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Author Topic: 756 PRO ll-- only 1 watt out.........  (Read 1507 times)
K2UX
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Posts: 13




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« on: January 23, 2008, 07:46:14 AM »

I'm only getting about one or two watts out on SSB and about five watts out on CW or FM, all bands. I think it might have happened while trying to tune up on 60m through an external antenna tuner. Any ideas? Pre-driver ic or driver xstrs or finals? Or maybe a passive component? Been like this for a while and not getting anywhere.
Thanks & 73--
Joe- K2UX
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N4ATS
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Posts: 810




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« Reply #1 on: January 23, 2008, 08:42:35 AM »

Typical Problem. It is an 8 pin SMIC that controls the power output. I can help you as it is an easy fix but am not home until Friday. Feel free to call me Saturday.  407-873-3070  OR http://n4ats.com
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N4ATS
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« Reply #2 on: January 23, 2008, 08:50:28 AM »

Actually, while I was sitting here in the hotel , I went to mods.dk and looked at the schematics. For the 756-PRO it is IC801 which is a UPC1678G. That should fix it as I have fixed many of them. The IC is 6 bucks at mouser.com OR ICOM has them. 5 minutes to install. Let me know...  http://n4ats.com
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N1EQ
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« Reply #3 on: January 23, 2008, 10:10:22 AM »

UPC1678G is IC151 on the RF-A Unit.

After replacement of IC151, I suggest checking the status of the RFRX-muting circuit (Q25, D22, D23 on the CTRL Unit). Q25 should have around 13.5-volts on the collector during RX, and near 0-volts during TX.
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N4ATS
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« Reply #4 on: January 23, 2008, 11:15:41 AM »

I can't believe this hotel let me download the 756 Pro II   Service manual from Mods.dk. The Internet connection here is mega slow. Anyways , it is IC151 , Thanks (above). I have changed at least 10 or 15 in several different radios and never experienced any other problems associated with common circuitry. But it is probably a good idea to look at the other components anyways.
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N1EQ
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« Reply #5 on: January 23, 2008, 11:26:46 AM »

The RFRX-muting circuit is easily overlooked (by the operator and the repair tech), as the symptoms are rather subtle. Usually, it causes an oscillation during TX, but only on 12, 10 and/or 6-meters.

I too have replaced many UPC1678G chips in 746/756-series rigs. And most of the time, the RFRX-muting circuit is also damaged (HRX-muting in the 746-series). A very common failure.
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N4ATS
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« Reply #6 on: January 23, 2008, 12:21:48 PM »

Good point , I will start to check that "if" I ever get another one in with the IC problem. Thanks for the tip.
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K2UX
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Posts: 13




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« Reply #7 on: January 23, 2008, 01:48:32 PM »

OK, I just ordered the IC from Icom. Heard about this IC some time ago but never replaced it. We'll see what happens. Thanks for the ideas and would still like to hear any others.
Joe
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AJ4CU
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Posts: 78




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« Reply #8 on: February 08, 2008, 03:13:03 PM »

Hi,

I had a recent failure just like yours, rig was working fine one night and the next day no more than 5 watts out.
I sent the radio to Icom in Wa. and they found the following, "thermal bias diode failed, PA drivers failed".
183.00 + s/h and the radio is fine now.

I wonder what could have caused this issue in my rig???

anyone?

73  DE AJ4CU

Hari
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K2UX
Member

Posts: 13




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« Reply #9 on: February 10, 2008, 10:11:59 AM »

Yup, hari, those exact components failed  me once before. Back to Icom for that but this time it was the common failure of IC 151. Tried to replace it myself but too tiny for me. I was lucky enough to have a local ham (KC2SGO) who could do it for me and the rig is fine now, full power output.
For such a great line of radios Icom does seem to have overlooked a couple of common failures and not corrected them. IC 151 was a common failure in the 746 also and all Icom did to solve the problem for the 756 was to turn it off on receive but it can still overheat on transmit.
73
Joe- K2UX
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NO5C
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Posts: 2




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« Reply #10 on: February 23, 2008, 02:58:47 PM »

I have just purchased a ic 756 pro, I get 100WATTS out on cw and fm but can only manage 75 -80 watts out on ssb no matter what the mic gain, compression levels are. ANy Ideas??
What does L261 Tx peak adjustment do/control???

Tnx
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K2UX
Member

Posts: 13




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« Reply #11 on: February 25, 2008, 07:15:30 AM »

OK Dennis--
This is just my own opinion but I don't think you have any problem there. Thats pretty much exactly what I get when the rig is working right (which isn't often). 100w out is what it should be on CW or FM but on the same meter it should only be about half that as the rig is rated at 100w PEP on SSB which is a peak reading and typically will read about half or a little more on an average or DC reading meter.
MEANWHILE.... THAT DARN IC-151 BLEW OUT AGAIN AFTER ONLY TWO WEEKS!!!!! So I'm right back to where I started but I know it didn't over heat because I was running the rig at only FIVE WATTS out when it blew. Icom has more than one problem with that IC. I don't want to replace it again because it will just fail again. If ICOM won't do anything about their FLAWED design I guess I will. I'd like to find some other device that is more bullet proof.
73
Joe- K2UX
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