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Author Topic: National NCX-3 PA tuning problem.  (Read 688 times)
KF4CZV
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Posts: 152




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« on: July 16, 2019, 12:40:19 PM »

I’ve been working on restoring this rig off and on for a while now. I’ve done many tube receivers but this is only my second transceiver. The first was a Swan 250 which came out great and is my main 6 meter rig. I am woefully knowledge deficient when it comes to the transmitter part. Without going into detail (I will if asked) it’s as electronically restored, parts wise, as it can get. Driver and finals are new and all other tubes are tested strong. Receive side works great. My problem is twofold: I have very little experience tuning up a tube transmitter and find the instructions in the manual confusing. Here is the manual excerpt:




I’m tuning up first on 20 meters into a 300 watt dummy load through an MFJ power meter. Following the instructions, turning the Exciter get’s me to 40 watts. When I turn the PA Tune even a hair from fully cow, it instantly drops to zero. I’ve tried multiple times with the same results. When trying to follow the rest of the instructions, turning the PA Load also drops the power, although not as dramatically. I’m stumped. What am I doing wrong? Or, what’s wrong with the transmitter? My gut is pointing me to the PA Load control.
I do not have an O-Scope nor know how to use one. Meters, signal generators, tracers and that sort of test gear I do.
Before trying to tune this rig I looked at all the threads in this forum on tuning and was watching the MFJ meter for max power. Any and all help will be greatly appreciated! I want to use this rig in my shack alongside the Swan.

I have 2 videos on my YouTube channel showing the rig and detailing the restoration up to this point. I will post the links if you think it would be helpful.

Terry
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KF4CZV
Member

Posts: 152




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« Reply #1 on: July 16, 2019, 04:38:35 PM »

When it rains…… well, you know the rest!
I believe I have a bandswitch problem and possibly a meter circuit problem and an oscillation problem. First, when I had it on 20M and tuned for loudest noise (antenna) it was not very loud and no S meter movement at all. I switch to 80M and the S meter almost pegs. When adjusted the noise had the S meter topped out. I switch to 40M and S meter reads half scale but as I adjust the noise loser I start to hears faint motor boating in the speaker. I put it in tune mode (on 80) and all of a sudden I’m getting no power out indication on either the radio’s meter or the MFJ. I’m done for the evening. My friend John will be here on Friday. In order to take myself out of the equation, I’m going to have him start from scratch, follow the book and try to tune it up after we verify the bias amps. I will video it all and post it so you can see exactly what is happening.

Terry
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WA2CWA
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Posts: 568


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« Reply #2 on: July 17, 2019, 12:09:47 PM »

I wonder how we were able to fix rigs back in the "good old days" when we didn't have video's, the Internet, You Tube, etc.  Cheesy
The mysteries of the 21st century.

Pete, wa2cwa
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KF4CZV
Member

Posts: 152




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« Reply #3 on: July 17, 2019, 01:01:28 PM »

I wonder how we were able to fix rigs back in the "good old days" when we didn't have video's, the Internet, You Tube, etc.  Cheesy
The mysteries of the 21st century.

Pete, wa2cwa

Radio schools and Elmers neither of which I've had the benefit of.

Terry
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KF4CZV
Member

Posts: 152




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« Reply #4 on: July 17, 2019, 01:01:44 PM »

I hate to come off sounding like a dummy (which is what I feel like now) but I need some schooling on properly tuning up a tube transmitter. I had no problem with the Swan I did but it had different instructions than this one. I’m having a hard time understanding this one.
I have set the bias to 55ma as per the manual. I tune for loudest noise. Then:
Step 1- I turn Exciter for max meter reading which happens to be the same spot as max noise.

Step 2 - Rotate PA Tune for minimum meter reading (dip). This is where I start getting confused. Exciter bring the needle to just below the 100 mark. I rotate the PA Tune CW and it goes just above 100 then drops back to 100. Is that the dip? If I turn it cow then it goes to 0 on the meter. That certainly qualifies as “minimum.” So which do I do, dip or minimum?

Step 3- no problem. Key the mic and the initial setting at 12 O’clock is the minimum.

Step 4- Same as step 1. Then rotate PA Load cw and dip meter with PA Tune until 300ma reading. Am I supposed to rotate both controls at the same time? I’ve been doing them Load first then tune and end up nowhere close to 300ma and almost no output. This is on 80 meters.

The problem I thought was with the meter was actually me. I had the antenna switch on the dummy instead of the antenna. Doh!

Maybe those new Chinese tubes are crap? I hope one of you fellows will simplify the tune up steps for me with what I should see on the meter each step. I think that would help to eliminate or confirm ME as the root of the problem.

Terry
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KM1H
Member

Posts: 5541




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« Reply #5 on: July 17, 2019, 01:24:35 PM »

I wonder how we were able to fix rigs back in the "good old days" when we didn't have video's, the Internet, You Tube, etc.  Cheesy
The mysteries of the 21st century.

Pete, wa2cwa

Factory service with fast turn around since the circuits were simple and inexpensive or in warranty. Other sources were local dealers or a Radio/TV guy out of his home that was also a ham. I worked part time at two Boston area dealers while at National on a wide range of brands.

Carl
National Radio 1963-69
Service Tech, Service Manager, Sr Engr Tech
Member of HRO-500, NCL-2000, NCX-1000 Design Teams
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KM1H
Member

Posts: 5541




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« Reply #6 on: July 17, 2019, 01:30:28 PM »

I hate to come off sounding like a dummy (which is what I feel like now) but I need some schooling on properly tuning up a tube transmitter. I had no problem with the Swan I did but it had different instructions than this one. I’m having a hard time understanding this one.
I have set the bias to 55ma as per the manual. I tune for loudest noise. Then:
Step 1- I turn Exciter for max meter reading which happens to be the same spot as max noise.

Step 2 - Rotate PA Tune for minimum meter reading (dip). This is where I start getting confused. Exciter bring the needle to just below the 100 mark. I rotate the PA Tune CW and it goes just above 100 then drops back to 100. Is that the dip? If I turn it cow then it goes to 0 on the meter. That certainly qualifies as “minimum.” So which do I do, dip or minimum?

Step 3- no problem. Key the mic and the initial setting at 12 O’clock is the minimum.

Step 4- Same as step 1. Then rotate PA Load cw and dip meter with PA Tune until 300ma reading. Am I supposed to rotate both controls at the same time? I’ve been doing them Load first then tune and end up nowhere close to 300ma and almost no output. This is on 80 meters.

The problem I thought was with the meter was actually me. I had the antenna switch on the dummy instead of the antenna. Doh!

Maybe those new Chinese tubes are crap? I hope one of you fellows will simplify the tune up steps for me with what I should see on the meter each step. I think that would help to eliminate or confirm ME as the root of the problem.

Terry

WHAT Chinese tubes??  The 6GJ5A was a USA TV sweep tube later replaced by the 6JB6 along with a Service Bulletin.

The biggest problem was the TX/RX relay which was initially wired in and then became a plug in. A kit was available for owners to do the work at home.

That relay is till a PITA due to pitting from the low B+ switching but last I looked was still available as it was a generic catalog type item.

Carl
National Radio 1963-69
Service Tech, Service Manager, Sr Engr Tech
Member of HRO-500, NCL-2000, NCX-1000 Design Teams
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KF4CZV
Member

Posts: 152




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« Reply #7 on: July 17, 2019, 02:47:38 PM »


WHAT Chinese tubes??  The 6GJ5A was a USA TV sweep tube later replaced by the 6JB6 along with a Service Bulletin.

The biggest problem was the TX/RX relay which was initially wired in and then became a plug in. A kit was available for owners to do the work at home.

That relay is till a PITA due to pitting from the low B+ switching but last I looked was still available as it was a generic catalog type item.

Carl
National Radio 1963-69
Service Tech, Service Manager, Sr Engr Tech
Member of HRO-500, NCL-2000, NCX-1000 Design Teams

Sorry, I was thinking of the 6146 tubes in the Swan. I've had this one on the back shelf for a while so the details are a bit fuzzy.

Terry
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K5SBR
Member

Posts: 12




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« Reply #8 on: July 17, 2019, 05:56:18 PM »

I still have one of the two NCX-3's owned since 1963.

Pull the power cord and make sure everything inside is cold (that is, no residual voltage). Use your VOM and common sense. Trace the schematic. No Buck Rogers stuff here. Just basic tube circuits.

Sounds like the PA tune cap may have a short when rotated? Ohm it out cold. That is not something to discover with your expensive new PA tubes.

When you do power up, use that VOM to check voltages. That relay clicking into the receive position is a good sound. And if that seems like backward logic. It is, but it works.

The only magic is that after setting the PA bias, treat it like a CW rig. Carrier balance acts like a drive control, along with the driver tuning. Mike PTT button doubles as the CW key.

If you can't pull almost 400 ma. plate current off resonance, then the driver tube is soft. Or the finals are too. Mostly the driver goes first. Or your power supply is inadequate.

Once you make sure there are no shorts in the output circuit and get it back under power, use a wattmeter and juggle PA Tune and PA Load controls for 300 ma. plate current and 100+ watts output. DO NOT SIT ON THE KEY/PTT BUTTON. This rig was built for SSB, NOT RTTY! 5 seconds on, 50 off. There are NO thermal overload safety features built-in like the new radios. You CAN (and will) melt things.

Once you get it perking, you can re-neutralize the PA so that the PA current dip and maximum output coincide like the books say.

Listen to the former National service manager. He knows his stuff.

73,
Ed
K5SBR
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VR2AX
Member

Posts: 1016




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« Reply #9 on: July 19, 2019, 06:37:14 AM »

The only issue I could take with mine back then was the carrier balance control needed a lot of attention.
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KM1H
Member

Posts: 5541




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« Reply #10 on: July 19, 2019, 09:55:17 AM »

The only issue I could take with mine back then was the carrier balance control needed a lot of attention.

As did others of those years. A little carrier was common all over the bands and the 7360 was especially sensitive to other fields around it.  Once excellent SS diodes were used that problem went away.

I went from a CE 10A, 10B, 20A, HT-37, to a CE-100V over very few years (a 10B and 20A went to driving VHF/UHF transverters into the 80's) and still use the 100V.

Carl
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VR2AX
Member

Posts: 1016




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« Reply #11 on: July 20, 2019, 04:07:19 AM »

Yes it did not detract from the capability to make contacts. Great rig back in 1969.

Wyn
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