maco 750 ssb problems

[fatcat3]

I was wondering if anyone has experienced this problem with the 750. on AM it sounds great, everyone I talk to praise the sound of my setup. but when I go to ssb it sounds like crap. I also hear a "rattle" coming from the box. I have tried readjusting dead key and output to get rid of it but with no luck. can someone guide me in the rite direction?

[CowboyCody]

I have no experience with those old tube amps, but my guess is there is no SSB circuit in it to keep the relay keyed up. I have heard of guys building their own keying circuit for some of those old style amps, I am assuming this is the reason why. Adjusting your dead key will not help in this situation because there is no carrier on SSB. Hope this helps.

[jessejamesdallas]

You're probably over driving the Amp on SSB...

I had the same problem with my Cobra 2000 GTL on SSB using my Kicker 500. The Kicker 500 is a 1x3 tube Amp.

On AM it sounded just fine, swinging to about 450w from a 3w DK...But on SSB everyone said it sounded horrible...The Cobra 2000 was swinging to about 20w's on SSB, so I adjusted the SSB ALC pod down so instead of swinging 20w's, it only swings about 6 or 8w's. This also caused me to loss some of my swing with the Amp turned on, from swinging over 500w's on SSB, to only swinging to about 200w's, BUT, Now my modulation on SSB is crystal clear with NO distortion...On AM I still have the same amount of swing as I did before adjusting the ALC...

What it is, these older Tube Amps were built back when radios didn't have that much swing (10-12w)...least that's what I was told, so by lowering your swing it clears up your signal...

It didn't hurt my modulation either by backing off the ALC because everyone still tells me I'm blowing smoke...

You didn't say...But what radio are you using?

[fatcat3]

I am using a cobra 148 gtl with this. the over driving is exactly what it sounds like. will have to take it to my friend and let him put it on his bench. thanks for the replies.

[jessejamesdallas]

I am using a cobra 148 gtl with this. the over driving is exactly what it sounds like. will have to take it to my friend and let him put it on his bench. thanks for the replies.

If it's a older 148 GTL, those have the same board in them as the 2000 GTL's...The SSB ALC is VR11 and is located close to the back/center of the board...You may have to move some wires to see it...

[fatcat3]

OK finally got ahold of him and he said am and ssb where both set at 12w. he did suggest I try something else though.

[jessejamesdallas]

12w's is fine for AM, but SSB needs to be lower if you want to clean up your modulation...

[silvereagle1]

Hey fat cat, when I first got my grant it was clipping real bad on sideband. I took it to Bluestripe and it was only putting out 5 watts max on sideband straight from the factory. He could tell it was clipping before I told him what was wrong with it. He fixed it though. one thing, if you take it to him better put some tape around the bezel!! he scratched mine a little. Not bad but enough to see. very happy with the performance now on the grant.

[Bluerunner]

On the older tube radios the capacitors often lose their capacity some. Maybe the capacitor that is used for the key up relay delay is weak and need replacement.

Looking at the Schematic on CB Tricks it is a 220uf capacitor right next to the SSB switch on the diagram.

Replace it or just add others to it to increase the delay time to what you like. Always good to use capacitors with higher voltage values than the minimum required in the circuit. Typically, for the smaller ones I double it.