HT-10 APC issues

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wtcmeng
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HT-10 APC issues

Post by wtcmeng » Wed Jan 23, 2019 9:56 am

I have been having issues with the APC circuit on a HT-10 Per Gates Air’s advice I recapped the BIAS and Screen Supplies. My assistant and I also gave the TX a good cleaning and inspected hardware. My measured AM noise was between -25 and -35 DB before the cap replacement. I remeasured the AM noise afterword and it didn’t change. I swapped out the exciter with a known good exciter and still no change with the AM noise level or APC circuit raising and lowering with modulation peaks. If I remove modulation the APC circuit functions normally. I have even tried using a different audio processor. No change. The final tube has been in service for 5 years. I am still making full power with some head room. I am wondering if the aging tube is the AM noise culprit? Has anyone experienced this problem.... Thoughts? Oh and did I mention the transmitter is the main and is 30 years old.
Last edited by wtcmeng on Wed Jan 23, 2019 11:04 am, edited 1 time in total.

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RGORJANCE
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Re: HT-10 APC issues

Post by RGORJANCE » Wed Jan 23, 2019 10:07 am

You might try bypassing the IPA and feed the final direct from the exciter to see if the AM noise changes to rule out an IPA problem. There should be an adaptor on the back rail in the xmttr to use for the bypass test. You will need to raise the output power of the exciter to get enough drive to the final. I have done this to bypass a failed IPA and it works fine. Then check your AM noise. If it is still present, I would then check neutralization, and possibly put in a new final.

That's about all I can think of.

Fossil

wtcmeng
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Re: HT-10 APC issues

Post by wtcmeng » Wed Jan 23, 2019 10:31 am

Fossil,

Thanks for the advice. I will try bypassing the IPA. I may bring one with me. This site is three hours from my house. I like to be armed with as much as I might need to address the issue.

Eric

TPT
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Re: HT-10 APC issues

Post by TPT » Wed Jan 23, 2019 6:14 pm

What kind of exciter? Digit exciters were shipped with some of the HT-10's
There are various problems that can develop with the digit exciters with age, including spurs that might show up as AM noise. Usual cure is to recap the exciter.

wtcmeng
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Re: HT-10 APC issues

Post by wtcmeng » Wed Jan 23, 2019 8:45 pm

It’s a THE1 exciter. I did swap out the exciter with a
bext XT-30 to see if the THE1 was generating the AM noise issue. It didn’t improve anything.

DaveSt
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Re: HT-10 APC issues

Post by DaveSt » Thu Jan 24, 2019 12:32 am

I do not know this transmitter model or your test setup, so I might be pointing out the obvious. Are you measuring the AM with a dummy load or into antenna? A narrowband antenna/filter will cause poor AM measurements. Especially so if the measurement point is not the forward port of a good directional coupler.

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Dale H. Cook
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Re: HT-10 APC issues

Post by Dale H. Cook » Thu Jan 24, 2019 6:35 am

I will second Fossil and Dave. I have seen AM noise increase with IPA problems, and you need to measure it into a good dummy.

Two questions, which may help find the cause: 1) Is this synchronous or asynchronous noise? 2) Have you looked at the noise on a 'scope?
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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Re: HT-10 APC issues

Post by TPT » Thu Jan 24, 2019 7:40 am

Believe most of those IPA's in the HT-10 are repairable by Silicon Valley Amplifier--if you are prepared to wait. Took almost a year to get one fixed for a transmitter I have to deal with.

While it was out we drove the transmitter with a Nautel V-1 (not VS series). Match was touchy, but we were able to get about 6 kw. As I recall, it takes around 175~250 watts to drive.

wtcmeng
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Re: HT-10 APC issues

Post by wtcmeng » Thu Jan 24, 2019 3:40 pm

Dale,

I did measure the AM noise will transmitting into the dummy load. I haven't looked at it with a scope but it appears to be synchronous as the RAISE and LOWER LEDs on the controller follow the modulation peaks.

ChuckG
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Re: HT-10 APC issues

Post by ChuckG » Thu Jan 24, 2019 6:09 pm

Mark Persons used to repair those by gutting the amplifer and installing a Broadcast Concepts module. Takes just a few hours and a fraction of the cost Take a look:
http://www.mwpersons.com/tech-tips/FM/H ... tt-ipa.htm

TPT wrote:
Thu Jan 24, 2019 7:40 am
Believe most of those IPA's in the HT-10 are repairable by Silicon Valley Amplifier--if you are prepared to wait. Took almost a year to get one fixed for a transmitter I have to deal with.

While it was out we drove the transmitter with a Nautel V-1 (not VS series). Match was touchy, but we were able to get about 6 kw. As I recall, it takes around 175~250 watts to drive.
<><><><><><><><><>
Chuck Gennaro
Central Wisconsin

Kelly
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Re: HT-10 APC issues

Post by Kelly » Fri Feb 01, 2019 11:47 am

It's been a while, but I recall those Harris THE-1 exciters were plagued with microphonic tendencies from blower motor vibration. Besides being a pretty crappy exciter, they should at least be mounted apart from the transmitter.
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jthorusen
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Re: HT-10 APC issues

Post by jthorusen » Sun Feb 03, 2019 4:05 am

Something seems just a bit wrong to me about the problem description. An automatic power control (I assume that is what we are talking about with APC.... good rule: always define your acronyms and abbreviations) is not supposed to follow modulation. If it does, then that is the cause of your AM noise right there.... changing the power level is changing the amplitude of the signal which is amplitude modulation.

Look at caps in the APC detector circuit. Make sure that its power supply is clean. There should be a long time constant in this circuit... at least one second and probably more. It sounds as though this is no longer the case. You need to find out why.

Just my $.02 worth; your mileage may vary.

Regards,
James K. (Jim) Thorusen
KB6GM
Central Coast Electronics
www.centcoast.com
NW Oregon Consulting Bdcst Eng.

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Re: HT-10 APC issues

Post by Kelly » Mon Feb 04, 2019 1:20 pm

There are so many variables. Synchronous and Asynchronous AM noise are different but similar animals. If the APC is running the screen too low or high from where the tuning is resonant, that could cause increased Asynchronous AM noise. As jthorusen mentioned;
if the APC is off and the transmitter tunes for low AM noise at operating TPO then the AM noise goes up after the APC is engaged, then chances are the APC is adjusting the screen voltage up or down and effectively de-tuning the PA. It would be more noticeable with an older tube.
If you couldn't get the AM noise down, even if the PA was tuned at licensed TPO, then I'd look at contributors of AM Synchronous noise like the exciter picking up vibration from the TX blower motor.
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