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  1. D

    Messy Maco 750 resurrection

    Played with it a bit last night, I removed that disc cap coming off that coil tuner which allowed a -102 vdc of bias to pins 5 on the drivers, tested it out for a bit then put my better driver tubes in since I had trust it wouldn't red plate them, it ran stable but of coarse it wasn't putting...
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    Messy Maco 750 resurrection

    Yes, that looks like this is exactly how this amplifier is setup, the cathode gets grounded when the relay is keyed. The wire coming off of that coil tuner has -104 vdc on it before that disc capacitor but after that capacitor it is actually +2 vdc until the radio is keyed then it only goes to -...
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    Messy Maco 750 resurrection

    Oh yes, absolutely I do know about that! I am actually good and careful with that subject and even though there are bleeders I check each and every time for present voltage
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    Messy Maco 750 resurrection

    I would be very good at the sheet metal work, have alot to learn on the electronics part of it
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    Messy Maco 750 resurrection

    Probably so linearone, if I knew as much about them as you guys do anyways, but I don't but would sure like too. I just get a kick out of fixing these old boxes that are older than I am ! I enjoy it once and if I get it figured out anyways and learn somthing in the process, however I am having a...
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    Messy Maco 750 resurrection

    Hi, I very well may try coming off the ac heater voltage with the diode and cap like you mentioned for the screen and try that out and see what happens! I was messing around with it a bit last night and noticed I have a - 104 vdc of negative bias on the wire coming directly off of that input...
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    Messy Maco 750 resurrection

    But that -48 volts of bias was also under modulation I forgot to mention that but you said with that high of screen voltage the negative bias would have to be 10 times higher than the -16 volts and that's nowhere close, is it perhaps possible to use the screen voltage off of the final tubes for...
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    Messy Maco 750 resurrection

    I tried it and grounded the screen grid pins and it worked but I was lucky to see 100 watts out of it so I wired it back up the way it was before and will start over. Ya know one thing I noticed last night after I keyed it down enough times and got it to start cherrying the plate I heard the...
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    Messy Maco 750 resurrection

    Pin 3 the screen voltage has 510 VDC on it
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    Messy Maco 750 resurrection

    However, what do you think about that burnt resistor, did you happen to see that ?
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    Messy Maco 750 resurrection

    That's ok Nomadradio, I thought that seemed kinda odd That's why I wanted to confirm that with you before I tried that procedure, yeah you may have very well seen higher voltage on Pin 3 earlier in this thread, I will check it out tomorrow and go from there ! Thank you very much for your help !
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    Messy Maco 750 resurrection

    There is also this 1/4 watt resistor right underneath the preamp relay that looks kinda burnt I am not sure what it is for but I do know the preamp works in this box, just somthing I thought I would mention, but yeah let me know if it is ok to disconnect that coil from pins 5 and 9 if so I will...
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    Messy Maco 750 resurrection

    Hi, this is the only wire feeding pins 5 and 9 on the driver sockets and it is coming from that input tuning coil or is it a bias adjustment coil ??? Is it safe to go ahead and disconnect there from that disc capacitor and put that wire to ground then as it connects all 4 pins in that driver...
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    Messy Maco 750 resurrection

    10-4 Nomadradio I will definitely try this and get back with you with the results, I am also thinking about installing a metal mesh screen between the drivers and final tubes on this because they are very close together, like alot of you have done on the Maco 300
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    Messy Maco 750 resurrection

    Since I swapped from 8950 finals to the 6LB6 tubes I will take wire on the socket pins from 5 to 9 as it won't hurt anything anyways
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    Messy Maco 750 resurrection

    I should also mention that the driver tubes do not start to red plate until keyup, either way I do not believe the negative bias is present
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    Messy Maco 750 resurrection

    The connection between 5 and 9 on the socket tubes is there on the 2 driver tube sockets but not the final sockets, however if I use a continuity tester on the 6LB6 tubes between pin 5 and 9 the meter beeps and shows continuity. But it is the driver tubes red plating, should I have that -16...
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    Messy Maco 750 resurrection

    Also, I just looked at the 6LB6 data sheet again and it says internal connection for pin 9 so I grabbed the ohm meter and checked for continuity between pin 5 and pin 9 on a 6LB6 and it did have continuity within the tube ‍♂️ so would it make a difference????
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    Messy Maco 750 resurrection

  20. D

    Messy Maco 750 resurrection

    Ok Nomadradio, you may have hit the nail on the head, this box originally had 2 6LB6 driver tubes so they have the pin 5 to pin 9 connections, but the final tubes were originally 8950 and I swapped them to 6LB6 and they DO NOT have the pins 5 and 9 connected together and only have that blue wire...