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AM DA question

W

WNTIRadio

Hi Brain Trust...

Here's the conundrum: I have a client with a 3 tower AM DA. All 3 are in a line, tower 2 is the reference tower and the pattern is a figure eight. Tower #3 tunes right up, day and night. Tower #1 is giving me problems, the ratio comes in but the phase won't no matter what I do. It's supposed to be .116 and +170. Right now, it's stuck at .119 and +145. Yesterday, trying to adjust it, there was a quick light at the end of the tunnel, where all of the sudden the phase came close. Problem is, it didn't stay that way for long. About 30 seconds after it went back to +145 again. Tried it 3 different times, each time returning it to the previous settings I had written down, and for a few seconds it started to look right and then reverted back to +145 no matter where I cranked on the phasor. In the end I put everything back to where it was. Needless to say, the daytime monitor points are hot because of this.

I switched inputs on the antenna monitor with towers 1 and 3, and the channel is working properly as tower 3 reads correctly in the tower 1 input. Checked and the tower is matched at 50+j0. Reflected power is about 5-10watts depending upon modulation. Nothing that has a red flag on that end.

There are 2 isolators on tower 1, with a unused FM antenna and a an unused STL dish.

So the question is... could it be the sample line? The toroid (this seems unlikely to me)? Or one of the isolators? Any ideas or input would be greatly appreciated.
 
If it jumps around that much, I'd suspect something with the sample line for that tower such as a connector or bad sample toroid. I'd also look at the terninating resistor on the antenna montior for that tower. You can swap out these with ones from the good tower and see if things change for the better.
 
You should see if the MP's actually change with adjustment. Sit someone out there and crank away.

If the phase adjust has a pair of coupled inductors is the coupling between them is solid? And they actually do move?

What's the make & model of the antenna monitor?
Try running the transmitter power very low and see if the readings changes. Readings shouldn't change much.

Potomac 19's had diodes become leaky causing all sorts of odd readings.

The 1900 series have a problem with an IC that when it fails compresses the phases ratios.
U1 "limiter". Old one was a MAX9690. Replacement's a CMP08. Don't put replacement in a socket!
 
I'll have to check that IC on the Potomac 1900. I swapped out the tower 1 and 3 lines on the back of the monitor to see if the problem followed the line or the input, and it followed the line to input 3. Tower 3 was spot on in the input for tower 1. The unit was calibrated a couple of years ago.

The phase adjust is 2 coupled inductors, and I did check the connections in the phasor as well as taking the back off and watching them move, so that rules that out. Could one of the mica caps going bad affect it? I've got a box of replacements so that wouldn't be too hard to do, but I'd rather not have to shut down everything if necessary.

This is the first time it's actually jumped around, any previous attempts it was pretty flat around +145.

I'll have someone go out next week with the FIM and see what happens when I crank on it. I'm just baffled that it comes in to tolerance, at least according to the monitor, for such a short time then goes back to the same readings.

Thanks for the replies!
 
You might want to check the ground connection on Tower #1. I've seen a similar problem. It was a bad ground.
 
If the ratio isn't changing too much, it could be a shorted input or output cap at the ATU. That wouldn't explain why the phase jumps back and forth.
Use your FIM to check the ground radial radiation near the tower base. If the radiation is quite a bit lower than the ground radial radiation on the other towers (taking into account the power relationship between towers, of course), it would indicate a bad ground connection. I'm still guessing a bad ground connection at the base of Tower #1.
 
I had a problem much like that several years ago, I do not know if it will help, but check to see if there is water in the isocoupler. The old Mosely gray Isocouplers were known for getting water in them, It is almost impossible to get water in the Kintronic.
 
That's a great idea. The one for the FM is the Kintronic and rather large (and mounted on the side of the ATU shack) but the one for the STL dish is one of those small ones that's on the transmission line near the base of the tower. I also discovered another one for an 2.4ghz link that hasn't been used in a long time.

I'm thinking of disconnecting the STL dish and the 2.4 yagi and letting them float on the tower. I'll keep everyone posted as to what happens early next week when I go back down there. I'll also get someone to run MP's with me and see if anything changes.
 
I know this may be "caveman" diagnostics, but this sounds like some sort of intermittant. Try taking a DRY 2 x 2 out there and "gently tap" on the tower and other feedlines while someone is looking at the phasor. If all those unused antennas are un-terminated, you might check shields and center conductors to station ground, or maybe even get brave and short them out. Might drop the AM power before trying that!
 
I'd certainly check into what others have suggested. With that being said, if you have one of the older Potomic monitor you may have some flaky mercury-wetted relays inside. I'd do as someone else suggested and swap sample connections on the back of the unit. At least that would tell you if the monitor itself is the problem or not. If you can scare up a nice OIB I'd see if things stay stable when the monitor readings change etc. Also, if you can get a Tdr to use and a dummy load I'd check the sample lines are ok (with the am off of course to protect the tdr).

I like the idea of banging around on stuff a bit too. That's likely to make the problem evident. Ultimately it's some kind of lousy connection regardless of where....
 
I've fixed or found a lot of problems with the "Fonzie Method" before, I'll be trying that this week.

Already swapped the sample lines on the back of the Potomac, the problem followed the line from tower 1 to tower 3, so I'm pretty sure that rules out the 1901. It was calibrated about 14 months ago as well.
 
Does the common point change or does it remain the same when the phase shift occurs? If it stays the same, sample line problem, if it changes look for bad connections/blown up parts in the ATU.
 
I'll let you know on Wednesday, that's my next visit to the site. I don't recall seeing the common point change, no matter when I looked it was between 10A and 11A (depending upon modulation) which is normal. 10.4A is nominal.
 
Here's the update...

I cranked around again and much to my surprise, everything slowly came into tolerance. I was amazed, and shortly afterwards, realized that it was "too easy".

That familiar smell of something electronic burning brought me to the back of the phasor cabinet. I opened it up and the input to the phasing coil was a little burned looking and a bit of smoke was coming from the coil where the feeder was touching the insulator holding the coil.

I put everything back to the previous settings, and was back where I started on the antenna monitor. But the smoke & heat is gone. So when things are in tolerance (.115 +170), it heats up, when it's out of tolerance (.119 +145) no heat/smoke.

Could it be the mica cap just prior to the coil? The coil? A bad feeder? There's a slight "kink" in the feeder the way it's routed, can that have an effect?

I have a photo of it, I can PM it to someone. I wish I could post it here directly.

Thank you to everyone with all the advice!!
 
If I've read this correctly, the phase shifter is two coils ganged together. Sometimes the roller on one of the coils can loose contact with the coil. That will throw the phase off, but usually will cause a change in the ratio for the tower. You can check that by moving the crank a turn or so in either direction and see if anything changes. You will hear some arcing if that is the problem. Since you smelled something hot in the phasor, I would check the caps on both sides of the phase shift coils. Look for splits, or signs of something oozing through the body of the cap. A multimeter with a capacitance checker will tell you if the value has changed. Since you mentioned that a cap is feeding the phase shifter, that suggests a +90 network, so there may be a cap on the tower end of the shifters. Check that one also. My gut says you have a cap that changes value when it heats up.
 
WNTIRadio said:
Here's the update...


That familiar smell of something electronic burning brought me to the back of the phasor cabinet. I opened it up and the input to the phasing coil was a little burned looking and a bit of smoke was coming from the coil where the feeder was touching the insulator holding the coil.
Click to expand...

I ran into problems with Bakelite insulators breaking down over time becoming resistors. If the the burn area is Bakelite suspect that. If they are porcelain...never mind.

The only capacitor problems I have run into were out at dog houses where lightning would take them out by shorting or making them lossy. The large caps are hard to check and don't always test bad on a regular cap checker. Substitution of known good parts always diagnosed and fixed the problem.

Rotary inductors can sometimes be problematic over time as they age and get tarnished/corroded. Cramolin (DeoxIT) usually brought them back to operation however I did have one that got so bad from repeated arcing it had to be replaced.
 
Got the can of Deoxit ready to go, they'll get sprayed out on Wednesday.

And yes, they're Bakelite insulators holding the coils. I'm going to loosen up everything, spray the Deoxit in there and tighten the nut holding the feeder back on after a good wipe down.

Hopefully that does the trick.
 
There are very few magical things in this world, and DeOxit is one of them.
Besides the marvelous removal of oxides and sulfides, the lubricant left behind is very light and non-gumming.
I left my 1950's daily-wear watch in a pocket which went through a whole wash cycle and while I was in the shower, and the watch was IN THE DRYER, I remembered...Uh-oh.
I unscrewed the back, blew gently "warm"" air it a while, and gave it a shot of deoxit , (being careful not get it on the hairspring)
put it back together, and 5 years later it is still keping perfect time. Thank you Bulova and Deoxit.
Any lubicant fine enougn to use in a watch is pretty darn good.
 
If you've had significant arcing between the coil and the rolling contact(s), you'll need to file down the rough spots on the coil and perhaps replace the rolling contact to avoid future arcing. At that point, you can use the DeOxit. Just use it sparingly. A little goes a long way.
DeOxit is the best product of it's kind that I've ever used. Far better than most "contact cleaners."
 
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