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Messy Maco 750 resurrection
- Thread starter Danzik
- Start date
My screen voltage is also at over 500 volts the 6LB6 tube data sheet shows the screen voltage should be at 200 volts ????? Is that what's wrong with this box
In a nutshell, what's wrong with it is that it's been hot-rodded in a way that you can't keep it from heading straight for the ditch every pass. Maco built them to deliver every last Watt possible, never mind how many times you can key it before the first Kaboom.
Civilizing it so it doesn't shoot itself in the face is something we have never really written up in detail.
They were built differently every production run, so a detailed DIY procedure can't cover every variation easily.
What we have done in the past is to convert the driver from grid drive to cathode drive, and feed the screen grids from the 12-Volt relay supply. Negative grid bias rectified from the heater supply takes the place of the higher bias voltage used for the grid drive setup.
But that's a big problem with the 750. The model number alone won't tell you how a particular amplifier is built. Too many possible versions.
73
This has to be the worst of all the versions they made. The circuit board along the rear of the chassis conceals the solder side of the board. The input coil looks as if one winding has been burned. More than once we have found a burned foil trace on that board once it's extracted.
We completely stopped trying to rehab that circuit board. It gets removed, the relay bolted to the rear apron, a new keying circuit added, and a cathode-drive input circuit takes the place of the tiny burned-up input coil.
More of a "rebuild" than a repair.
It's just the way they built it.
73
We completely stopped trying to rehab that circuit board. It gets removed, the relay bolted to the rear apron, a new keying circuit added, and a cathode-drive input circuit takes the place of the tiny burned-up input coil.
More of a "rebuild" than a repair.
It's just the way they built it.
73
Yeah, that's unfortunate . I believe the input coil got burned like that when the driver screen wire came off and contacted other spots in the amplifier, which black soot can be seen in the case in various places. All in all the box is keying up and putting out power, when the tube damage occurred on the final section it was in standby and not even being keyed, it is possible the 2 tubes were close to being on there way out but tested real good before hand. I think I am going to swap the finals over to 6 volt to use 6LB6 in case it decides to pop anymore . If I end up having issues with that tuner and rear circuit board I already have a Nomad keying circuit board on hand. Is that screen voltage going to the drivers at 500 volts ok ???? Should I or couldn't I lower that with a larger power resistor or something? I did order some new neon bulbs for that I hope the ones I ordered will work in there
I also ordered new large capacitors for that section and I already replaced the 2 caps for the final screen section that was feeding them 110 vdc they looked like they had been hot, dunno just trying here, I think once I get it going I can get her to run putting very low input watts into it I hope, thanks for your advice as it is appreciated!
Ok, what's the best way to test that coil ? Can it simply be done with a ohm meter and turn the slug with a Allen wrench and look for a change in ohms even with the amp off or should I place a swr meter between the radio and amplifier and check out the swr between the units and see if adjusting the coil changes the swr ??? Chances are if it's burned up the swr will be sky high and won't adjust, correct? Thanks
Hi , I rewired the box for 6 volt 6LB6 tubes, I replaced the large caps with 3 220uf 450v caps, I installed new Neon bulbs and replaced a couple of those old carbon resistors, I currently have a swr of 2 between the radio and box and have not attempted to adjust the input tuner yet, my main SWR is 1.3 and drops to below a 1.2 with modulation added. There is a weird 3 position standby switch on this box, at first I had flipped it into the upward position and I noticed the 2 driver tubes quickly starting to red plate, then I noticed if I put the switch into the downward position they did not red plate ! I am starting to think that might have been what was going on before when it destroyed the old tube, I am going to have to check out why that switch is setup that way .
The problem with the input coil is that when the insulating varnish burns, the wire turns short to one another. An inductance meter is the only way to test an inductor. Don't have the inductance value you should read on file, though. The tuning slug requires a non-metallic tool, pretty sure the flats are 1/10 of an inch apart. This was a standard "alignment tool" size for years. Using an Allen wrench that's 'almost' the right size runs the risk of cracking the brittle thing.
If the input-side SWR is that low, just put a nail in it. You can't expect to improve it enough to be worth any effort.
An amplifier that old almost certainly contains some modification or another. Sounds like that standby switch has a 'suicide' mode.
73
If the input-side SWR is that low, just put a nail in it. You can't expect to improve it enough to be worth any effort.
An amplifier that old almost certainly contains some modification or another. Sounds like that standby switch has a 'suicide' mode.
73
As always, @nomadradio is correct. People get hung up with input swr. I agree we should all strive for perfect but any amplifier will pretty much show no difference in power with a perfectly flat input and a 2.0:1 input. The only time this can jam you up is with much bigger boxes.
With 4w forward a 2:1 input is less than half a watt in reflected power. Half. A. Watt.
Even mini 8 coax can safely dissipate this half watt of heat. It matters when you have hundreds of watts of reflect on your coax between drivers etc... meaningless in small boxes.
Yeah, the swr readings are good between the box and radio and after, with modulation applied it drops even more, however if I flip the box on high I still have the low SWR but the driver tubes start to red plate right away on keyup, not real sure what my next step on this unit will be maybe a match and diesel fuel
maybe a match and diesel fuel
What is your take on the red plating, Oscillating or ???? How would one check the bias on a box like this if it would happen to be out of spec ? Thanks
Chris (nomad) is your best hope for help in this project, I was just injecting humor into the mix.
If a service manual was available for it that would provide some help, but they never wrote one, he has years of experience with these old amps.
73
Jeff
If a service manual was available for it that would provide some help, but they never wrote one, he has years of experience with these old amps.
73
Jeff
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