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What causes a Maco 750 to swing backwards?

Feedback/RFI and excessive deadkey into the box are a couple things that come to mind if you're swinging backwards... Check your deadkey, make sure you're not putting some nasty high deadkey into the box, 3-4 watts deadkey is all you need for a deadkey input, and let it swing.

Check and make sure all coax connections going to and from the box are solid and well soldered (check all jumpers inline). Grounding the box (if it doesn't already have a grounded plug) helps too. Make sure any inter-connections that involve coax cable inside the box are not kinked and are in good shape.

Check all tubes, make sure they are good on emissions, grid leakage, and no gas.

As dumb as it sounds, make sure your SWR is good in front of the box, and after the box.

Give this all a try, tell us how things work out.

Regards,
Rick (Triple 7)

I had a diode go up in smoke, so I've got to get it replaced before I can get back to the swing issue. That is if the diode is why it stopped keying up.

I replaced the diode that went up in smoke and a capacitor that I broke the lead off of. I also placed a mesh shield between the 2 drivers and the finals to help with the oscilliation of the tubes. The amp has an onboard swr tuner that I made sure was tuned to the lowest setting. That is not what it is called, but what it does.

Now my problem is this. With 2.5-4 watts dk going into the amp, the amp will dk from 13-20 watts and swing between 500-600. It is possible for me to tune the amp to a higher dk but then the amp starts to swing backwards. I cannot get a higher dk without losing forward modulation.

Any ideas?

I have one other thing to do to the amp that somone suggested doing and that is to make sure the shield on the coax in the amp is grounded on both ends. This is to prevent rf problems somehow. I don't think it has anything to do with the low dk.

Any suggestions will be greatly appreciated.

Thank you!!!!!

Most of the time when you see the meter swing backwards its caused by overmodulated output by wich i mean you start to clip the output. with out the amp on what is your dk and what is the swing. Also what does the maco need for forward swing. it is possible that you have to much modulation. If it is a low drive amp like i am guessing that is what you have 1.5 to 2 watts dk is all you need. then with audio you should have about 10 watts swing is all the more you can drive into them before you run into overdriving the linear. I am not familiar with the maco but i will find some specs and see what they are.

MIKE

lower the DK on the amp...let it swing...

I could be wrong but if an amp is swinging to 600 watts and only deadkeying 15-20 watts that is a heck of alot of modulation. Personally i would like to see a higher deadkey. I could see 150 watts for your deadkey and swing to 600 but 15 watts and swinging to 600 that sound a bit silly. I dont know call me crazy. I think that when you get that kind of ratio it makes you sound kind of funny at least with past experience. With your carrier is that low when talking great distances the person on the other end would only here the modulation and no carrier. Dont know guess it comes down to a matter of how you like to set up your equipment. I would personally try to get my dk higher. You will have to start with your tranceiver and see what you are getting out of it then go from there. Also make sure you dont have any limiters cut in your radio. This is not good for your linear since this will only make you sound like you are talking in a tin can. Especially on a linear amp.

mike

I would like to get the dk up as well. That is what I am trying to figure out, why the amp's dk is so low. Right now I have 2.5 dk going in, with 3.5 dk coming out and swinging to 450.

At one time it was keying 600 and swinging backwards, now the dk is low and I can't get it to key the 600 and swing backwards any more.

Frustrating indeed.

Thanks

Sounds like there is still a problem with the amp. Make sure that you dont have a cap going bad in the input. You said that you had a blown diode that concerns me a little. What caused the diode to blow if you know. Also check the drive tubes you might have a bad tube giving you problems.

MIKE

I am gonna admit to defeat on this project. I am gonna load it up and take it to the local amp guru tomorrow, which is probably what I should of done from the beginning. No, I take that back, what I should of done in the beginning is to go with solid state, but I had this tech tell me that if your'e not running glass, you have no class. I know what I'd like to do with all this glass right now. Thanks to all for your advice and suggestions on this project.

73's

Two days ago I spent 1 hour with the local amp guru. I say local, he is actually in another state. Anyway, after an hour, 10 replaced parts, some of which were missing entirely, the amp is now working. For $54.50 I could of saved myself a lot of headache. The amp will now dk anywhere from 0 to 300 and swing up to 1200. I guess he did well.

Thanks to everyone for your help. I might of been able to do the repairs had all the parts been there from the begining.

73's