Maco 300 gets 47-year tuneup.

[nomadradio]

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

 

[BJ radionut]

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

NICE work...NOMAD...clean!!!

 

[nomadradio]

Here's the continuation.

The original HV rectifiers are 1N4007 rated for 1 Amp. Never underestimate how big the surge a Maco can create inside.

The original HV filter caps have to go. Here we see the "Maco Bonus" feature. No bleeder resistors.

Overkill in the defense of reliability is no vice.
(with apologies to the ghost of Barry Goldwater ex-K7UGA)

Yes, those are 6-Amp rated rectifier diodes. The added cost on the repair total is a lot less than the aggravation of having JUST ONE of the original diodes break down.

The HV filter pack is the one we sell on fleabay. The 220uf filter caps are more apropriate than the 100uf parts the factory used.

The yellow wire feeds the 16 VDC relay power to the screen grids of the 8950 tubes. This is a sorta "nitrous" trick for these tubes. We make two mods to this hookup. A series diode gets added coming out of the filter cap. This way, if a tube shorts and shoots a few hundred Volts back out those two pins on the tube socket, the 12-Volt keying circuit won't get fried. And at the far left you can see a 10k 2 Watt resistor from the yellow wire to ground. This is to prevent a tube from running away if the screen grid gets hot enough to go into secondary emission.

Maybe. Cheap insurance, anyway.

The original grid-bias filter cap has to go. For more than one reason.

Can't read the rating until it's removed.

This capacitance value is a compromise. Too small to properly remove the rectifier ripple (hum) when the carrier is turned down. The capacitance is just big enough that when the radio modulates the amplifier hard enough, the tube's control grid wil rectify the audio peaks, creating a high negative DC voltage on the tube's grid. This causes the carrier to "dive", being suppressed by the negative bias, until the capacitor bleeds back down to negative 16 Volts DC where it was before you screamed "FOUR!" into the mike. This "diving" is a problem. It also screws up the audio waveform, but the fix is simple.

The 3300uf filter is more than enough to smooth out the negative DC bias voltage. No more hum with the carrier turned down.

The rectifier diode in the center of the three has its cathode (banded end) connected to the 12 Volts AC heater supply The anode now feeds negative DC to the 3300uf filter cap. The top of the three diodes feeds bias to the grids of the driver tubes. Any increased "grid-leak" voltage appearing on this pin of each driver tube will stay here, and won't charge up the filter cap. It's called a "blocking" diode. The lower one does the same thing for the four final tubes.



As advertised, each of the four final tubes now has its own parasitic-suppression choke.

It's agonizingly close. Still need the Faraday shield between the driver and final tubes before I try to fire it up. Half the trick with the 300A is to keep it from oscillating.

No idea how long that will take. Got stuff on the calendar for the next couple of days.

Film at 11.

Or maybe 12:30

73

 

[nomadradio]

IT (was) ALAYEEEV!

For about a dozen keys of the mike, and one of the 1976-made 8950 tubes flashed and died.

Forever.

The owner says he has some tubes, so we'll pick up where I left off once he drops them off.

I'm about ready to call this "Maco Syndrome". Works great until 48-year old power tubes get some stress, followed by a POOF!

There is a Maco 750 in the shed that we rebuilt and it snuffed four 2057 tubes, one by one until we were one tube short to run it.

Gonna need a better way to screen customers' tubes before tearing into an amplifier. A tube tester just doesn't work them hard enough.

The older the tubes get the riskier they seem to get.

73

 

[linearone]

I seem to remember opening up an amplifier like this a long time ago and somebody had used some kind of a tin can as a shield for one of the tubes I don't know if it was a mako but I remember seeing a tin can bolted over the deck over a tube like literally a small soup can. It might have had holes in it I don't remember

 

[nomadradio]

This project got sidelined when one of the customer's six 8950 tubes went "FLASH!", 'snap' and died on the spot. He had said he had 'spare' tubes, but when he brought them in, they turned out to have nine pins, not twelve.

Oops.

Back up and punt time. Turns out I had a pair of 6LB6 tubes in the "we don't stock those" stock. Had to rewire the driver sockets, first to put the two heaters in series, and move the control and screen grid wires. The 8950 is wired the same from pin 1 to 6, but not from pin 7 to 11.

The Faraday shield got installed, and the new driver tubes don't look all that different.

Upside is that he still has five good 8950 tubes, with four of them installed in the final stage. No telling how long until another 1977 tube goes "SNAP!". When it does, he'll have one remaining spare to take its place.

Probably ought to mention the issue of making the Plate Tune control peak properly between its max and min extremes. The final coil (up top) gets tweaked until the Plate Tune control shows a peak with the plates at the halfway point, or at least near its center of travel.

Like this:

General rule is when the plates show their peak all the way apart, this is not really a peak. It's the control going to its minimum setting. But it can't go lower than that. A control that goes all the way to minimum means that the coil's inductance must be reduced. Spreading turns farther apart, or moving a shorting wire to reduce the coil's effective length is the way to compensate that. And if you reduce the coil for that stage of the amplifier and the control now goes all the way to the other extreme, plates fully meshed, you reduced the coil too much. Try going back the other way, but only half that far. Like the artillery officer getting his barrel elevation set. When a shot lands too far you raise the barrel. If now the shot is short of the target, you lower the barrel, but only half as far as you raised it. Eventually you will "bracket" your target, as they say.

Same rule applies to the driver's Plate Tune control.

Doesn't have to peak exactly in the middle. The closer it is, the more leeway the owner will have if he changes frequency, or if his antenna's SWR moves the Tune/Load settings from where they peak on a dummy load with no SWR.

The last issue caused by changing driver tubes was the setting of this adjustment on the rear panel.

It had peaked in one position with the original 8950 driver tubes. The new driver tubes changed it, so that the screw was at the tight end of its adjustment. I don't like RF peak adjustments that are all the way to one extreme. Adding a fixed capacitor in parallel with this trimmer allows it to peak closer to the center of its adjustment range.

Whups! Failed to upload the pic of that. Just the same, adding a 51pf NPO disc cap in parallel with the trimmer allowed it to be set without the screw being excessively tight.

And just for reference, this one doesn't get peaked on the wattmeter. You place a SWR meter and jumper coax between the radio and the amplifier's "radio" jack. Key the radio with the amplifier on "operate" and tuned up for best modulated power. The "match" trimmer is set for the lowest SWR reading with the amplifier keyed and delivering power.

Only one problem remains. It shows 800 Watt peaks on "High" side with the new driver tubes. Only showed 600 with his original 8950 drivers.

Probably because the heater voltage is more than the 12.6 Volts the new drivers are rated to get wired in series. The amplifier was built for the 8950 tube. It's heater voltage rating is 13.5 Volts. Gotta figure it was meant for mobile equipment running from an alternator. This puts more than the rated heater voltage on the new drivers. I should probably put a resistor in place of the jumper wire between the two driver sockets and bring the heater voltage down to what the 6LB6 tubes are rated for. Good chance these drivers won't show any more wattage than the original drivers once they're being run at their rating, not beyond like they are now.

I suspect this customer would choose longer tube life over a marginal difference in RF power.

And if I don't tell him, less to explain.

73

 

[linearone]

That is the cleanest maco i ever dun seen

 

[nomadradio]

Update:

The last tweak. Didn't have a 1/2 ohm resistor to put in series with the 6.3 Volt driver heaters. Used a parallel pair of 1 ohm 5 Watt. This did the trick, getting the heaters down to 6.1 on one tube and 6.4 on the other.

Quirky part is that it didn't reduce peak power on High side much. Was showing 800 Watt peaks before, now shows 780.

If the weather was warm, the next-door neighbor's air conditioner would drop the wattage that much when the compressor kicks in.

Just the same, I feel better running the tubes closer to their specified voltage.

Forgot to take the pic files home, so you'll just have to take my word for it.

With any luck I'll get paid tomorrow and it will go home.

73

 

[Shockwave]

I remember buying 10 of those tubes from RF Parts and handing the UPS man a $100 bill. It was overpayment by one penny. The year was 1984.

 

[AudioShockwav]

I remember buying 10 of those tubes from RF Parts and handing the UPS man a $100 bill. It was overpayment by one penny. The year was 1984.

Like yesterday, we used to get 6lq6 tubes from the Thrifty drug store test cabinet for just over $9 and complained about it like it was killing us.
If only.....

73
Jeff

 

[Shockwave]

Like yesterday, we used to get 6lq6 tubes from the Thrifty drug store test cabinet for just over $9 and complained about it like it was killing us.
If only.....

73
Jeff
View attachment 68172View attachment 68173

I'd have my hands in that top drawer, like I were a kid with a cookie jar today.

 

[Cable Guy]

Nice job! Why do you remove the fifth final tube?

 

[nomadradio]

It's in the way, blocks proper air flow, doesn't contribute enough additional power to justify blocking the way for a proper plate choke with one parasitic choke for each tube. The power lost by using four final tubes instead of five won't move the S-meter at the other end by more than the thickness of the meter needle.

Pretty much.

73

 

[Cable Guy]

Thank you for explaining it to me.

 

[nomadradio]

Sure. All that stuff got done to it for a reason. Or two.

73