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Author Topic: IC-756 Pro 3 receive help needed  (Read 3009 times)
K9WJL
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Posts: 183




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« on: August 14, 2010, 03:46:19 PM »

My 3YO 756 P3 has developed a problem on the receive end. Received reports are still good.

When I first turn the thing on lately, the receive is greatly attenuated. I took it apart the other night, and while listening the entire time, I tapped on the little relays closest to the antenna jacks, and had no change, still attenuated. I then shut it off and put the cover back on, plugged it back in and turned it on the receive was good again.
 
I just had the problem again, and cycled the power again, and its good now.

I've seen the receive return with a actuation of the mic key, but lately that dosent seem to help. cycling the power seem to be the fix now. The receive has never attenuated with operation, once the receive is good, it stays good until the next time I turn it on.

IC-746 on the same antenna dosent exhibit the same problem, I've traded the feedlines between the rigs and no difference.

I'm looking for a little experienced guidance on how to proceed here. I don't want to send it out if I dont have to.



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K8AC
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Posts: 1477




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« Reply #1 on: August 15, 2010, 06:30:13 AM »

In the Pro III, there are several relays in the receive signal path between the antenna coax connector and the receiver.  I would first try connecting an antenna to the RX antenna jack on the rear panel (RCA jack), making sure to select the receive antenna by holding down the antenna select button.  From the schematic, it appears that will bypass three or four relays and I'd bet that one of those is your problem.  I've not been into that portion of the Pro III, so can't say how difficult it might be to replace the relays.  If the problem can easily be duplicated,  you could inject a signal into the signal path using a signal generator, starting at the antenna jack and moving forward. 

If you're not comfortable doing the work yourself and don't want to send the rig off to Icom, you might check with AES in Milwaukee to see if they would service it for you.  They used to have a good service department and it looks as if you could drive there and back in a day.  About the same cost as shipping it, and certainly safer!

You might also want to check the connectors on the board that contains those relays.  I don't recall how the boards are interconnected, but probably via small connectors that you could wiggle or re-seat.  There's always the chance that one of the wires isn't making good contact with the connector pin. 

73, K8AC

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K9WJL
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Posts: 183




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« Reply #2 on: August 15, 2010, 09:54:13 AM »

Hiya Floyd,
 Thanks for your reply. Alot of good ideas there thats for sure.

Just tried your idea with the RX antenna, and switching between ports using a switch and the rigs control didn't make a difference.

Keying the mic made the receive come back as normal. It's looking like a relay more and more all the time.

I didn't look at the schematics very much, so I couldn't find the relays in question, but I know from swapping the antenna to ANT2 made no difference either. That should eliminate the antenna switching system right, Or does it use the same relays for T/R?

Anyway, I'd bet I cant just pop the cover off of any of the offending relays for a quick shot of contact cleaner. I'd bet I have to desolder and replace it. Is that correct?

Thanks for the help again.

73,
 Bill K9WJL
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W4KVW
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Posts: 501




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« Reply #3 on: August 15, 2010, 12:38:09 PM »

There are also receiver pre-amps which can fail.I had one fail in my 756 PROII after about 15 months & ICOM replaced it for "FREE"! ONLY issue I have EVER had with the rig since 2003 when I purchased it NEW & I only use it EVERY DAY! }:>)

Clayton
W4KVW
 Smiley
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K8AC
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Posts: 1477




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« Reply #4 on: August 15, 2010, 12:57:16 PM »

There's only one relay that is in the signal path of a received signal for both the RX and ANT connectors and that's R101.  It's located on the rear of the RF B unit, which is the board that contains the XVRT and RANT jacks.  If you were looking at the component side of that board, the relay is the first one to the left of the XVRT jack.  I've never been into that part of the Pro III, so can't tell you for sure about the relays, but from the size I see in the service manual, I'd say it is likely a DIP size unit in a plastic shell.  Sometimes you can lift the housing off of one of those and sometimes they seem to be glued on.  A new one would probably cost just a few bucks from Icom.

I was just looking at the schematic and see another couple of possibilities.  After the receive signal passes through R101, it enters the attenuator circuitry.  There's a 6 dB and 12 dB attenuator in series and each is bypassed by a relay.  When you switch in the attenuator, one or both of those relays is activated, inserting the associated attenuator into the path.  If one of those relay contacts is flaky, then the received signal level would drop.  I've seen that happen in another Icom rig I once owned.  The attenuator relays are labeled R102 and R103.    A bit farther down the line are some other amplifiers and attenuators that have to do with whether you're using VFO A or VFO B.  You might want to try using split mode and see if the problem exists with both VFO A and VFO B.  If so, that part of the path is probably OK. 

I also see that the band pass filters are in that path and they are diode switched - no relays.  Does this problem occur on just one band or on all bands?  If just one, there could be a faulty switching diode (there are two per band) which could be a problem.  If the bandpass filter for one band were not switched in due to a faulty diode, then there would likely be very high attenuation for just that one band.
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K8AC
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Posts: 1477




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« Reply #5 on: August 15, 2010, 01:07:22 PM »

Clayton - just checked the schematic again and think maybe the receive preamps you're talking about are in the area I was talking about, past the attenuators.  There's a dual set of those - one for each signal path - VFO A or VFO B.  With your Pro II, was it just one of the VFOs having the problem, or both? 

My experience with two different Pro IIIs was similar to yours - virtually no problems.  This Pro III has the new MOSFET PA board in it and has been troublefree.  My previous Pro III also had the MOSFET PA board, but produced only 80 watts output on all bands instead of the 100W it was supposed to.  Since the service manual and adjustment procedure for the new board wasn't available yet, I had to ship it off to Icom for the final to be properly adjusted.  They refused to honor the warranty and made me pay for their time before they would return the rig to me.  That was because I told them I had lifted one end of one resistor to check the idling current in the PA transistors. They insisted there was nothing wrong and there was no adjustment necessary.  When I got the Pro III back, the output was 100W as it should have been in the first place!  Clearly, some of the early units with the MOSFET PAs left the factory not adjusted properly as several other folks noted the same problem. 

Outside of that minor glitch, I've never had a problem of any kind with either unit.

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K9WJL
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Posts: 183




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« Reply #6 on: August 17, 2010, 05:06:55 PM »

Hi Guys, Thanks for the input.
Just got back in the shack after 3 days of being away, and the rig turned on just fine......
I'll take your advise Floyd, and look into those things next time it PO's me.
Thanks and 73,
 Bill K9WJL
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N3LKA
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Posts: 7




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« Reply #7 on: January 27, 2011, 01:51:51 PM »

Just wondering if you fixed your issue or not. I wanted to ask one question, do you have an amp in-line? I had an issues where I had a bad relay in an amp inline with my proII, putting rf through it whether the amp was on or off made the receive come back.

Just thought I'd throw my 2 cents in.

73,

Brian
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W4VR
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Posts: 1194


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« Reply #8 on: January 28, 2011, 08:45:35 AM »

Don't blame it on the radio.  It's probably a bad coax connector somewhere down the line.  I recently had this same problem...all connectors looked fine...I replaced them all (2) and the problem went away.
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