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Author Topic: The Icom 735 problem  (Read 1594 times)
BG1SB
Member

Posts: 5




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« on: March 13, 2002, 10:45:04 PM »

I got a used icom 735 rig, but the rig have some problem. Pls help if you know what happend with my 735.
1.The sensitivity is too low.
I have had compared it with a icom 751A, it like the icom 751A's ATT on state, and the S-meter didn't swing. I had checked the RF connecter inside and outside, no problem. The machine haven't been repaired before.
2.The LCD display is too weak, but can display all the numbers and word, how can i adjust it?

Thanks for your help and sorry for my bad english.
                  VY 73! de BG1SB/ A newbie form China

 
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KE9EH
Member

Posts: 18




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« Reply #1 on: March 14, 2002, 12:09:11 AM »

Check that small gray jumper on the back of the radio for proper function. Cable is about 3-5 cm long with a phono (RCA) male connectors on each end. This cable simply connects the receiver input (front end) to the TX/RX relay. Removal of jumper allows use of a seperate external receiver preamplifier or receiver. If cable's center conductor is open it could cause desensing of your receiver.

Simple check, otherwise desense problem could be any number of things.

Regarding LCD adjustment, access to the potentiometer is located on the left side of the radio near the front.  

In case the two problems are related, are you supplying the radio with 12 Volts DC?
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NO9E
Member

Posts: 382




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« Reply #2 on: March 14, 2002, 10:43:01 AM »

The sensitivity of 735 is lower than that of 751A. ARRL review shows about 8 db difference. On the other hand, my 735 did not show any intermods whether the preamp was on or off even on 40m so I kept it on all the time.

With display, there is an adjustment of backlighting  via a screw at a side of the radio (I think). Did you try it?

Ignacy, no9e@arrl.net
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N4LRA
Member

Posts: 4




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« Reply #3 on: March 14, 2002, 03:23:11 PM »

You did not give an indication as to just how bad the sensitivity problem is. I have owned several of these radios and one problem experienced is that there are a number of diodes on the RF input that can blow if the rig is hooked to an antenna, while another rig nearby is operating on a different antenna (like at Field Day).

These diodes are light duty and can not take the overload. The best indication of this being the case is that some bands will give you good receive, while others seem almost entirely dead. The noise will be very low on the bands that have the bad diodes in them.

You can buy heavier duty replacement diodes at Radio Shack, but some care must be taken when replacing them, as you have to lift the RF board up and out of the chassis. There are also a number of plugs and coax jumpers internal to the 735 that must be removed and then reconnected after you have finished replacing these diodes.

Best bet is to remove and replace all of the diodes towards the back of this board. This way you don't have to try and figure out which ones are actully shot and which ones aren't. Just be careful to get the polarity correct when intallling the new diodes.

Good Luck

Bill
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N4LRA
Member

Posts: 4




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« Reply #4 on: March 14, 2002, 03:29:48 PM »

Sorry about referring to Radio Shack. I guess there are none of these stores in China. You certainly can buy the diodes locally, if you determine this is the problem.

Again good luck to you

Bill
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BG1SB
Member

Posts: 5




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« Reply #5 on: March 19, 2002, 10:08:47 PM »

Thanks for all for your kindness.
I have compared my icom 735 with a FT-80C(ft-747), the sensitiveness is as much as ft-80c's.
It's seem no problem, but the s-meter can't show the signal(or show too weak, such as when the signal is S9+20db showed by ft-80c, the icom 735 only show S2), How can I adjust it?

Thanks again for your help

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