The AM Forum

Guys,

     Well, I found a Nice multi Elmac AF-68, with the companion receiver PM-R7 ( I know PM-R8 is the right matching pair), anyways, this rig could be a nice exciter for a bigger transmitter.

      I found this rig only with the P.S. for the receiver ( PS-115), unfortunately the power transformer is burn, (at least I can use the "S" meter circuit), so:

Any Ideas for a new homebrew P.S. for use with both rigs, and update the 6146 for a 6146B ( More Voltage ).

And some Ideas for change the annoying 22V  ??? Battery in the modulator?

Thanks in Advance

Pedro XE1YZY

Good evening Pedro,

I used to have the AF-67 and PMR-7 a long time ago. Nice transmitter. I used the 5881 tubes for the modulators instead of the 6L6's. I heard someone say that they used a wall wart to take the place of the 22 volt bias battery. Might be a thought.
73
Leo WA6MTZ

Hey Pedro

Long time no see

I had a friend who built up the bias supply with a small xfrmr, diode, cap and a zenier

Hope life in Qto is good for you and the family and that business is good. Have not been to MX city for close to a year but hope to get down there this June. Enjoyed our eyeball last time

73 mi amigo

Carl /KPD

Hola Pedro, como te va?

I think you can get rid of the biass battery by replacing it with a properly polarized zener diode so the tube develops its own biass.  I'm sure someone will bring forth the details but as I recall it was outlined in Electric Radio some time ago.

A Heathkit HP-23 power supply would be just the ticket for that pair!

73 John

Hi Pedro,
There have been a few threads about the Elmac Tx. I posted some info on building a P.S. from an ER magazine, or try Ebay for an old Heathkit high voltage P.S. with a bias output. The 22 volt battery is still around. There were some mods to get rid of the driver transformer in the ER mag also. You might want to do a "search" and you will see the results of many postings. I using a highly modified AF-67 to drive an Ameritron AL811A linear. 200 watts of carrier and a lot of nice audio reports. I use it on 80 and 40.
Fred

Do a search above on this page on "AF-67".   There has been much advice in the past that will also apply to the AF-68.   My AF-67 has a 22V zener in place of the battery.    Get a 5W 22V zener, take out the battery clamp, drill a hole to mount the zener.   Ground the center tap of the driver transformer, the lead that went to the -22V connection.    Disconnect both cathodes of the modulator tubes and connect them the to cathode of the zener, the anode connects to ground.  If I recall you must insulate the zener from ground using the mica insulator.  Bypass the cathodes of the modulators to ground, probably with a .1 uF.     Some say this takes away 22V from the operating curve of the modulators, but I find there is plenty of reserve.    There are other methods to eliminate the battery as you will see if you do the search for AF-67.

A quick mod for the PMR-7 receiver.    They have a capacitor across the primary of the audio output transformer, I suppose it was to limit the audio bandwidth.   Snip off one lead of the cap and the audio bandwidth sounds much better when used with a wide range speaker.   You can always solder the cap back for "communications quality" audio.

Hello Guys!

  Its nice to see you all again !, Carl , everything its ok here tnx, I has been traveling a lot to Europe for my business, and I on my way now, but you now, once the bugs bites you is a life time disease  ;D, when you come back to Mexico again?

Hi Leo, nice to meet you and thanks for replay!

Hola John!, Hows my favorite boatanchor mentor goes?, thanks for the info, are you going to Dayton this year?.

Hi fred, TNX for the info, yes my idea is driver an amplifier with this rig, I heard once a multi elmac in the air and sounds good, I would love see a picture of your modified multi elmac! BTW, is the stock VFO stable? or you modified this too?

Hi Robert, nice to meet you, wow, tnx for the information, Im going to follow your tips, I like a lot your ideas for get rid of batteries, I let you now about my progres!

Take good care all of you, again many thanks!

Pedro Arnaud XE1YZY

One thing I have found with several AF-67, 68's, they don't like any more voltage than originally used. The plate tuning capacitor has minimal spacing already, and more voltage under modulation isn't a good thing.

73
Don, W2DL