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THE-1 exciter problem

R

rew

I have a THE-1 fm exciter that drops out of lock when it feels like it. It may work for days fine and then go out of lock several times a day. It always locks back up after 10 to 20 seconds. The PLL voltage is where the book says it should be and I have run it higher and lower with no change. Has anyone had any similar problems or have any suggestions. I know finding these problems that occur at random can be a real PITA. Hope someone has been down this road and remembers what the problem was.

Thanks
Robert
 
The format is AC, I dont know about the hi pass filter, I will check on that. The processor is one of the older Orban Optimod units. I just cant remember the model right now. Gray front, LCD screen, about the size of the 8100. I am feeding composite out. I have also been told to check for any DC voltage on the processor output. I will be back on site in a few days and will check all of this.
 
chriscollins said:
Make sure your processor has a 30Khz HPF on. Could be subsonic if you play currents.
Click to expand...
That's why I asked. I'm pretty sure that's your problem. Although I'm not familiair with this exciter, there are exciters that can't handle a fair amount of bass>30Hz either... This is an older model isn't it?
 
I think the exciter and the processor are both dated back to the mid 90's. I hope the fix will be as simple as turning on a hi pass filter. My luck is not usually that good.
 
I think you are going to be lucky this time... This is really turning into a pretty common problem. Thankfully, the HPF cures it. I had it on both my CHR and Hip-Hop stations. At first, I had a hard time tracking it down, until I realized it was triggering on the same song, in the same spot.

Now, when I see a post like yours, I suggest it and so far, 100% of the people have had success... Don't pull down the average. LOL.
 
Sounds like you're using an 8200.... yes, as everyone else said, make sure the 30Hz filter is on. The THE-1 doesn't have a dual speed loop and winds up losing lock with heavy bass transients. Sometimes, even with the 30Hz filter on, it can still get bumped with really heavy bass. Been there, done that, bought the T-shirt.

Are you using this with a Harris transmitter too? Just curious.
 
Just to note that the broadcast AM/FM/FM-stereo transmission of audio frequencies much below ~50Hz cannot accurately be reproduced as acoustic waves by a huge majority of those receive systems.

So - limiting the low-end, transmitted AF response of the program chain to ~50Hz has a negligible affect to those listeners, overall, AND can reduce/eliminate AFC unlocks produced by most analog FM exciters of that era when presented with AF waveforms having substantial audio components below 50 Hz.

RF
Harris Broadcast Division RF Systems Engr, 1980-1999 (retired)
 
I believe this is an 8200 processor. The transmitter is a Harris HT-5. Chris I hope i dont pull your average down. This station is about a 150 mile drive for me so I cant listen to it and tell what is happening from where I am and dont want to depend to much on what the people there are telling me.
Thanks to everyone for the ideas. I will let you know what the problem was when I get it fixed.
 
8200 has the HPF, if I recall correctly. That will fix you up. I have an 8300 and 5500 on my two stations here that had the problem and once I toggled that setting, it has never happened again.
 
Years ago same issue with an older BE FX30 (not modified).

Reciently had an issue with a lot of DC on an analog audio source. The Digit didn't un-lock but the PCL6030 did.
Every time they played a recorded traffic report from the DigiCord the audio would go away.
Not sure when the 30 Hz HPF got turned off but that (and later fixing the audio source) did the trick.
 
Answering Kmagrill's query: The THE-1 and MX-15 AFC cards are very similar, if not identical dual speed designs. The THE-1 has this weird "drop-in" edge connector--probably specially designed so they could re-use MX-15 AFC cards.
 
Kmagrill said:
Don't the MS-15 and MX-15 exciters have dual speed AFCs? If so, shouldn't the THE-1 have the same dual AFC?
Click to expand...

I believe you are partially correct. Best I recall, the MS-15 is not dual speed. Someone correct me if this is wrong (Richard Fry?)> Geoff Mendenhall designed the AFC/Mod Osc in the MX-15.
He also designed the AFC/Mod Osc in the BE FX-30. Both are dual speed. I think he did the FX-30's first and then the MX-15's when he returned to Harris. I may have this bass ackwards.
Anyhow left over MX-15 AFC/Mod Osc modules were used in the THE-1.
Open one up and you will see them mounted sideways in the chassis.
Really all the THE-1 had to offer over an MX-15 was size reduction and higher output power.
They even used those awful push button switches left over from the MX-15.

When I first used Texar Prisms/8100 w/ RFC-1 into an MS-15, the AFC would unlock. I upgraded it to an MX-15 and the problem went away.

I never had an unlock problem with a MX-15 or FX-30, driving them with all types of
processing, some without HPFs or they were turned off.
Of course, low frequency content in music was not what it is today.

I'm thinking this may be an DC offset problem.
 
I looked at the manual and the afc pulls the R/C filter out of the afc line until the comparator locks and then puts it back on. It speeds up lock time from a cold start.

The time constant looks like 30Hz would be a good point to high pass. I don't know the 8200 however since it is digital it should have FIR filters.
 
MS-15 was first, MX-15 second; the AFC and modulated oscillator cars in the MX are different from the MS; but the remainder of the exciter is very similar (just rebuilt an MS that is now using the MX AFC/Mod Osc. modules--the donor MX was other wise mostly toast from overheating).

The THE-1 followed the MX series and would put out 50 watts instead of the typical 15 or so from the MS/MX series exciters.
 
I went to check on this exciter this afternoon and found the AFC voltage was as the book says at 7.5 volts. As I was watching it I noticed it would sometimes go up as high as 10 volts and sometimes dip as low as 5 volts. Never noticed that before. Take the composite signal loose and it was stable at 7.5. The 30 hz HPF was already turned on in the processor. I did notice a lot of noise coming out of the processor so I am now thinking the unlock problem may be caused by a processor problem. This trip was not planned. I just wound up close by and went over to check on it and didn't have the equipment with me to check on much. I will go back next week and change the processor out and see if the AFC stabilizes.
Thanks for all the ideas, I will let you know what the fix is.

Robert
 
I'll pass this info on because it was the only problem we ever had with the THE-1 in almost 20 years of operation. The AFC unlocked one night taking the transmitter off; however this was not an intermittent, it stayed off. The fix was to replace the 2 large electrolytic caps in the power supply.

Maybe that will help....good luck.
 
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