathode circuit (R16 and R23).

Once you have it wired this way, try pulling V4, the 6K6, and measuring the voltage at pin 5. It should be zero volts with respect to ground. Then plug the 6K6 back in and measure the voltage at pin 5.

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Last edited by stevebyan on Apr Sat 04, 2020 4:41 pm, edited 1 time in total.

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