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[HAMHOCK75]

I just began a restoration of an old AL84. The RF choke to the plate caps was found lying loose as shown inside the box. The wire at the bottom is broken.



My first question is did the RF choke have any kind of metal ring around the base of the choke like in other designs to which the wire was attached? There was a piece of cellophane tape around the bottom as you can see from the discoloration at the base. I removed the tape but wondered if the tape was put there at the factory or if someone used tape because a part is missing.

Also, I found 1.04 KV with a scope at the nearest point to which the RF choke screws into the pcb but see no hole for any wire from the choke to the HV in the pcb. How did the factory connect the choke to the HV?

[W1QJ]

Often times there might be a short insulator and that would be a tie point between that wire on the choke and the B+ from the supply and a bypass cap to ground.  You can see that the ceramic is busted on the bottom so it could be it had a metal ring where the B+ came in and the cap to ground.  You will have to improvise now.  That is not a good choice of amp to restore.

[HAMHOCK75]

I am quite familiar with the Dentron GL1000 series, the Amplifier Supply Co. LA1000 series, and the feelings most hams have for the AL84  .

They are still fun to work on because they are so simple. When I do a restoration though I do try to make it as close to factory stock as possible warts and all, so I would still be interested in how what the original factory setup looks like. There is a big difference between a restoration vs. a repair. 

[KM1H]

You might find a decent photo of a GI-7B or some other tube swap in that amp, there was a EL-509 Russian sweep tube that was a popular swap for awhile also. I havent touched one for repair since sweep tubes went thru the roof.

Carl

[HAMHOCK75]

This is a restoration to factory stock for historical accuracy so no changes contemplated but if someone has a photo of the factory RF choke and how it is mounted would be nice.

[KM1H]

This is a restoration to factory stock for historical accuracy so no changes contemplated but if someone has a photo of the factory RF choke and how it is mounted would be nice.

I understood that the first time and suggested looking at mods which usually have before and after photos.

[HAMHOCK75]

Ah, misunderstood what you meant to say but no luck. Found conversions for Dentron GLA1000 and Amp. Supply LA1000 but few photos of before and none for the AL84.

Also, checked the "for sale" AL84's which often have the covers off to show the insides but usually the tubes are in place so the bottom of the RF choke can't be seen.

[W1UO]

If you search on eBay under completed listings,

"Ameritron AL84 160-15 meter RF Amplifier"

There is a side view picture.  It looks like the choke is mounted directly into the PCB.

[W1BR]

It does mount directly center between the four tubes.

https://www.google.com/search?q=al-84+ameritron+images&tbm=isch&source=iu&ictx=1&fir=1nYkJ9ESMiEQYM%253A%252Ck0C28K-DANuL6M%252C_&usg=__XU9KE5m66sMDYytqe3D-ztw4eA0%3D&sa=X&ved=0ahUKEwiDw8-buNLXAhVExoMKHd0RBLoQ9QEIOjAI#imgrc=kH_wcGTCgTmE9M:


For Ameritron Amps, I would go here and ask the question:

https://groups.yahoo.com/neo/groups/Ameritron-Amps/info

If there were different revision levels, they may have change the FP-144 choke design, that
is also something to watch for.

You could try calling the factory and seeing if they still a choke in inventory.

Pete

[HAMHOCK75]

I always wondered if it was possible to see closed auctions or sales on eBay. I did find that eBay photo and another one in a PhotoBucket series of photos at this link.

http://s74.photobucket.com/user/arlags/library/Ameritron%20AL-84?sort=3&page=1



Both photos show the fiber reinforced tape at the base of the RF coil. I have some similar tape. The base of the choke is being rebuilt with structural epoxy.

Meanwhile, a plate cap that was torn off one of the four tubes was reattached with JB Weld epoxy and soldered back in place. The tubes had shipped inside the AL84 with two sustaining damage as shown below.



The relay frame was bent so the contacts could not move at all. That has been corrected.

The high voltage measures 1,040 volts with a high voltage scope probe but 850 volts on the front panel. R6 the resistor supplying the current to the front panel meter was correct in value so the meter's sensitivity seems to have dropped. Now working on checking and calibrating the front panel meter so it reads accurately.

[HAMHOCK75]

The choke was successfully restored with epoxy. The base of the choke was rewrapped with fiber reinforced tape as was found there but removed earlier. The lead from the RF choke fits in the same hole as the orange ceramic bypass capacitor. When one lead of the capacitor was lifted, a small remnant of coil wire was found.

[HAMHOCK75]

Measured the plate current of each of the remaining tubes in operate mode with no drive. One measured 20 mA, the other two 10 mA so one tube is dissipating 20 watts since the high voltage measured was 1 KV. Bias is created by 7.5 V zener.

Dentron's GLA1000B seems to use a 24V zener. Wonder what the bias current is with that large a bias?

[W1QJ]

Less idle current with higher operating bias

[HAMHOCK75]

Was just thinking that my Yaesu FT101EE runs 30 mA bias to each of it's sweep tube finals to achieve -34 dB IMD3 ( ARRL measured method ). So 10-20 mA seems a bit light but it must be pretty bad with 24 VDC bias and even less bias current.

[HAMHOCK75]

Next up but more challenging is the broken plate tuning capacitor. The ceramic in the front is broken in two places and there are some slightly bent plates. Alignment of the three ceramic pieces is actually not that difficult due to the hardness of the material and sharp breaks. If you have ever glued a ceramic dish you know what I mean. Probably epoxy again for repair. The front and rear mounting plates may not longer be parallel. I noticed that of the two mounting tabs at the bottom, the rear one does not lie perfectly flat on the table.

Re-alignment of the plates seems to be the real challenge. Thinking of cutting pieces of cardboard and placing them between the plates during reassembly so rotor and stator plates stay in position during re-assembly.

Any ideas would be appreciated. The capacitor is still available from Ameritron is all else fails.