Harris 25K

[brandonb959]

Hi, I'm new to this forum. I was wondering if anybody here has had any experience with the old Harris 25K's we have one as a back-up transmitter that I am working on getting back to full operation as now our "main" transmitter is also having issues. I have now gone through 2 screen blockers in a short amount of time. They are around $400 a piece so its not a trend that I hope to continue. This screen blocker is simply two sheets of copper with a piece of kapton in between, has anybody here had any luck with fabricating their own to spec?

[BroadcastDoc]

Welcome!

I hope someone will be able to give you some ideas. Unfortunately I never had to deal with the famous 25K (though I did have a 20H3 for a while) but I seem to recall that what you're talking about is a pretty common problem.

[RGORJANCE]

Brandon:

Please be a bit more patient. I know a few things about the rig, but not your issue. I have "called in a marker", namely an old friend who knows the box perhaps better than Harris. I hope to hear from him soon, and will hit him with your problem, and also invite him to join our "group of merry men". If I can get him to join up, he will be a valuable source of info for all of us.

Fossil

[RGORJANCE]

Brandon:

I suggest you go the Harris web site and get yourself a "Premier Account". Sign up is real easy. There is an FM TRANSMISSION SECTION. Go to that and scroll through the data to find the FM-20/25K section. You can download a huge .pdf file containing lots of service bulletin data on the FM25K.

I saw two areas of interest. The first related to a tuning issue. The first thing to check is that the PA cavity is tuned and loaded tightly to the load/antenna. Briefly and generally speaking, minimise pa current with plate tune, and maximise output (fast rising side of the peak in output) with pa load. Fine tune for maximum output on the "fast rising side of the dip" in plate current. Use your factory test data sheets for voltage/current plus other values. (if you need a sample factory test data sheet, let me know as I have quite a selection. Specify your operating frequency and TPO so I can choose one as close as possible to your situation.

Second area - there is on page 28 and more on page 39 regarding the plate blocker. They had a problem with arcing of the kapton on the early models caused by sharp edges on some hardware near the blocker. The fix was to round off the edges with different hardware.

More as I get it.

Fossil

[RGORJANCE]

Wow! It's getting harder and harder to string more than two consecutive thoughts together.

A third idea is to see if you can get access to a good spectrum analyzer to see if the rig is producing any funny stuff that might be causing some high circulating currents in the pa section. Don't know if this is a valid idea, merely an additional thought.

Fossil

[n9lhm]

I worked on one once that someone else had been into..... that had lost several screen blockers. Replaced it once more and checked the safety gap and found it a lot wider than spec. Reset it and have not heard about it happening again. Might want to look at it. I can't remember what the gap spec was though, but it's in the book.

Brian

[RGORJANCE]

Brian has made a great point about the arc gap. If you need the specs for the gap, they should also be in the support data at Harris I mentioned in a previous post.

Another item that might help. I forgot about this one. If you kill the primary AC power, you can go into the power supply cabinet and move the big switch from the high power to the approx. have voltage position. This will allow you to do a lot of cranking on the tuning at half voltage and provide a lot of protection during the tuning procedure. I don't recall if there is metering, but you might want to watch the IPA VSWR and tweak it according to the manual.

An additional advantage of operating at about half power is that if you can get close to half your normal output power, you won't see any appreciable differences in your primary coverage area.

I got the call from the fellow who was super knowledgeable on the 25, and he had never lost the kapton. He ventured the thought that you might have been bad batched. Other than that, he suggested looking at the PA tuning.

Sorry that we don't have more to offer at this time.

Fossil

Fossil

[brandonb959]

Thanks to RGORJANCE and others for all of the responses. I started looking into the arc gap theory and cannot find a spec on what the spacing is supposed to be...I checked the manual and the service bulletins on the Harris website..Does anybody know what the spec might be?. I just received two more screen blockers from Harris today. I am going to THOROUGHLY clean both pieces that contact the screen blocker and look for anything that could be causing it to arc before trying another one. With the previous screen blocker I ran the transmitter at 50% for a day with no problems, but the next day when I went up to 80% it only lasted for about an hour before it arced through that screen blocker.

Another issue that I have been unable to get an answer from Harris on is why the transmitter will not operate in "normal" mode, the plate will only come on if its in "local" only.

[ChuckG]

I still maintain one of the 25K's. Are they blowing holes around the mounting studs? A little dab of silicone sealer underneath the shoulder washers was recommended to stop that.
Not operating in "normal"- there is a failsafe input that needs to be jumpered to the +12 volt control supply for the transmitter to operate by remote. If that jumper is missing the filament will start but the plates will not unless it's in "local". If that doesn't do it, try replacing the logic gate associated with the plate on command.

[RGORJANCE]

Chuck is correct on the jumper for the failsafe. It should be located on the strip associated with the remote control wiring.

Fossil

[w4cl]

Had a FM25K back in the mid 80's. Had a plate blocker go. That was no fun to replace. Just make sure the surfaces of the kapton and the metal is clean, clean, clean and there are no burrs sticking up to poke a hole in the kapton. ANY breech of the kapton and the first time voltage hits it, it is lamp shade time for it.

[brandonb959]

Update.....had a few hours at the site today to work with the Harris:

Changed the screen blocker and found that the arc gap was out of place....found where the previous engineer had marked it.

The plate will come on but the transmitter will not make any power (I am using an external silicon valley IPA) I have bias, filament, and screen. The transmitter just idles and shows a little bit of plate current, but very little power (maybe 1K at best). I suspect that the grid input tuning capacitor has stopped turning as it has been acting funny recently.

Note* I have tried two different tubes today and carefully seated the tubes. I suppose I will try checking components and signal flow through the grid input circuit on my next trip maybe on Sunday night.

[RGORJANCE]

Grasping at straws now.

Not sure if that rig uses a feed thru cap to get drive into the pa grid. If there is one, and it opened, you would lose drive.

PA screen voltage could be too low for tube to conduct. Check the screen rheostat motor to see if it is working, check screen rheostat for an open, check to see if screen is turned down and only needs to be raised. Look for an open to the screen in a location past the rheostat.

Aha, another possibility - not sure if that rig has an "underdrive relay", but if it does and it didn't engage, there wouldn't be any drive present.

I'm done.

Fossil

[ChuckG]

Check the 25 ohm screen resistor, it's in the cavity. That transmitter should make more than 1kW with the screen turned off though. Not uncommon to run them that way in severe icing conditions.
More likely there is little to no grid drive, a mistuned grid cap might be the culprit, or a short somewhere in the circuit. IIRC the grid circuit is affected by the screen blocking plate...I think there's a grid blocker in the vicinity too. Been a while since I had that apart.

[brandonb959]

Cleaning of the L4 Grid tuning with contact cleaner has allowed the transmitter to now make some power, but it is jumping around at times....it is also somewhat difficult to move back and forth...probably time to replace..