AF-67 - Need advice

[K3ZS]

I am trying to restore a real rough AF-67.  I have one operating in mint condition in the main station but I thought I would try to get my bad one working for the basement station.  I noticed that the modulator driver transformer is open, apparently a common problem.  I have heard that the interstage single ended to push-pulll audio transformer sold by Antique Radio Supply is a good substitute.  There was an ER article in April 1991 (before I was a subscriber) by K5DUT on a modification to eliminate this transformer.  Would it be appropriate to ask if there was a copy of the article available or are there old ERs avaiilable to buy.   Another common problem mentioned on an internet site is a sticking meter.  The site stated that this is easliy solved but doesn't say how it is solved, anyone know about this?  My junker has that problem also.  I originally thought this one was a basket case because the bandswitchiing mechanism was so rusted it couldn't be changed.  With a lot of WD-40 I got it to work.  It was encouraging to find the VFO was working and on frequency.  I have seen the mods by W7ZFB and most of the others avaiilable on the internet.  Thanks for any advice.  73, Bob K3ZS

[flintstone mop]

HELP from ER magazines for the AF-67

ER 24 Pg 28 Modulator Fix , get rid of the interstage transformer
ER 22 pg 27 audio mods for better audio
ER189 Pg 30 extensive article to "optimize" the Af-67,  you can filter out what you want to do with this article as the author is not very informative about some of the procedures. He is Chuck Felton. He "hot Rods" R390A's and probably would like you to send your unit to him so, he can make the mods and charge. Most of his article is clear enough to do most of the important mods, though.
I have one of those little guys myself and am considering the mods listed here. I need  to build the PS for mine. I can't remember the ER number, but there is an article that lists parts and schematic using two inexpensive transformers to come up with the B+ and current and filament for the AF-67. I can get you the ER number if interested, it's not close by the compooter right now.
Good luck

Fred

[K3ZS]

Fred, thanks for the info.  I have all issues since April 1998 of ER.  I remember the power supply was within the past few years.  I was lucky enough to recently pick up a Lambda regulated lab supply for a few bucks at a local ham estate sale.  I don't think I would pay anyone to restore this one, it would be cheaper to buy a mint Ranger on ebay.  I haven't seen the 1991 article on eliminating the transformer so I have no idea how it was done.  However, the NC-183 I just restored has a phase inverter tube stage driving the push-pull 6V6's on the output.  The same sort of circuit or some newer solid state equivalent probably will work.  I am suprised that I have never seen the simple circuit of putting a 22V zener in the common cathodes of the modulators and bypassing the cathodes with .01 or so.  Seems to me this is the simplest mod to get rid of the bias battery. 

[flintstone mop]

ok Bob, on the power supply.
It was the same ER 189 after the Chuck Felton article that has the economy power supply. I wasn't looking to well earlier this morning.
I was going to look for a DX-100, but I think I'll save the bucks and upgrade my Elmac. On 40M and up who's going to tell the difference betwen a 50 watt and a 100 watt transmitter? The antenna is the heart of the station!
75 M and 160M need the juice, usually.
You'll love how simple it is to rid the audio circuit of that transformer. 6 resistors and 3 capacitors and an hour of your time. Goodbye frequency limiting and phase shift!
my intserstage was replaced with something (?), but I'm going to gut it anyway and go with the mod.
Fred

[Pete, WA2CWA]

You can find all the past articles in the Electric Radio Index:

http://home.wi.rr.com/n9oo/ersearch.html

[Tim WA1HnyLR]

Hi Bob .In regard to the AF 67 ,I have worked on quite a few  The modulator tubes are driven into grid current to make full modulation . You must use a driver transformer. If you use different modulator tubes such as a pair of 6DQ6 tv sweepies  you can set then up for AB1 operation  You will want to use fixed bias . You can also use 6CA7/EL34 tubes in AB1 with a minimum of 250V on the screen. The stock modulation transformers impedance match will allow you to make 100% modulation. Using cathode bias will subtract from the overall available plate voltage on the modulators. The best tube to use if you insist on a form of cathode bias try the 8417. This tube is very high gain and does not require much bias with the usual 250V on the screen. There is enough room to install a larger modulation transformer. By minor relocation of the grid coils I have fit in the ART 13 modulation transformer. This piece of iron has good response and will allow you to make 125% positive peak modulation. A better transformer is the mod transformer that was used in the T-195 I did up an AF67 for Brian W1LYD/KL7 Mobile using a pair of 6DQ6 sweepies in AB2 with 150V regulated on the screens with 3- 9 volt batts for- 27 volts of biass. The transmitter does better than 150% positive . Perfect for mobile work!. Being that the transmitter is in rough shape makes it a perfect candidate for a high performance model. Good luck De Tim WA1HnyLR

[K3ZS]

Thanks to all for the info.  Tim, thanks for the details, I have heard you a few times running mobile, I think you were  using the the AF-67.  I listen a lot but don't talk very much as I am usually doing something else while listeniing.   I have two AF-67's and a set of NOS tubes for them.  Also have the moblile vibrator supply and a PMR-7.  Just need 500V to run mobile.  It probably won't happen though, I just don't drive to much to bother with any kind of mobile rig.  When I get the junker running on CW then I'll see about AM.  I have a mod transformer from an old business band Johnson amplifier (PA-85).  It had a transistor push-pull modulator driving a industrial type 6146.  I was thinking of driving the AF-67 with a 50W PA amp replaciing the 70.7V line XFMR with the PA-85 mod transformer.  I also have an old stereo equalizer that I was  thinking of adding to the mess and building a compressor into it.  So much for the long range plans.

[flintstone mop]

Here's a news FLASH to get rid of the bias voltage on the mod tubes

I had a nice conversation with Hal, K6DPZ. He wrote several articles in ER, (ER 189 The Economy PowerSupply For The Af-67") and owns a communications company. And they sell, repair, refurbish Elmac equipment and he is very familiar with the AF-67.
He informs me that the 22.5 bias can be eliminated completely by:
1. removing the connection of B+ to the screens PIN 4 of the mod tubes.
2. removing the green interstage transformer wires from the control grid PIN 5 of the mod tubes
3. GROUND pin 5 of both tubes
3. RE-connect the green wires to the screen grid PIN 4 of the mod tubes.
4. It's done!
SLICK?
Fred

[N8ECR]

K3ZS:  for your infomation, Many of these driver transformers have been repaired with a little patiences.  There is solder connection that have oxided where the leads are connected.  This is because of the bias battery alwaays in the circuit.  Remove tthe transformer, and carefully with a exacto knife cut into/and remove the outer paper.  Clean the connections resolder and tape back up and your in business.... This is a common failure.  Many have fixed it this way.

N8ECR/Mike
 net control Op. of the Mighty Elmac net Wedesday nite at 8:00 pm 3880 kcs.(except the first Wedesday of the month)

[flintstone mop]

Hello again,
This thread has been very helpful to me and it's nice to knw that there is a net that meets in honor of our little AF-67.
My question is about the modulation audio. I'm not looking for B'cast quality audio, but what am I to expect from my Elmac?? I might run my audio chain into it or use my K7DYY modified D-104 into the mic inpoot.

Hal, who does repair and refurbs theses things says that the mod iron is not going to give more than communications audio....period. It's not worth going into the audio circuits and changing any caps to "open up the audio", it ain't gonna happen.
Your thoughts?
Thanks
Fred

[N8ECR]

Fred:  Listen in tonite and see what some of them sound like,  they are not to bad, and many I have heard are quite loud and full of audio, but with less quality when cranked up, but not unlike the typical plate modulated rigs of that time.....Mike/N8ECR

[Carl WA1KPD]

Hi Bob .In regard to the AF 67 ,I have worked on quite a few  Tim WA1HnyLR

Tim,
I overheard you buying the AF 68 at Hosstraders and thought you said you were going to put a TCS 12 mod xfrmr in there. I picked up the AF 67 on the table and since I have some TCS-12 transformers wonder if you would suggest the same for it?
Thanks

Carl