The AM Forum

Greetings!  I've recently acquired an Elmac AF-67 for running 10 meter Yeaaa..M.  I've got it going but it won't play using chrysteeals. 

Are there any instructions for removal of the VFO compartment anywhere?  It looks like it could be a bit on the hairy side!

Remove the VFO??? That would be a disaster!!
I'll look at some info I have. Getting the crystal socket to work shouldn't be too hard. I imagine that the drift is pretty bad on 10M.
Great little TX for the upper bands. Output power will be struggling on 10M.
Fred

Have you tried a known good replacement 6AK6?

Are your crystals not oscillating at all or do they oscillate but provide insufficient drive?

Are your power supply voltages OK?

Battery dead?

C

The battery is used for bias in the modulator. If it won't work with crystals, it is most likely the oscillator tube (6AK6?).

Are you using fundamental or 3rd overtone type crystals?

Biass batteries in most circuits that use them are actually being charged under load. (that is why they last practically forever) They actually become more of a voltage reference than an actual battery. And in many (most) cases they can be replaced with a string of zeners and forget about them forever.

They were usually used in the modders and/or audio circuits.


I actually have had a fairly clean Yellmac AF-67 sitting on the shelf for around 10 years and have never gotten around to playing with it.

I'm glad to know I'm not the only one who does that.

I have an Elmac and with the single 6146 and a pair of 6V6's is just a mobile version of a Ranger, without the drifting VFO.
The output tank circuit leaves a lot to be desired in the Elmac. There were issues with improper caps in the  loading that you would never get rated power out of it.

Fred

Ive got 5881's in mine, I think thats what they came with. Dont use it much, too many rigs and I hate crystals!

I also have one. Its been sitting on the floor in the corner of a room for about 3 years.  With The power supply.   The big problem is that tank and the insulators.  Once repaired they jump way up in power. My buddy has one that had all the ER mods done and it does a solid 65AM now. Those little rigs are getting popular now I hear them all the time on 40 meters.  I guess a cheap way to get on AM with a decent sound.

C

i have a couple of these. always wanted to put one in a mid 50 car and make a total vintage mobile station. Its a complicated setup. just goes to show how easy we have it nowadays. One of the big problems is that the output is setup for 36 ohms. The mods are
to move it closer to 50 for modern antennas. I believe the mods are listed in ER...

That 36 Ohms is a perfect match to a good 1/4 wave vertical with no ground loss. A roof mounted ground plane would be ideal.

I am having a hard time visualizing how the 22 volt zener is connected in the Elmac to replace the 22.5 VDC battery.A schematic would help on how it is connected.I have two of the AF-67's and one A-54H.

http://en.wikipedia.org/wiki/Linear_regulator

The top diagram "Simple Zener Voltage Regulator" shows a basic practical circuit.

You would need to tap an AC voltage from elsewhere in the AF-67, then rectify it and drop it to close to 22.5v.  The circle at left represents that source.  R1 at top will be chosen to provide adequate current for the regulator Zener and the load (which is the bias and should be very low.  You may need to check the tube spec sheet for bias current).   The zener (or two or more in series) will be chosen for the proper voltage, i.e. close to 22.5v. 

The replacement battery is readily available and costs about $10.  I'm told they will last for years since the load is so small.   I believe Amateur Electronic Supply of Phoenix carries them.

bill

This is from N1ESE,
To eliminate the biass battery EXTREMELY simple

It's to eliminate the 22.5V battery to give bias to the 6L6's.  It works on both AF-68's and 67's First, ground the center tap of the driver transformer.  Then disconnect the 6L6 cathodes from ground (pins 1 and 8 ), keep both cathodes connected together, and put a 22 volt Zener Diode between the cathodes (pins 1 and 8 ) and ground with the arrow of the diode pointing toward the 6L6 cathodes  connection.  That will reverse bias the Zener Diode and it will develop 22 volts across it, plus side at the 6L6 cathodes and minus side at ground.  So the cathodes will be at +22 volts with respect to ground, but the grids will be at ground potential since the driver transformer center tap is grounded, and thus the grids will be at -22 volts with respect to the cathodes and will have the proper bias on them.  This modification does reduce the effective B+ voltage across the 6L6's (Plate to Cathode) by 22 volts, but doesn't seem to effect their ability to modulate the rig 100%, so it's no problem.
 
Hope this helps.

- JT

Thanks, Fred!! I will get that mod done on both of the AF-67's this weekend.