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HARRIS DIGIT EXCITER

Posted: Thu Aug 02, 2018 6:27 am
by apache45
I have one of the original Harris Digit (not CD) exciters that will not put out any more than one watt. I have rebuilt the A-2 power supply section and all 4 voltages are correct. The APC voltage is also within tolerance. The manual troubleshooting chart suggest "debugging" U-8, U-3, U-7 in the A-2. What exactly is meant by "debugging"? And yes, the PA-amp fuse is good as well. Any suggestions? Thanks gang.

Re: HARRIS DIGIT EXCITER

Posted: Thu Aug 02, 2018 6:47 am
by RGORJANCE
Not specifically knowledgeable on the Digit.....I think a bit more info would be helpful.

Is the exciter showing any serious VSWR? I assume you are hooking it up into a good dummy load with wattmeter.

I experienced two problems with an MX-15 exciter showing similar problem that was traced to two areas. One was a bad interconnecting coax cable and the other was a contaminated output BNC connector on the exciter itself. Replaced both items and exciter came back to life. You might want to look at those areas as a start. I am sure you will get some other ideas from the group.

Only other thought at this time is the condition of the filter caps.

Fossil

Re: HARRIS DIGIT EXCITER

Posted: Thu Aug 02, 2018 7:44 am
by apache45
Thanks for the response but, no, there is no VSWR as I am using a thru-line wattmeter into a dummy load.
Checked all cables & connections and they appear to be solid and working. I may have to get back into the A-2 card for more serious inspection, however.

Re: HARRIS DIGIT EXCITER

Posted: Thu Aug 02, 2018 1:36 pm
by DaveSt
Check the electrolytic caps in the PA unit, in particular C29 (82u/63V). If a 47u is fitted, change it. You can break the old one so that you do not have to remove the board.

Re: HARRIS DIGIT EXCITER

Posted: Thu Aug 02, 2018 2:27 pm
by grich
DaveSt wrote:
Thu Aug 02, 2018 1:36 pm
Check the electrolytic caps in the PA unit, in particular C29 (82u/63V). If a 47u is fitted, change it...
C29 is mentioned in service bulletin FM-580-WCF. Get ahold of that and bulletin FM-535-WCF from the GatesAir website.
DaveSt wrote:
Thu Aug 02, 2018 1:36 pm
...You can break the old one so that you do not have to remove the board.
Done that more times than I'd care to admit over the years. Sometimes faster in an emergency, sometimes less damage done doing it that way. And, it's the way the service bulletin tells you how to replace C29!

Re: HARRIS DIGIT EXCITER

Posted: Thu Aug 02, 2018 3:07 pm
by awsherrill
Is C29 one of the caps responsible for the infamous Digit "whistle"?

Re: HARRIS DIGIT EXCITER

Posted: Thu Aug 02, 2018 4:56 pm
by grich
awsherrill wrote:
Thu Aug 02, 2018 3:07 pm
Is C29 one of the caps responsible for the infamous Digit "whistle"?
One of several. Previous thread link below...

viewtopic.php?f=3&t=5907&hilit=digit