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Author Topic: IC-751A display:LSB ,FM and RTTY at the same time  (Read 434 times)
HG5BZR
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Posts: 20




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« on: April 22, 2007, 05:26:40 PM »

Hi!

I have a problem :My IC-751A works fine but sometimes If I switch my rig on I can see FM LSB and RTTY mode at the same time on the display and no RX,no TX,just whistling noise from the speaker.Only 40m band RX,but only in FM,I can not change mode to LSB.I noticed that If I switch off and on my rig or I turn VFO knob a little ,my rig will works fine.I noticed an other problem:After about 10 minutes the TO-9 transformer (DC-DC converter) is very hot on the display unit.I can not touch it by hand,after 1 seconds my finger will be burnt if I touch TO-9 transformer. Is it normal? Thank you very much for your help.

                                 Laszlo
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KA1MDA
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Posts: 543




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« Reply #1 on: April 23, 2007, 09:04:49 AM »

Although I am not familiar with this particular radio, I have seen many DC-DC inverters overheat and fail in VCR's. The main cause being electrolytic filter capacitors on the input and output side of the inverter. When these capacitors are heated for a long time, they begin to dry out and lose capacitance. As the capacitance goes down, the inverter draws more peak current and runs hotter. This dries out the caps even more and creates a thermal self-destruct cycle. I have seen DC-DC inverters in VCR's get so hot they even char the circuit board before failing.

Check if there is an electrolytic capacitor on the input and output of the inverter. Try replacing these and then recheck the inverter temperature.

73, de Tom, KA1MDA
www.ka1mda.org
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N1EQ
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Posts: 166


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« Reply #2 on: April 23, 2007, 01:01:23 PM »

KA1MDA is right. Your problem is probably due to dried-out electrolytic caps in the DC-DC inverter circuit, and noise from the DC-DC inverter is causing the logic circuits to glitch.

Also check the electrolytics near the regulator chips on both the logic and PLL boards. Go to this site for more info:

http://www.qsl.net/icom/oldicomfaq/ic-751_51a.html
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HG5BZR
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Posts: 20




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« Reply #3 on: April 23, 2007, 03:19:00 PM »

Dear KA1MDA and N1EQ!

Thank you very much for your answer.I ordered a new DC-DC converter from ICOM and I will change it. I will change some electrolyt cap. on the display unit.

                            Thanks HG5BZR  Laszlo
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EI6IL
Member

Posts: 21




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« Reply #4 on: February 14, 2008, 11:00:28 AM »

Hi Folks,
   Does anyone know where I could buy the TO-9 DC-DC convertor Transformer for the display unit on my ICOM IC-R7000 receiver.

Thanks,
Don EI6IL
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