Maco 750

[donemad]

Can anyone tell me how to adjust the driver cathode on this maco 750 under it where there is the adjusting screw, square looking adjustment?.....thanks for any help

 

[donemad]

Can anyone tell me how to adjust the driver cathode on this maco 750 under it where there is the adjusting screw, square looking adjustment?.....thanks for any help

These are all 8950 tubes....thanks for any help anyone......

 

[Grim Reaper]

There should be a small flat tip screw slot in the square adjustment

 

[donemad]

There should be a small flat tip screw slot in the square adjustment

Yes it is......but I need to know the procedure of doing this! I have been reading some post, but I can,t find how its done. I think what I do is take 2 clips & clip one on each side of it, going to my ohm meter, using a insulated screw driver, set the meter to ma, key it up, & set it to 400 ma. doing this as quickly as possible. Thanks for any help.......

 

[Eastside]

My brother had one of those maco 750s back in the 1970s....very nice linear.

 

[Shockwave]

The part you are describing sound like a trimmer capacitor used for RF tuning. If it is white porcelain it is probably a cap and has nothing to do with bias. If the part is connected to the driver cathode, it will be a input VSWR adjustment and should be adjusted for minimum VSWR with a meter between the amp and radio. If it is connected between the driver and final stage it should be adjusted for maximum output. To the best of my knowledge these amps did not have adjustable bias. It is very likely that the bias is fixed on this amp.

 

[donemad]

The part you are describing sound like a trimmer capacitor used for RF tuning. If it is white porcelain it is probably a cap and has nothing to do with bias. If the part is connected to the driver cathode, it will be a input VSWR adjustment and should be adjusted for minimum VSWR with a meter between the amp and radio. If it is connected between the driver and final stage it should be adjusted for maximum output. To the best of my knowledge these amps did not have adjustable bias. It is very likely that the bias is fixed on this amp.

Hi Shockwave, yes thats where its connected, goes to pin 2 & pin 6 on the pre driver & the other two drivers, then has what looks like a choke on the line going to the 6 finals also pins 2 & 6. How do I adjust it or can I? the guy that had it before me has turned the screw. thanks for any help...

 

[Shockwave]

Your description of where this cap is located is not making sense to me. The way you describe it, you have the part between the same tube pins in different stages. This is unlikely but in any event you would be safe to tune this cap for maximum output on your power meter.

 

[donemad]

Your description of where this cap is located is not making sense to me. The way you describe it, you have the part between the same tube pins in different stages. This is unlikely but in any event you would be safe to tune this cap for maximum output on your power meter.

Thanks Shockwave, you have been a BIG help......

 

[Shockwave]

Thanks Shockwave, you have been a BIG help......

That's what the forum is for. Keep us posted on how you make out tuning this circuit. Don't overlook the potentially lethal voltages inside. Work with only one hand ever in contact with the equipment when adjusting this. Do not hold a metal mic in the other. Plastic stock mics are safest for this. If you must use a metal screwdriver, only hold the insulated handle and be careful not to short anything out since you will have to tune this one live and we want you the same way after.

 

[donemad]

The part you are describing sound like a trimmer capacitor used for RF tuning. If it is white porcelain it is probably a cap and has nothing to do with bias. If the part is connected to the driver cathode, it will be a input VSWR adjustment and should be adjusted for minimum VSWR with a meter between the amp and radio. If it is connected between the driver and final stage it should be adjusted for maximum output. To the best of my knowledge these amps did not have adjustable bias. It is very likely that the bias is fixed on this amp.

Thanks Shockwave..it works great......only now I have i think a bad relay, it don,t my let my recieve come in after i key it up, i have to flip the hi/lo switch to off to recieve. I have tryied to clean it but to no avail. Do you happen to know the volts & amps of this relay? They are usely marked on them, but not this one. Thanks for your help on this.........