So here is a rare treat. A Maco 300 with low miles. Upshot of this is that the owner has six good tubes. It's the 7-tube version, but no problem. I won't send a Maco 300 home with seven tubes in it. The fifth final tube comes out. More on that later.
Naturally each of the four final tubes will get its own, separate parasitic choke.
At least we know its birthday.
The two driver tubes each have a proper parasitic choke.
Fortunately only one wire has to get spliced when the center tube sockets gets removed.
You'll need a tiny plate to cover the empty tube socket hole and support the plate choke.
The new plate choke occupies the center of the four driver tubes. Provides a stable anchor to attach four parasitic chokes. This is the same choke used in the Palomar 300A and the Heathkit SB220. Honest.
So here is my pet peeve with Maco. They didn't ground the shield on this coax. It leads from the Load control to the output side of the relay.
The shield is just left loose at the front end, too.
The receiver preamp just goes away. The entire circuit board with the white relay on it.
Abracadabra! It's gone.
Just one problem. This board also contains the keying circuit. But not a problem for long. Got keying circuits. On hand.
This pic shows three issues fixed. The output coax is now teflon and has the shield grounded to boot. There is a tiny keying-circuit board visible under the radio jack and a rectifier diode has been inserted between each set of tubes and the relay's center pole.
Didn't shoot any "intermediate" pics before all three fixes were in place.
Yes, the output coax braid is grounded at the Load control, too.
There is a visible rectifier diode leading from the final tubes' cathode circuit to the center pole of the relay.
The cathode connection on the two driver tubes has a 3-Amp diode between it and the relay's center pole. The four finals have a 6-Amp diode between them and the relay. This serves to isolate the two separate cathode circuits. Maco simply tied them together at the relay. Sounds simple, but we had a recurring problem that a driver tube would "FLASH!" when you unkey. Not healthy for the tubes. Stumbled across this fix. Someone suggested it to me, but 35 years later I have no idea who to give credit. Long story short, this stopped the flash problem.
Yeah, the original hookup was causing some kind of oscillation, but I don't know what. A fix that works is where I move on to the next chore.
Oh, and speaking of oscillation. That's what leaving the braid on the output coax "floating" with no connection will cause. This turns the coax braid into an antenna, beaming unwanted feedback into the amplifier's input circuits. This is our preferred fix. Yes, this coax is skinny, but it's coax. The losses are low enough it will handle whatever four sweep tubes can deliver.
There are more chores remaining on this one, both upstairs and downstairs, but it was time for dinner.
More when it gets finished.
73
Naturally each of the four final tubes will get its own, separate parasitic choke.
At least we know its birthday.
The two driver tubes each have a proper parasitic choke.
Fortunately only one wire has to get spliced when the center tube sockets gets removed.
You'll need a tiny plate to cover the empty tube socket hole and support the plate choke.
The new plate choke occupies the center of the four driver tubes. Provides a stable anchor to attach four parasitic chokes. This is the same choke used in the Palomar 300A and the Heathkit SB220. Honest.
So here is my pet peeve with Maco. They didn't ground the shield on this coax. It leads from the Load control to the output side of the relay.
The shield is just left loose at the front end, too.
The receiver preamp just goes away. The entire circuit board with the white relay on it.
Abracadabra! It's gone.
Just one problem. This board also contains the keying circuit. But not a problem for long. Got keying circuits. On hand.
This pic shows three issues fixed. The output coax is now teflon and has the shield grounded to boot. There is a tiny keying-circuit board visible under the radio jack and a rectifier diode has been inserted between each set of tubes and the relay's center pole.
Didn't shoot any "intermediate" pics before all three fixes were in place.
Yes, the output coax braid is grounded at the Load control, too.
There is a visible rectifier diode leading from the final tubes' cathode circuit to the center pole of the relay.
The cathode connection on the two driver tubes has a 3-Amp diode between it and the relay's center pole. The four finals have a 6-Amp diode between them and the relay. This serves to isolate the two separate cathode circuits. Maco simply tied them together at the relay. Sounds simple, but we had a recurring problem that a driver tube would "FLASH!" when you unkey. Not healthy for the tubes. Stumbled across this fix. Someone suggested it to me, but 35 years later I have no idea who to give credit. Long story short, this stopped the flash problem.
Yeah, the original hookup was causing some kind of oscillation, but I don't know what. A fix that works is where I move on to the next chore.
Oh, and speaking of oscillation. That's what leaving the braid on the output coax "floating" with no connection will cause. This turns the coax braid into an antenna, beaming unwanted feedback into the amplifier's input circuits. This is our preferred fix. Yes, this coax is skinny, but it's coax. The losses are low enough it will handle whatever four sweep tubes can deliver.
There are more chores remaining on this one, both upstairs and downstairs, but it was time for dinner.
More when it gets finished.
73