Bought a Maco 300 several weeks ago to use with my President Lincoln 2+. The receive amp is non functioning...I knew that at the time of purchase. I also bought a small set meter to put between the radio/amp to keep an eye on things. Hooked everything up and all seemed well for the first couple keys. All of a sudden, bam...I popped both finals and the am regulator in the radio. Checked the swr meter and it was dead shorted so I thought that’s what happened to the finals. Replaced the bad finals/regulator...fired the radio up and talked barefoot for 15-20 mins no issues. Put the amp back in line and after a few keys...bam, blown finals/regulator. Replaced the parts again and the radio is just fine again. Swr is great (1.1-1) before and after the amp. Only thing I can see is after a few keys the amp relay starts kicking in and out. Any ideas would be appreciated!
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Maco 300 blowing radio finals
- Thread starter Jeremy Hull
- Start date
On standby? Or with the amplifier keyed?
Or either way?
Years ago we would see Cobra/Uniden base radios that blow a final when the owner brings home a Palomar 300A. Never figured out exactly why bad relays in that model were so consistently toxic to the old-style bipolar final transistors. Would fry the first time the amplifier gets keyed in "operate" mode.
Until the 300A got a new set of relays.
After the first couple of disgruntled customers I learned to ask "did you key it into a 300A?" whenever a President Washington, Madison or Cobra 2000/142 had a blown final in it.
If you haven't cleaned the big relay in the Maco, maybe it's time.
A coax jumper that has the shield braid just 'folded over' inside the plug and not soldered can cause mischief.
Use of pop rivets to mount the coax sockets on a linear can turn into an intermittent ground connection for that socket.
The non-functioning preamp leads me to wonder if the standby side of the preamp relay is cutting out. Usually you get a hint of this when the receive signals drop in and out, or when you unkey. That alone can do this.
I always recommend bypassing the preamp with a jumper wire on the main relay.
And if the SWR is high only when the amplifier is keyed, that will have to wait for another day.
A 35-Watt radio is a bit large for the Maco 300 with two driver tubes. If yours has only one driver, you need a radio about half that size to match it.
73
As I recall there’s no issue in stand by. Just after a few key ups with the amp in operate...the amp relay chatters and blows the radio finals. If I could pip point what’s up I’m sure I could fix it...there really aren’t any techs near by me that service the old tube amps. Probably the best thing would be for me to take it to someone and have it fixed even if it were a drive. I’m in northwestern NC so if anyone knows a GOOD tech let me know please. Thanks for the response.
Also, swr is fine between the amp/radio and after the amp so I’m sure it’s an issue with the amp itself.
Check C11, the plate blocking cap for the driver. Because of the cathode switching on the main relay, high voltage can be on the center wire of the input coax when the amp is unkeyed and in receive mode. L1 is supposed to stop this voltage from appearing on the input if C11 shorts but I've seen this choke get blown open before by a C11 failure. Also, since the main relay has contacts for both TX/RX switching and separate contacts for cathode keying that are somewhat connected by C11, a smaller momentary voltage spike can occur (the charge across a properly working C11) on the input coax if only L1 is open and the contacts are dirty or the timing between the contacts isn't in sync because of age.
So, the amplifier never really does "key" and hold the relay closed. Just chatters until the radio fails.
I'll guess that the SWR that the radio is feeding into while the relay chatters is off the chart. Good way to pop finals.
Leaves two possibilities.
The keying circuit may be hosed. Not so rare. We seldom leave the stock keying circuit in a Maco, and install the one we sell on Ebay to prevent chatter.
More likely the amplifier's input impedance may be so low that it pulls down the RF voltage from the radio. Pulls it down below the level that holds the keying circuit active.
Makes the keying circuit shut off.
But now the relay falls back to its standby position. No SWR, plenty of RF voltage to activate the keying circuit. So it begins feeding current to the coil of the antenna relay
Again.
But not for long, since as soon as the contacts close, the input impedance drops like a rock, the RF voltage to the keying circuit falls below the minimum threshold, and it unkeys.
Again.
Lather, rinse, repeat.
The input-impedance adjustment is one of two kinds, but only if it came with one at all. Not every production run of Maco amplifiers had one at all. Either a tiny brass screw slot on the rear panel, *OR* a small slug-tuned coil with a tiny access hole for turning the slug on the rear panel.
Or no matching circuit at all, with a capacitor that goes straight from the relay's input side to the cathode pin of the driver tube. Best to use only a radio with a tube for the final driving one of those. Input SWR tends to run three-to-one or higher with that setup.
We used to have a 23-channel Kraco AM base radio that had a SSB final transistor in it. Power supply was turned down to 12 Volts. Was bullet proof. You could key it into any amplifier, no matter how messed up and it wouldn't suffer damage. Our "bulletproof" bench radio got retired some time ago.
Sounds like you need one of those, just to get the amplifier's input SWR low enough that your modern radio can cope with it. On some old amplifiers that used a tube to sense the radio's RF, the input-matching adjustment would be set for "minimum chatter" from the relay. Won't work with a radio built with SWR-sensitive MOSFET finals.
You could just jump across the keying circuit with a gator-clip lead, and hold the relay closed while adjusting the input match low enough to use. I would recommend an external SWR meter patched beween the radio and the amplifier's input to do this. A radio's built-in SWR meter may or may not be accurate enough to help. Should probably turn down the carrier to a couple of watts or 3 just to reduce stress on the radio finals.
And if you can score an old Cobra 29 with the bipolar final, it would be safer to use while you civilize this amplifier's input circuit. Once it behaves, you can severely overdrive it with the fragile radio.
Only question would be for how long?
73
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Actually, it does key fine the first couple of keys. This last episode, I started on a.m., keyed up with about 3w and 28-30w swing (from the radio) and worked the amp fine. Amp showed about 150w key and 550 modulation. Unkeyed, paused, keyed again...no issue. Filled to sideband, keyed/modulated, no issue. Switched back to a.m. and keyed, relay kicked out once...let off the key, rekeyed, amp chattered and finals blew.
Sounds like the amp would be better suited for someone with a different radio! Maybe need to sell it and get a pill amp...anyone interested in a Maco? Lol
Saw you posted that you are in NW NC.
Screwdriver is in Statesville, NC. He works on and builds tube amps. He used to work building Elkin amps many years ago. Friend of mine bought a MACO 300 from him about six weeks ago that Screwdriver had fixed.
Send me a PM, and I will hook you up with his contact information.
Screwdriver is in Statesville, NC. He works on and builds tube amps. He used to work building Elkin amps many years ago. Friend of mine bought a MACO 300 from him about six weeks ago that Screwdriver had fixed.
Send me a PM, and I will hook you up with his contact information.
BJ radionut
Supporting Member and 6m addict
Jeremy: You must remember when that amp was built radios most generally only put out 5 watts MAX...So hitting it with 30+ watts is like driving it with a Sledge Hammer.
All the Best
Gary
All the Best
Gary