Need help with a Al811 amp

[Jerry-n5ugw]

I need some ideas of where to look. I have a al-811 lashed to my Ft-902dm in my trailer. I can not get the amp to work on 80m or 160m it will not allow the drive through the tuned input circuit. It works on 40m -10m.
I have tried to adjust the slugs and no effect. It is clamping the input drive. Where should i be looking? Wished it did not have the tuned input circuit at all, the ft-902 would load into it no problem I beleive. I did put the tuner between the amp and xmitter and it loads up FB. any ideas??

[Jim, W5JO]

Does that thing have a variable ALC circuit?

[w4bfs]

Hi Jerry ... sounds like tuned input trouble, most likely bandswitch or capacitor ... look over the switch... direct sub on the caps, unless you have a grid dipper and can check resonance ... 73  .. John

[AB2EZ]

Jerry

I think you are saying that the amplifier loads up if you put a tuner between the transmitter and the amplifier. This suggests that by including the tuner you are providing a DC path to ground (or removing a DC path to ground) at the interface between the transmitter and the amplifier. I don't have the schematic of the Al-811, but I would suggest that you look for something like a shorted coupling capacitor on the input tuned circuit, or an open inductor (running from the input to ground) on the input tuned circuit... for the bands giving you problems.

Stu

P.S. I found the schematic of the Al-811 on the Ameritron/MFJ web site... and I also took a look at the schematic of the Ft-902dm on the Bama web site.

The FT-902dm has a "safety choke" at the output of the pi network... so it does provide a DC path to ground that might be eliminated when the tuner is placed between the FT902dm and the Al-811 (depending on what type of tuner it is). However, it is not apparent (to me) why the problem would only occur on 160 and 80 meters. If the Al-811 schematic on the Ameritron web site is representative of your amplifier, I don't see why the problem would only occur on 160 and 80 meters. Then again, maybe the tuned input circuit in your amplifier is not the same as the tuned input circuit shown in the schematic of the Al-811 that is available on the Ameritron web site... either because Ameritron changed the design, or because someone made a modification to your amplifier's tuned input circuit.

[Jerry-n5ugw]

Al-811 scmatic is the same as shown on the web site. I think they have had some problems with the two caps and coil. I am going to just add a small tuner inbetween the 902 and amp for now. That seems to solve the problem. I'll add a good ground andsee how it goes. Thanks

[W1AEX]

Jerry,

As I recall that amp has an extended shaft that gangs the tank circuit bandswitch with the input bandswitch. Sometimes the coupler loosens up and the input shaft gets out of synch on some bands. You might want to click through and watch what happens at both ends of the mechanism.

73,

Rob

[Jerry-n5ugw]

Will do... good idea didn't occure to me.

[flintstone mop]

Try to double check the bandswitch. I heard that it might get out of alignment and some bands will not work.
I have an AL811A and it's pretty nice for scraping by with 150 watts of AM carrier. If you have three pubes then you have a little more juice.

Fred