.the amp relay chatters and blows the radio finals.
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