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Author Topic: Modulation issue Clegg Zeus  (Read 1215 times)
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wx3k
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« on: August 27, 2023, 09:18:23 PM »

Hello AM foners

It has been awhile since i posted here.

I operate my Clegg Zeus & Interceptor on a weekly 6 meter AM net. Its been a great rig for years !

A recent development is the modulation pattern gets what i call more inflexible to modulation characteristics in my voice. Less peaks and less full looking on a bandscope. There is also a feedback issue that creeps in that seems to coincide with what sounds like arcing in thr final cage. I just recently changed the final to a 4CX250 and put it back on the air. Looked great for a couple of transmissions and this mod issue returned.

Hmmmm

The arcing sound occurs separately from the mushier modulation issue.

Hmmmmm

What i suspect is a component is heating up/breaking down after several minutes of operation. Im just not sure where to start.

Power output is solid the entire time this issue develops until i hear the arcing which does show up as fluctuations in plate current

Perhaps my first step is the observe what ends up arcing if this is possible.

This rig is a challenge to troubleshoot because it has an AGC circuit for modulation which works well until you remove the bottom cover of the transmitter and all hell breaks loose LOL The lack of shielding plays havoc with the modulation so monitoring voltages is nearly impossible.

Any suggestions would be helpful
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Stephanie WX3K
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K8DI
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« Reply #1 on: August 28, 2023, 07:05:12 AM »

No familiarity with the Clegg…but I chased my tail for months with a “warms up and then all hell breaks loose” problem with my RCA broadcast rig. It turned out to be loose hardware on a doorknob style cap. It was a matter of take it out, clean where it sat, put it back with a new lock washer, problem solved! But it took forever to find.  I’d suggest you go through the thing and make sure it’s mechanically tight as a reasonable first step.

Ed
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Ed, K8DI, warming the air with RF, and working on lighting the shack with thoriated tungsten and mercury vapor...
KD1SH
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« Reply #2 on: August 28, 2023, 09:25:15 AM »

  Hi, Stephanie. Sorry to hear you're having problems with your Zeus. I've been running a Zeus/Interceptor for five years now, and I absolutely love it— it's the centerpiece of my station.
  Having said that, I can't promise to be an expert in working on the rig, simply because it's been so reliable that I haven't had to do much with it, other than re-capping it when I first brought it home, and replacing the 4X150 with a 4CX250, as you did. Re-neutralizing was touchy, but otherwise that went well.
  As far as the arcing, at least it sounds like it's confined to the final cage, which narrows it down and reassures you that it's not happening in the secondary of the mod transformer, which wouldn't be good news at all. I'd have to agree with Ed: check all the hardware first, especially since you were recently in there replacing the tube. Maybe revisit the neutralization, too—the RF PA suddenly breaking into oscillation could very well cause the arcing that you're hearing.
  And the "mushy" audio—could be a lot of things, of course, given that the Zeus's modulator is a bit more complicated than some, with its nifty phase-inverter driver and clipper. You might want to start by taking a good look at the 6C4 cathode follower which controls the bias to the back-to-back diode pair. This comprises most of the clipper/compressor circuit; any changes to the 6C4 or its peripheral components would change the point where clipping begins, and potentially compress your audio into the mud, and a failure of either of those two back-to-back diodes would probably leave you will some very asymmetrical audio.
  Another thought; something I have experienced: check the interconnect cable between the power supply/modulator and the transmitter. Blast the pins with DeOxit—both the cable and the transmitter and modulator— and make sure both ends are making good contact. There's a lot of stuff going on in that 21-conductor cable, including low-level audio and signal grounds.
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« Reply #3 on: August 28, 2023, 09:58:46 AM »

I'm going to go out on a limb here and say your mod transformer may be 'transforming' (no social correctness intended!). If I recall, that uses 811's as modulators. Pete, WA2CWA was the Clegg fellow on here so he may have some input. But that 'mushiness' in the audio reminds me of something similar. Good luck.
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Mike(y)/W3SLK
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« Reply #4 on: August 28, 2023, 12:04:33 PM »

I have no experience with the Clegg but you might want to air spray (from about 12" so as to not bend the plates) of the tuning caps and adjacent areas.

I have had little critters crawl in tuning caps and also to collect dust and webs that are, believe or not, conductive. Crispy critters  Smiley can make havoc near HV.
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