Maco 300

[lmoreau]

Hi all...long time CBer who has drug some stuff out of the closet...

I have a Maco 300 that used to work fine. Well, I drug it out of storage and tried to fire it up. It powered on, and all tube filiments glowed fine. On key up, however, the relay that is between the radio/antenna connections started to buzz/click very fast and was very noisy. Meter in the front of the unit would peg out and fall off in sync with the relay opening and closing.

After some fiddling around with the load and tune knobs, and just general trying to key the thing to awaken it from its slumber, I could get a solid contact when I keyed the mike. But when I started with the audio, the modulation light would stop glowing and the meter would fall off back to 0...then when I stopped talking, the meter would swing forward and the mod light would come on again. If I really got loud into the mike, the relay would start the chatter again.

Any ideas? This thing used to talk great back in the day! I pulled the bottom off and everything was very clean...I didn't notice any leaking caps or anything.

Thanks!!!

['Doc]

Just a rough guess but I think you may have two things 'not right'. One of those is correctly tuning the amplifier. Probably the simplest method to do that is to adjust those knobs for maximum power out. The 'catch' to that is that you need to keep the drive power under whatever the amplifier's max input power is. Not always easy to do specially if you don't know what those max input number and the radio's max output numbers are (don't ask me, I don't know).
The second part, the 'modulation light' changing and the meter moving backwards, means you are over modulating, the radio is producing more than 100% modulation for whatever reason. When you have more than 100% modulation you actually reduce the output power, you sure don't gain anything from that. And that's why the meter moves backwards, less power going through it. Not exactly what's claimed for high modulation figures is it? Sorry, still the way it is.
- 'Doc

[lmoreau]

Yeah, that makes sense...playing around with it, I seemed to get it to work 'okay'. I dont remember every having this problem when I used to use it years ago. I was able to transmit thorough it and talk to people, so it's doing something. I dont have my wattmeter anymore so, I can't tell what it's doing. There can't be much wrong with it.

['Doc]

A watt meter is a nice thing to have if you are going to use an amplifier, makes things much simpler to do. Depending on the metering on the amplifier, if any, you can tune it by using them. How you do that tuning depends on exactly what those meters are measuring though, and typically it isn't just a tune for max thingy. Just too many things I don't know about that amplifier so I won't even try to tell you how to do that.
There are a few good general guides for tuning amplifiers here and on other forums. It'd be a good idea to go through them at least to get a general idea of what/how thigs should be done. Since those guides are only general in nature, I would make sure they 'fit' your amplifier before using them.
If you have a stock radio I would think that you would stand a better chance of 'under-driving' that amplifier, rather than 'over-driving' it, which is good. Under-driving isn't going to be damaging where over-driving would be.
Good luck.
- 'Doc

[385]

You really need a swr/watt meter for that maco. I have one too and they have an ALC in the back marked "match" and takes a small allen wrench to adjust. It really made a lot of difference for me. The swr/watt meter needs to read at least a 1000w. I've seen 1100 out of mine on sideband but thats runnin it real hard. Does it's best at about 500-600 AM or SSB. I run a RCI-2950 (non DX) at 2.5w DK and 20w swing on am and 15-20w on ssb. Nuthin but flowers all day. I got a little more info on it if your interested, Just takes me a while to type.

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[lmoreau]

Thanks for the info. I have had the amp for about 15 years, believe it or not. I never had any problems with it before until I tried to fire it up the other day. I dead key and tune it to max with the tune and load knobs...the relay locks in and the mod light comes on ok. But if I start to modulate, the relay starts clicking rapidly, so the amp is kicking on and off of boost every time you say something. A reply I got from a radio check was that it sounded 'chattery'.

I don't have the nice rig like I used to. I lived in the country back then...now I'm in the suburbs, and don't really have anywhere to put up a nice antenna. I was testing this on a Lil Wil with a 25 LTD.

I really want to sell it, as I have no use for it anymore, but I wanted to make sure it was working correctly before I did.

I LOVED this box back in the day. It would black out every TV in the house when I keyed it up. My neigbors would complain once in a while It really got out there with no problems. It won't drop the maul like some of these guys out there, but I wasn't a mud duck!

[385]

I think that "lil wil" mag mount antenna is only rated at 300w, which could cause problems like that. Mine has a tune knob on the right side near the rear that adjust the driver stage. This is what I was told by a guy in Memphis TN that used to work for MaCo. You could list it on that big auction site as limited testing but probably won't get as much for it though. You could take it to a trusted tech that could test it for you and know for sure what it's doing power wise, or buy a radio shack swr/watt meter for around 30-40 bucks and antenna and use the meter that you bought to set the swr's on it and find out yourself what its doing. Hope this helps........385

['Doc]

Get that watt meter. If/when you sell the amplifier, include the watt meter along with it if you don't wanna keep it. Ought'a work.
- 'Doc

[lmoreau]

Excellent Idea....the meter would be a good investment.

[Slowdraw]

Didn't the Maco 300 have an input tune capacitor on the back? It may be out of tune. You will need an SWR meter between the radio and the amplifier to tune it properly.

There is also a filter capacitor for the 12v supply that may be bad. You would hear a 60 cycle hum from the relay along with the clicks if this was the problem.

Always double check the way your coaxes are hooked up. If they are backwards, you will get relay chatter when you key up.

[ironchief]

High swr will causes the same problem as well as a bad coax jumper connection. I myself need a antenna relay for my maco 300. Does anyone know where or how I could find one?

[Punkin Head]

Ironchief, you may do better if you start a new topic regarding needing that relay. With this thread being so old you may not get a reply on it. I know there are several companies that carry parts for amps but I couldnt point you in the right direction as I dont know who would have it. Hope you do get what you need to get it running tho.

High swr and bad coax jumper will cause that type of problem with a tube amp. It can cause the relay to "chatter" and cause the amp to not perform or could cause damage to the amp.