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Low Grid on Johnson 500




 
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Author Topic: Low Grid on Johnson 500  (Read 708 times)
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Detroit47
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« on: January 19, 2018, 02:35:54 PM »

I finally got my 500 back together. I was wondering if this is a common problem. On 75 meters I have tons of grid drive, but as I go up in frequency it gets a little low. It isn't an issue till I get to 10 meters.When I get to ten the best I can see is ten mills I know this is what it calls for but it wants to swing backwards as I modulate.I know from past experience that you need all the grid you can get even if you aren't using it. I went thru the alignment procedure and noticed it peters out as I go up. I have seen this on other transmitters. I will mainly use it on 75 and 40 but I want it right. I have a Yaseu peak reading watt meter that I calibrated against my bird. I am seeing about 350 watt dead key with swing over a grand so that appears ok. That is till I get to 10 meters. I still have the same dead key but it only swing about  750. If anybody has any suggestions please share. I have neutralized the driver and final properly so that isn't an issue. This has been a shelf queen for 25 years so I am really excited to get it back together. I bought it off the Yellow sheets as a basket case with a bad mod transformer.

Thanks John N8QPC
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N1BCG
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« Reply #1 on: January 19, 2018, 06:48:48 PM »

Admittedly, I'm not familiar with the "500" although I feel as though I've worked through every conceivable issue with it's younger brother, the Valiant. Valiants seem to have the same issue, that is, a drop in drive as the frequency increases. This is normal to an extent as the oscillator frequency is getting multiplied and each higher harmonic is a bit weaker.

I found the majority of drive issues with Rangers and Valiants stem from weak drive tubes, which are also the multipliers. I was going to mention neutralization, but you checked that off the list.

Not sure if you've already checked that but I wanted you to get at least some possible course of action.
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WD8KDG
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« Reply #2 on: January 20, 2018, 08:25:19 AM »

Hello John,

Check the easy stuff first, the driver tube (5763). Substitute another one. I haven't tried the higher frequencies on my 500 since restoring it in 2004-2005. But do remember the 15 meter band seemed to have the lowest output/grid drive.

The 500 has always played well on 75 & 40 and this is where I've used it since.

Craig,
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Detroit47
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« Reply #3 on: January 20, 2018, 01:22:11 PM »

Thanks for the response guys. I will see if playing musical tubes changes anything. I have to find anther 5763 first.

John
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KD6VXI
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« Reply #4 on: January 20, 2018, 03:28:46 PM »

http://amfone.net/Amforum/index.php?topic=43576.0

Ask and Ye shall receive!

Multiple 5763s.


--Shane
KD6VXI
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Dave K6XYZ
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« Reply #5 on: January 20, 2018, 07:07:13 PM »

You can go to BAMA and dwnnld the manual....it is probably different if you have an original.
The BAMA manual has pages from more than one manual date.
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WD8KDG
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« Reply #6 on: January 21, 2018, 09:47:18 AM »

Me thinks there are several of us with basket case Junkston "500" transmitters. None of which came with an original manual or any manual. So BAMA to the rescue. I have to wonder how many different original Johnson 500 manuals were printed. Were certain pages updated and not all? Those with original 500 manuals please chime in!

I have a copy of The Johnson Production Figures from Electric Radio, #27 July 1991. There were 865 produced during 1957-1964. Also have a brochure, date unknown. It states the 500 uses a 4-400A tetrode.

So back to the BAMA manual for the 500; page 39 states to set the CW-AM bias at -50 volts using terminal 3 of R168. Page 22 states the different types of PA tube that can be used, but nothing about bias.

My Junkston "500" was set at -50 volts back in 2004-2005, haven't looked at it since.

Regards,
Craig
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Dave K6XYZ
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« Reply #7 on: January 21, 2018, 12:06:20 PM »

.
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KB2WIG
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« Reply #8 on: January 21, 2018, 12:37:59 PM »

.



Yes, 100%.

KLC
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W9BHI
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« Reply #9 on: January 21, 2018, 02:12:12 PM »

This thing is just an overgrown Ranger.
L107 in the tank of the 6cl6 osc. buffer needs to be adjusted for max driver current on 10 meters.
Once you have peaked this coil, you will have more than enough drive on 10 thru 40 meters.
I have seen this problem on Rangers and Valliants.
It it were mine, I would do a complete alignment per the manual.
I have done several 500's, it's not hard to do and doesn't take all that long.
It really perks them up.
Don
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Detroit47
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N8QPC on your AM dial


« Reply #10 on: January 21, 2018, 04:18:41 PM »

I finally got my 500 back together. I was wondering if this is a common problem. On 75 meters I have tons of grid drive, but as I go up in frequency it gets a little low. It isn't an issue till I get to 10 meters.When I get to ten the best I can see is ten mills I know this is what it calls for but it wants to swing backwards as I modulate.I know from past experience that you need all the grid you can get even if you aren't using it.] I went thru the alignment procedure and noticed it peters out as I go up. I have seen this on other transmitters. I will mainly use it on 75 and 40 but I want it right. I have a Yaseu peak reading watt meter that I calibrated against my bird. I am seeing about 350 watt dead key with swing over a grand so that appears ok. That is till I get to 10 meters. I still have the same dead key but it only swing about  750. If anybody has any suggestions please share. I have neutralized the driver and final properly so that isn't an issue. This has been a shelf queen for 25 years so I am really excited to get it back together. I bought it off the Yellow sheets as a basket case with a bad mod transformer.

Thanks John N8QPC

Don read the first post.
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W9BHI
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« Reply #11 on: January 21, 2018, 08:12:10 PM »

I guess I didn't read your post close enough, sorry.
Eimac calls for 12 ma. of grid drive for a 4-400@ 3kv.
I wonder why Johnson says 10 ma.?
You may be losing drive in one of the multiplier stages.
Time to get out the Hickok and start testing some tubes.
Just trying to be helpful.

Don W9BHI
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Detroit47
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N8QPC on your AM dial


« Reply #12 on: January 21, 2018, 08:57:30 PM »

Thanks Don I already did on my TV-11. I just went through a couple of thousand tubes and found some new 5763 and 6clc's. I will pop them in and realign it. If it is still low I will start checking cathode resistors and such stuff. I went through it with my Huntron Tracker
and nothing screamed bad. I have the urge to just shotgun the whole strip and be done with it.

John
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