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Author Topic: IC-751 Logic board  (Read 443 times)
KI4EU
Member

Posts: 14




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« on: November 30, 2017, 07:37:44 PM »

I have determined that my IC-751 Logic board is defective.  When turning the decoder (Tuning) dial, the display never changes by 1, but 2 instead.  For example if the frequency displayed is 8.222.2 the next change going up is 8.222.4.  The decoder is working correctly because I can "see" it change on my scope, but display won't change until it sends another change.  I can store an odd number frequency in the ram card using another radio, switch the card to the 751, and it will display the odd numbers.  But when the dial is turned, it will only change by 2 numbers.  This is true for all digits in the display.

The radio works, I can receive, but determining the frequency is difficult because for example, 7.180 would read 6.080.  Also in general receive mode, 6.180 would also read 6.080.  While watching pin 36 on the microprocessor chip (scope), I can see the voltage shift and hear a corresponding freq change on the receiver, although the display does not change.

I have built digital display units from scratch and understand that the decoder for the display only does one digit at a time, and the info is scanned into the display.  So I believe that the "1" bit in the decoder  processor (not the dial) is not getting changed, either because it is open, shorted to common, or latched to high.  That's the part I can't determine from the technical service manual.  It appears that it comes from insde the microprocessor chip on the logic board.

I have found a replacement board for a 751A, but it does not appear to be interchangeable.

Huh
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W9GB
Member

Posts: 3152




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« Reply #1 on: December 01, 2017, 10:49:57 AM »

Contact Scott Malcom at Malcom Technical Support.
http://mts-icom.wixsite.com/hamradiorepair
He may have a spare board (correct one) in his parts collection,
OR advise you about your Icom.

Scott worked at Icom America service, when the IC-751 was new (trained on model),
and years of Icom bench experience.

The Personality of Icom radios, in this era, programmed by EX-314.
http://www.ab4oj.com/icom/w9gb.html
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KI4EU
Member

Posts: 14




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« Reply #2 on: December 01, 2017, 06:29:56 PM »

Thanks.  Yes, I have communicated with Scott several times on other issues.  He is a really helpful person. 
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