THE-1 exciter problem

FM does it with frequency!
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REW
Posts: 23
Joined: Sun Aug 05, 2012 8:38 am

THE-1 exciter problem

Post by REW » Sat Aug 10, 2013 8:45 pm

I have a THE-1 exciter that is going out of lock when it wants to. The PLL voltage is where the book says it should be. I have not started any trouble shooting on this unit yet. I am waiting on a loaner unit to get here first. Has anyone run into this problem before or have any suggestions.

kcbooboo
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Joined: Sat Dec 10, 2011 4:53 pm
Location: East of the Mississippi

Re: THE-1 exciter problem

Post by kcbooboo » Sun Aug 11, 2013 7:05 am

The manual has some useful troubleshooting info, including random AFC unlocking.

I worked on an MX-15 exciter and during testing I had the same thing happen. In my case it was due to very low frequency audio input at the composite jack. A DC offset voltage at that input will also cause the same problem. The AFC lock can correct frequency variations faster than about 10 Hz, but slower than that, and it will think the VCO is actually operating off frequency, hence the inability to follow it and the AFC fault.

Due to the age of these units, I'd start with the power supply and measure all the voltages checking for hum and ripple. The filter caps in the PS and on the individual boards are likely suspects too.

You probably should go through the alignment of the entire unit. Also, exercise the frequency-determining DIP switches on the AFC board could be dirty/intermittent; exercise them.

Bob M.

REW
Posts: 23
Joined: Sun Aug 05, 2012 8:38 am

Re: THE-1 exciter problem

Post by REW » Sun Aug 11, 2013 7:45 am

Thanks for the ideas. I have been unable to find my book on this exciter so I downloaded a copy last night and have been reading through the troubleshooting tips.
A friend at another station here told me he had the same problem years ago and the problem was DC voltage on the processor output. I will be back on site later this week, I am trying to get a list of things to check for.
Thanks and keep the ideas coming.
Robert

kcbooboo
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Re: THE-1 exciter problem

Post by kcbooboo » Sun Aug 11, 2013 2:44 pm

The composite input is almost (if not truly) DC-coupled, so any DC or real low-freq AC here can cause the VCO to be modulated to an extent that the PLL/AFC thinks it's going off frequency (effectively, it is), and it can only correct so much error. Eventually it gets to be too great and you lose lock. At normal audio freqs, the AFC circuit's low-pass filters never see a problem.

A lot of two-way radios use similar PLL circuitry, but they need to get low-frequencies through without losing lock. Many will send the audio to both the VCO and the reference oscillator, modulating both, so the output signal has the proper deviation, but since both inputs to the PLL are being driven by the same low-frequency signal, the PLL doesn't see the low-frequency transitions as loss of lock.

The full technical manual on Harris' site is incomplete; it doesn't have the power amplifier module. However there is a separate schematic for that. I get the feeling that Harris outsourced the PA. Just mentioning this; it shouldn't have an effect on your AFC Lock problem.

Bob M.

radiootto
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Joined: Fri Jun 21, 2013 10:08 am

Re: THE-1 exciter problem

Post by radiootto » Sun Aug 11, 2013 3:09 pm

I have several of these particular (or peculiar) exciters. Just about all of them have developed odd problems all traced to those silly edge connectors that clamp onto the mod osc and the AFC boards. Take both boards out, clean the edge on the boards, clean the connectors. This may hopefully solve your problem. Just another Harris innovation!!

pinetree5447
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Joined: Sat Sep 07, 2013 12:00 pm

Re: THE-1 exciter problem

Post by pinetree5447 » Sat Sep 07, 2013 12:34 pm

I'm presently working on a THE-1....for the second time around. This one also loses AFC lock when it feels like doing so, even when no modulating input signal is connected. The AFC voltage is normal and does not appear to drift at the instant that it unlocks. I replaced all most-probable parts three months ago including electrolytic capacitors and IC's. It ran fine for months, but has now re-developed the same problem. I will take the suggestion from this forum and try cleaning those odd edge connectors on the modulated osc. and AFC modules.

Steve K.
Green Bay, WI

REW
Posts: 23
Joined: Sun Aug 05, 2012 8:38 am

Re: THE-1 exciter problem

Post by REW » Sat Sep 07, 2013 10:25 pm

My problem seemed to be in the processor. No unlocks after I put my spare processor in. The processor that caused the problems had a changing DC voltage on the output. Parts ordered and hope to have it back in service in a few days.

Doctor oldies
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Joined: Sat Jan 14, 2017 8:54 pm

Re: THE-1 exciter problem

Post by Doctor oldies » Sat Jan 14, 2017 8:59 pm

Hello guy, someone have the schematic RF amplifier of THE-1 fm exciter of harris?

Lee_Wheeler
Posts: 62
Joined: Sun Jul 24, 2016 1:53 pm
Location: Kansas City

Re: THE-1 exciter problem

Post by Lee_Wheeler » Sat Jan 14, 2017 11:19 pm

I might have one. I have a THE-1 driving a standby transmitter at one of the sites so, in theory, I should have a manual for it. Assuming that I can get into the site in the next couple of days I will look. We are supposed to have a huge ice storm tonight and I try to avoid the place while it is shucking ice.

If I can find it I will drop you a PM and send you a scan.

...Lee
Last edited by Lee_Wheeler on Sun Jan 15, 2017 10:57 am, edited 1 time in total.

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kkiddkkidd
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Re: THE-1 exciter problem

Post by kkiddkkidd » Sun Jan 15, 2017 7:18 am

I have one that looks pretty good. Send me a PM & I Will fwd. It is over 5meg.
--
Kevin C. Kidd CSRE/AMD
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AM Ground Systems Company
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