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Author Topic: Elmac AF-68 20mtr issue  (Read 5715 times)
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KF4NU
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« on: July 07, 2021, 06:43:55 PM »

I'm hoping someone may have an answer to this one.  My AF-68 tunes up beautiful on all bands except 20 mtrs.  I can't get the plate current to dip below 130ma.  I'm tuning into a dummy load, so that eliminates the antenna.  I did the driver-buffer alignment, and grid current is good. Frequency is dead on, so no oscillator issues. This is something that started recently.  I normally get 30 watts output on 20mtrs, but now it's only putting out 22, with too high a plate current.  Normal initial dip was usually around 50ma. All other bands tune normal and has normal output.   Any ideas?

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Mickey KF4NU
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W1NB
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« Reply #1 on: July 07, 2021, 08:35:24 PM »

C48a is completely removed in the 20 meter setting and, if C48b were damaged the your dip will have changed dial location on 15m and 10m. I would suspect the tapped connection on L46 may have either become disconnected, or its tab on the band switch is not making connection with its contact, or the tap somehow got mashed so its touching the coil where it should not.


* 90E05A0F-0E93-4A5B-B20D-B3FB36C6838D.png (1478.36 KB, 2048x1536 - viewed 250 times.)
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KF4NU
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« Reply #2 on: July 07, 2021, 08:53:10 PM »

Thank you but I already checked the tuning cap, center tap on the 40/20 mtr coil, and wafer contacts.  I really do appreciate that though.  Any other ideas, please spit them out at me.  I'm really scratching my head on this one.

What's really throwing me is all other bands tune normal. 

73's
Mickey KF4NU
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W1NB
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« Reply #3 on: July 07, 2021, 09:27:25 PM »

Mickey,

Just confirming that when you say the other bands tune normally, that includes finding the dip at exactly the same dial location as prior to the 20 meter problem?

If everything else checks out I would try a different 6146.
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KF4NU
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« Reply #4 on: July 07, 2021, 09:37:33 PM »

80,40,15,10 all tune good.  Initial dip goes to 50-60ma on those bands.  But on 20, initial dip is only going to 130ma. I thought of a bad tube, so one at a time I swapped, the buffer/multiplier, driver, final.  No change.  I was really hoping it was a tube going and causing the symptom on only one band.  I'm really running out of ideas.  Please keep throwing them at me.  I think tomorrow I'll spray the wafers and coils with contact cleaner and work them.  May there's a piece of dust, that I cant see, and with voltage to it, it becomes ionized.  I'm really grasping now Hi Hi.

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Mickey KF4NU
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Pete, WA2CWA
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« Reply #5 on: July 07, 2021, 11:47:49 PM »

80,40,15,10 all tune good.  Initial dip goes to 50-60ma on those bands.  But on 20, initial dip is only going to 130ma. I thought of a bad tube, so one at a time I swapped, the buffer/multiplier, driver, final.  No change.  I was really hoping it was a tube going and causing the symptom on only one band.  I'm really running out of ideas.  Please keep throwing them at me.  I think tomorrow I'll spray the wafers and coils with contact cleaner and work them.  May there's a piece of dust, that I cant see, and with voltage to it, it becomes ionized.  I'm really grasping now Hi Hi.

73's
Mickey KF4NU

DON'T SPRAY THE WAFERS WITH CONTACT CLEANER!!!!! Sparingly wipe the contacts with the contact cleaner. Residue on the wafer itself can cause you problems.
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
KF4NU
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« Reply #6 on: July 07, 2021, 11:56:52 PM »

Pete, Thank you for the advice.  I appreciate it.  I'll let you guys know if that fixes the issue.

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Mickey KF4NU
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KF4NU
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« Reply #7 on: July 08, 2021, 03:09:55 PM »

Hi all that's been following.  I did a close look at the 40/20 mtr coil, center tap, leads going to the wafer.  checked continuity.  Everything seems good.  I cleaned the contacts on all wafers as well.  I tuned it up on 80,40,15,10 all those tuned up as normal.  I tuned it up on 20, and got a slight improvement.  Initial dip is down to 115ma.  Still not right.  All other bands, initial dip is down to 50 or so ma.  Any other ideas would be greatly appreciated.

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Mickey KF4NU
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W1NB
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« Reply #8 on: July 08, 2021, 10:31:27 PM »

Mickey, are you actually getting a dip or are you just seeing lowest current at fully meshed or fully open position of the plate tuning capacitor?
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KF4NU
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« Reply #9 on: July 08, 2021, 10:48:49 PM »

It is an actual dip.  This issue started recently on 20mtr only. I've had a few solder joints I've had to touch with a hot iron.  Guess those questionable joints, after 60 years, need a little touch up.  Tomorrow I'm going to take an iron to the center tap, and the wafer. 

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Mickey KF4NU
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KF4NU
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« Reply #10 on: July 09, 2021, 03:37:00 AM »

I hit the center tap and the wafer with a hot iron.  Nice looking solder joints now.  Did a continuity check from the actual coil to the wiper on the wafer, all good.  Put it back together, and tuned it up on 20, and still the same thing.  There isn't that much to look at, so I'm missing something probably really simple.  Thank you all for all the ideas, I greatly appreciate it.  I'll let you know if I come up with something.

73's
Mickey KF4NU
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Tom WA3KLR
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« Reply #11 on: July 09, 2021, 12:17:13 PM »

Mickey,

Is the loading capacitance for 20 meters the proper range? I presume you can load up the other bands to 100 - 120 ma. also.

I’m wondering if the final is operating at several frequencies when on the 20 meter band – a spurious frequency also or an appreciable amount of sub-harmonic drive at 7 MHz? 

Do you have a spectrum analyzer and power attenuator to look at the output? Or have someone who can bring a portable spectrum analyzer over along with a power attenuator, or can you take the transmitter and power supply to a guy with a spectrum analyzer.  That can help figure out what is going on there.

GL OM.
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
KF4NU
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« Reply #12 on: July 09, 2021, 12:35:10 PM »

All bands except 20 are loading normal.  Output frequency on 20 is spot on.  Something changed in the last 2 weeks or so, where my initial dip on 20 will only dip to 130ma. Prior to this, 20 was tuning normal, like all the other bands.  Other bands, initial dip is around 50ma, and load to 100ma easily, with about 30w out except for 10 which is a little lower, normal for 10.  All normal except on 20mtrs. 
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Tom WA3KLR
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« Reply #13 on: July 09, 2021, 03:30:20 PM »

Do you have 3 - 4  ma. of final grid current?

Is the meter movement r.f. bypass cap good?
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73 de Tom WA3KLR  AMI # 77   Amplitude Modulation - a force Now and for the Future!
KF4NU
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« Reply #14 on: July 09, 2021, 03:41:23 PM »

Yes, on grid current.  Meter is working properly.  I redid the buffer/driver alignment 3 times, thinking that maybe there's 2 peaks on 20, and I keep missing one of them. I did the full alignment in sequence on all bands, to rule out an interaction on 20 with another that is slightly off.   I'm starting to think that maybe L33 has changed value.  I'm going to do a closer visual on it.  maybe a small chip out of the core. 
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Pete, WA2CWA
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« Reply #15 on: July 09, 2021, 05:35:23 PM »

I hate looking at just a piece of the schematic. Below is the complete schematic in one piece.

I would check L33 for continuity and check the core
I would check R26 and L22
I would check SW 41C in the 20 meter position to make sure it shorts out L45.

With power on and in transmit mode:
If you have a scope, couple it to Pin 1 of V2 and check each band for any screen anomalies
Then put scope on pin 1 of V3 and check each band again for any anomalies.
Remove 6146 and couple scope to pin 5 and go through each band.

If none of this finds a solution, remove all power, lift the transmitter with both hands, shake well several times, retest  Grin




* af68-sch1.jpg (617.03 KB, 4020x2784 - viewed 265 times.)
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Pete, WA2CWA - "A Cluttered Desk is a Sign of Genius"
KF4NU
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« Reply #16 on: July 20, 2021, 06:53:25 PM »

Problem solved.  Cleaned out the inside of L33 with a cotton swab and a touch of denatured alcohol.  Got some black residue out of it.  Also hit the solder joints of L33 as well as the wafer switch SW31.  Tunes like a champ on all bands now

73's
Mickey KF4NU
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w4bfs
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« Reply #17 on: July 20, 2021, 10:34:46 PM »

sounds like fried roach ... nice troubleshooting
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Beefus

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to see ourselves as others see us.
It would from many blunders free us.         Robert Burns
KF4NU
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« Reply #18 on: July 20, 2021, 11:44:39 PM »

Yeap, tiny roach or an ant Hi Hi
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