Harris DX-10 Fold Back Power Issue

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radiovigilante
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Harris DX-10 Fold Back Power Issue

Post by radiovigilante » Wed Dec 04, 2019 2:44 pm

I have been called to work on a Harris DX-10 that is folding power back to 1,000 watts. We have checked the load presented by the tower which is correct. The Bandpass filter light is red on the transmitter after you pass about 2200 watts and it folds back the power to 1,000 watts. Everything in the tuning section of the transmitter is correct and without damage...Any other ideas what could cause this?
This all started after a lightning hit a few week ago.

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RGORJANCE
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Re: Harris DX-10 Fold Back Power Issue

Post by RGORJANCE » Wed Dec 04, 2019 4:47 pm

I had a very similar problem some years ago. It might not be the same thing you are looking at, but here is what was found.......The coax flange - 3 1/8" clamping body was not properly mated with the outer conductor of the coax. Cuts were folded over, not v-notched. Numerous lightning hits had created carbon spots which caused fold back. Load was fine in this case.

I would suggest you get a bright light, look at the caps very closely to see if there are are hairline cracks in the epoxy/porcelein. That definitely would cause what you are experiencing. Also check coax for any carbon tracking.

That concludes my thoughts. Been too many years since I had to work on one of them to remember much else. Maybe one of the other experienced guys have some additional ideas. Good luck!
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Kelly
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Re: Harris DX-10 Fold Back Power Issue

Post by Kelly » Wed Dec 04, 2019 5:22 pm

radiovigilante wrote:
Wed Dec 04, 2019 2:44 pm
I have been called to work on a Harris DX-10 that is folding power back to 1,000 watts. We have checked the load presented by the tower which is correct. The Bandpass filter light is red on the transmitter after you pass about 2200 watts and it folds back the power to 1,000 watts. Everything in the tuning section of the transmitter is correct and without damage...Any other ideas what could cause this?
This all started after a lightning hit a few week ago.
If you select "Reflected" on the output power meter and slowly ramp up the power from zero, do you see the meter rise as the output power increases?
Skype:kellyalford Twitter: @KellyAlford

RodeoJack
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Re: Harris DX-10 Fold Back Power Issue

Post by RodeoJack » Fri Dec 06, 2019 12:21 pm

Tuning is important, but if you haven't been twisting the load/tune controls and nothing in the ATU has changed, that part of the transmitter is pretty stable, and mistuning shouldn't show up as a bandpass fault. As noted though, if those controls were someone's "go-to-first", you'll need to get that under control.

The indicator you see mentions bandpass and envelope errors. Bandpass is usually the less-likely

If bandpass: A couple of things will cause the bandpass fault light. The obvious would be a failure with a cap in that network, but that problem is rare (one exception described below).

There's a process for calibrating the metering (antenna net / nul), but needing to do that on an otherwise reliable transmitter would indicate that something has changed elsewhere.

This last example is more rare, but I did have it happen once. There's a minimum clearance number for the tubing coming off of the bandpass capacitor (Variable - C101?) and going down to L101 below it. I have a DX-10 where that tubing was too close and eventually arced to the side of the cap, frying it, as well as the fiberglass plate it sits on. Again, the book specifies the clearance. Look for it in the frequency change section.

If you are unfortunate enough to have had this happen to you, Gates will sell you one for $4 to $5 kilobucks (priced 5 years ago). I found a surplus Jennings cap for about $1.5k.

Resist adjusting the bandpass capacitor. If it was OK last week, adjustment probably won't fix the problem... especially since you're folding back that far.


If Envelope error: This one is more likely to be your problem. You'll get the same indicator and the same foldback. Simply put, it lights when "out doesn't equal in", meaning the modulator is trying to turn big step PA cards on and they are not responding. There may be a driver chip failure in the modulator card. There's a procedure for testing it in the book. If that fails, call Ralph at Gatesair. He'll walk you through it.

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