POW KW+ keying circuit

[nomadradio]

I do have one suggestion. The coax leading from the Load control to the relay does not have the shield braid connected. This was a bad habit somebody must have learned from Maco. Connecting the braid to a ground lug on the Load control, and running a wire from the rear end braid to a ground lug on one of the Antenna jack screws tends to make the driver tuning more stable. I just unwrap a half inch or so of the black jacket at each end without taking anything loose. A short piece of stranded hookup wire gets wrapped around the exposed braid and soldered. The other end of each wire gets grounded at that end.

Whether or not the driver Tune control gets twitchy when you turn it, we ground each end of the output-coax braid as a preventive measure.

73

 

[Trainman 8274]

I do have one suggestion. The coax leading from the Load control to the relay does not have the shield braid connected. This was a bad habit somebody must have learned from Maco. Connecting the braid to a ground lug on the Load control, and running a wire from the rear end braid to a ground lug on one of the Antenna jack screws tends to make the driver tuning more stable. I just unwrap a half inch or so of the black jacket at each end without taking anything loose. A short piece of stranded hookup wire gets wrapped around the exposed braid and soldered. The other end of each wire gets grounded at that end.

Whether or not the driver Tune control gets twitchy when you turn it, we ground each end of the output-coax braid as a preventive measure.

73

Hi Nomad,

Thank you for the suggestion! I was wondering if one end would work or both. I will do both, hopefully this weekend. Does the wire size matter when grounding?

73

 

[linearone]

Hi Nomad,

Thank you for the suggestion! I was wondering if one end would work or both. I will do both, hopefully this weekend. Does the wire size matter when grounding?

73

Do a ground connection at each end of the coax. Use whatever you have on hand.

 

[nomadradio]

We use #22 stranded for amplifiers that size. A solid wire is fine so long as you get it to wrap around the exposed end of the shield.

And like the man said, both ends.

73

 

[Trainman 8274]

Thanks for the help! Next question… How much drive will this thing take? Not that I’m going to drive it that hard.. Tubes aren’t cheap! I’m just curious.. I tried it with a Grant LT and Galaxy 86V on am.. It doesn’t seem to swing very much.. I may not have it tuned right… It has a knob on the side and tune and load on the front.. So, what is the correct way to tune it?

Thanks!

 

[nomadradio]

First thing to check is for the "twin peaks" on the Plate Tune knobs.

With the Load control set for max modulated power, spin the driver tune on the side. It should show a peak at *TWO* separate spots in one 360-degree turn of the knob. The peaks might be close together, or they might be 180 degrees apart. The important thing is to see that you have two peaks. If there is only one, a glance at the plates on that control will reveal that they are at one or the other extreme of adjustment. Plates fully apart, or fully meshed together. When you see this, the coil attached to that control has to be adjusted. This rule also holds for the front-panel final Tune control.

Until both Tune controls show those two peaks at a spot in between the two extremes of adjustment, you'll typically lose some power.

Running it with either Tune control at "just one" peak can be hard on the tubes.

The tiny adjustment screw on the rear panel gets set with a SWR meter and a coax jumper added between the radio and amplifier. Once it's tuned up for max modulated power, see what the drive-side SWR meter reads. The rear-panel screw is the input tuning and should be set for minimum SWR while the amp is keyed. Makes life easier on the radio.

Any of those can hold back the power.

73

 

[Naysayer]

Amplifier layout and chassis reminds me of the Elkin amps.

 

[fishingdutchman]

If you can only find AC in this amplifier, you've found the problem with the original keying circuit. The amplifier must have DC to run its keying circuit.

As I took a moment to look at your picture, I can confirm the reason why you have lost dc. Look at the small gray electrolytic capacitor mounted to the terminal strip Behind The Rocker switches. It's positive terminal is snapped off of the anode side of the diode. Leaving Raw DC that will not operate any of the relays properly. Check that capacitor see if you can solder it back or if it needs to be replaced.

Hello I'm an old time CBr with an old sweet tube linear been dragging it around for 30 years hardly used great shape tubes but did a dumb thing it wouldn't key up very easily so I manually tapped the sensor or switch and shorted it and now I can't find anyone to work on these.Pow KW+ Maco?Thanks for any help

 

[nomadradio]

I manually tapped the sensor or switch and shorted it

Not sure which "sensor" you mean. The relay in the back between the two coax jacks, maybe? It's a switch that's operated by an electromagnet. Called a relay. When the magnet gets current, it pulls a spring-loaded contact lever down and closes the linear's operating circuits. If manually pressing that part caused an overload surge, this is a sign you have at least one failed tube. Maybe more.

If all the parts are original, don't assume all the tubes are okay. Sure, you want them to be, but the odds are not in your favor. Best of luck finding a tech with those skills. Fewer and fewer of them still around every year that goes by,

One thing we have seen more and more often the last ten years or so is that tubes that tested good on a tester will work in the linear for the first dozen keys of the mike, and then there is a "snap" sound and a blue flash from inside one tube. The surge that caused the flash permanently disables that tube. If you don't have more of them it's now dead in the water. Sometimes the surge in the one tube damages one or more of the soldered-in parts, or causes "fratricide" and clobbers another tube alongside it.

Odds are you'll also get advised to change all of the electrolytic capacitors in it. This is because the chemistry inside that type of capacitor just goes bad after a decade or two on average. Yours is closer to four decades or more old. When those parts fail, they can become a short circuit that causes further damage.

Don't know if you have ever been involved in making a 40-plus year old car back into a daily driver. Rubber parts will have to get changed from age alone. Other parts will have to be renewed based on mileage. Your linear is not so different in those respects. Doing just part of the restoration can leave you stranded on the side of the road, so to speak.

Here's a post that shows what was needed to put a Maco 300 back on the air. It's not the same as your POW, but it's roughly the same age.

https://www.worldwidedx.com/threads/maco-300-gets-47-year-tuneup.268043/

73