The AM Forum

I pulled my Apache out after a ten year hiatus to operate a little "yay-yem". I'm having an issue while loading up and was hoping one of the gurus here could give me some starting points. Setting the drive (5ma - 6ma) gives me no issues. But when I start to load the plate, after advancing the load, when I dip it drops to 40ma. I'll advance the loading more and it dips to 40ma. Regardless of how much load it always dips to 40 ma, until the meter jumps and I get a screeching noise from inside the transmitter, then SOMETIMES it will drop back down to the 40mas. I cleaned the tune operate switch thinking it may have cruded up during it's vacation. This transmitter was completely re-capped before I stopped using it, and no issues were present at that time. I plan on pulling the final tubes and throwing them on the tester along with all the others and see if that reveals anything. My question is, what else should I be looking at? I've done a lot of searching of the intarwebs and here without much luck. This is the first time I've ever had this type of issue with a transmitter so I'm in the dark here. Narrowing it down will help as I've recently had cervical spine surgery and tossing this beast around is a big no no at the moment, thusly I have to get my son to come over and move it for me so naturally I'm trying to keep that to a minimum. Any help is appreciated and thanks in advance.

73

It appears that the output stage is looking into too low a value of RF load impedance. This could occur, for example, if there is an open circuit somewhere in the path leading from the output of the pi network to the antenna or the dummy load. Check for DC continuity between the antenna-facing side of the RF output coupling capacitor (a.k.a. the plate blocking capacitor) and the center conductor terminal of the SO-239 RF output connector.

Stu

First of all and hopefully you are loading the transmitter into a non-reactive dummy load.

Remove the fan and the cover of the RF cage.

Then I would check for bugs and spiders in and around all the components from C51, tuning caps C52, C53, and the condition of the Bandswitch.

And specifically, any burn marks or 'flashover" tracking in or around C51 and the Bandswitch.

If C51 is original, one might suspect it is breaking down during Keydown.

My experience with the Apache is that those components in the RF cage are fairly robust and rarely fail with a good fan and proper air flow.

Phil - AC0OB

Sounds like a short in the loading cap area. Do the caps all show an open circuit when tested with a VOM?  In many cases, shorted loading caps will have a different effect on different bands as sets of caps are added or removed from the circuit. Check the coax too.

I'm loading it into an MFJ Cantenna.

I've sometimes experienced unexpected results when loading up into a very nice lab grade dummy load I have. Yet, into an antenna, all is well. Perplexing. The old 100 watt incandescent light bulb as a dummy load is worth trying. At least it gives visual feedback that there's output.

A few ideas:
-Exercise every knob and switch.
-Try loading on a different band.
-Check all tube filaments are glowing/tubes getting warm.
-If its been a while since you've operated similar equipment, re-familiarize yourself with the operating manual procedures.

Now for my guess. One of the 6146's has crapped out. But, it really could be so many other things. As for troubleshooting, I believe the construction manual has voltage checks and other info. It's a good place to start.

Good luck,
Don

As Don said try one final tube at a time and try both final sockets one tube at a time. Check that the plate voltage and screen voltage is where it should be - screen voltage around 150 to 170 volts as this is controlled by the 6AQ5 clamp tube circuit.

Thanks guys, I'll let you know what I find.