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Author Topic: please help icom 746 low tx power  (Read 6456 times)
KB5NMA
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Posts: 4




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« on: December 08, 2013, 02:35:57 PM »

I have a 746 and it will only put out around 15 watts on any band in any mode not sure where to start any help would be appreciated
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K7KBN
Member

Posts: 2766




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« Reply #1 on: December 08, 2013, 06:08:10 PM »

Is this measured into a known-good dummy load or an actual antenna?
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73
Pat K7KBN
CWO4 USNR Ret.
N4ATS
Member

Posts: 800




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« Reply #2 on: December 08, 2013, 08:28:02 PM »

In standby mode , look at the current on your power supply , if it is above 3 amps , usually this is an indication this is an older unit and both 2SC3133 OR 2SC1972 transistors are shot. You can tell by the current draw that they are oscillating. If it is the newer 746 , there is one SMT FET right before the driver that goes south all the time.

Service manual is free to download all over the place.

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K9ZF
Member

Posts: 76


WWW

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« Reply #3 on: December 09, 2013, 06:37:34 AM »

I had the same problem with my 746 a few years ago.  I had it repaired, and I don't remember the exact details, but it followed what N4ATS is saying.

If you do a google search, you can find a lot of details of common 746 problems.

Good luck,

73
Dan

--
K9ZF
Amateur Radio Emergency Service, Clark County Indiana. EM78el
former K9ZF /R no budget Rover ***QRP-l #1269
Check out the Rover Resource Page at:
<http://www.qsl.net/n9rla>
List Administrator for: InHam+grid-loc+ham-books
Ask me how to join the Indiana Ham Mailing list!
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--
K9ZF
Amateur Radio Emergency Service, Clark County Indiana. EM78el
The once and future K9ZF /R no budget Rover
 ***QRP-l #1269
Check out the Rover Resource Page at:
<http://www.qsl.net/n9rla>
List Administrator for: InHam+grid-loc+ham-books
Ask me how to join the Indiana Ham Maili
N4NYY
Member

Posts: 4742




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« Reply #4 on: December 09, 2013, 06:51:03 AM »

I had a similar problem. It was either the dreaded IC-151 chip, of the 2SC1972 drivers. Since I can repair things myself, purchased all the parts for $60, and replaced both.

FWIW, the 746 pro and non-pro line was notorious for IC-151, drivers, and backlight failures. Icom turned a blind eye to it. Get the rig fixed, then sell it and buy something better.
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N4ATS
Member

Posts: 800




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« Reply #5 on: December 09, 2013, 03:05:46 PM »

N4NYY , good point

"I had a similar problem. It was either the dreaded IC-151 chip, of the 2SC1972 drivers. Since I can repair things myself, purchased all the parts for $60, and replaced both."

When I replace IC-151 , I add the protection diodes which keeps it from blowing again. Here is a very clean website of the fix(s)

http://www.qsl.net/k5lxp/projects/746Pro/746ProNoTX.html
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N4NYY
Member

Posts: 4742




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« Reply #6 on: December 09, 2013, 08:02:09 PM »

N4NYY , good point

"I had a similar problem. It was either the dreaded IC-151 chip, of the 2SC1972 drivers. Since I can repair things myself, purchased all the parts for $60, and replaced both."

When I replace IC-151 , I add the protection diodes which keeps it from blowing again. Here is a very clean website of the fix(s)

http://www.qsl.net/k5lxp/projects/746Pro/746ProNoTX.html

I totally forgot about mentioning the diodes. I DID add them to the 3 repairs I did. Here is a schematic of the mod: http://www.angelfire.com/biz2/mts/IC151Diodes.pdf

One thing about the drivers if this is a pro version. You will like not have the equipment to remove them. I would send them out to be repaired, like to N4ATS. The drivers on the non-pro (2SC1972) are way easier to remove, and can easily be done.

I also think there is a change in the idler current cal. I do not have the info, but it is available.


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N7TEE
Member

Posts: 39




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« Reply #7 on: December 12, 2013, 12:56:24 PM »

KB5NMA,

You have to remember that the IC-151 was destroyed by heat build up.  I have put a fan on the back of the radio and now can leave it on all day and no adverse effects.

Dave
N7TEE
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N4NYY
Member

Posts: 4742




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« Reply #8 on: December 15, 2013, 04:07:18 PM »

KB5NMA,

You have to remember that the IC-151 was destroyed by heat build up.  I have put a fan on the back of the radio and now can leave it on all day and no adverse effects.

Dave
N7TEE

From what I have read about IC-151, heat is not the cause of failure. And adding a heatsink was not been known to do much. The input protection diodes seem to be the trick.
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LA9XNA
Member

Posts: 104




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« Reply #9 on: December 18, 2013, 02:44:03 PM »

In standby mode , look at the current on your power supply , if it is above 3 amps , usually this is an indication this is an older unit and both 2SC3133 OR 2SC1972 transistors are shot. You can tell by the current draw that they are oscillating. If it is the newer 746 , there is one SMT FET right before the driver that goes south all the time.

Service manual is free to download all over the place.


On the older units there is an issue with the (pre) driver board it have two transistors Q1 and Q2 (2SK2975) these can become to hot/ damaged and give problems with low or non tx.
The issue is related to lack of proper heat sinking. I have pulled a few of these in the past and replaced the transistors. There is a fix on the internet somewhere in a document called SEN-015-03.
There is also an unofficial extra fix putting on a heat-sink on top of Q1 and Q2.
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N4NYY
Member

Posts: 4742




Ignore
« Reply #10 on: December 18, 2013, 04:09:46 PM »

In standby mode , look at the current on your power supply , if it is above 3 amps , usually this is an indication this is an older unit and both 2SC3133 OR 2SC1972 transistors are shot. You can tell by the current draw that they are oscillating. If it is the newer 746 , there is one SMT FET right before the driver that goes south all the time.

Service manual is free to download all over the place.


On the older units there is an issue with the (pre) driver board it have two transistors Q1 and Q2 (2SK2975) these can become to hot/ damaged and give problems with low or non tx.
The issue is related to lack of proper heat sinking. I have pulled a few of these in the past and replaced the transistors. There is a fix on the internet somewhere in a document called SEN-015-03.
There is also an unofficial extra fix putting on a heat-sink on top of Q1 and Q2.

Those 2SK2975 drivers are not removable with standard soldering equipment. They are a bitch! Highly advise a professional repair center for those. The 2SC1972 are easy to replace.
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LA9XNA
Member

Posts: 104




Ignore
« Reply #11 on: December 20, 2013, 04:25:05 PM »

In standby mode , look at the current on your power supply , if it is above 3 amps , usually this is an indication this is an older unit and both 2SC3133 OR 2SC1972 transistors are shot. You can tell by the current draw that they are oscillating. If it is the newer 746 , there is one SMT FET right before the driver that goes south all the time.

Service manual is free to download all over the place.


On the older units there is an issue with the (pre) driver board it have two transistors Q1 and Q2 (2SK2975) these can become to hot/ damaged and give problems with low or non tx.
The issue is related to lack of proper heat sinking. I have pulled a few of these in the past and replaced the transistors. There is a fix on the internet somewhere in a document called SEN-015-03.
There is also an unofficial extra fix putting on a heat-sink on top of Q1 and Q2.

Those 2SK2975 drivers are not removable with standard soldering equipment. They are a bitch! Highly advise a professional repair center for those. The 2SC1972 are easy to replace.
They are possible to remove with hot air soldering tool.
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N4NYY
Member

Posts: 4742




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« Reply #12 on: December 21, 2013, 04:29:08 AM »

Quote
They are possible to remove with hot air soldering tool.

Not sure it will work. The heatsink behind it totally sucks! Still, I did not have that tool.
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N4ATS
Member

Posts: 800




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« Reply #13 on: December 21, 2013, 08:02:48 AM »

If you have a good Metcal or Weller Ceramic system , just heat the center lead of the FET on the side away from the 3 pins, once it flows , go to the three pin side and simply touch the outer pins while lifting with an Exacto and it will pop off. Total time to remove this FET should take NO longer than 3 to 5 seconds. If it takes you longer , forget it and send it to someone.

As soon as I get back home (on travel) I will upload a video to Youtube showing you how to get them out safely.
« Last Edit: December 21, 2013, 08:06:28 AM by N4ATS » Logged
N4NYY
Member

Posts: 4742




Ignore
« Reply #14 on: December 21, 2013, 04:14:45 PM »

If you have a good Metcal or Weller Ceramic system , just heat the center lead of the FET on the side away from the 3 pins, once it flows , go to the three pin side and simply touch the outer pins while lifting with an Exacto and it will pop off. Total time to remove this FET should take NO longer than 3 to 5 seconds. If it takes you longer , forget it and send it to someone.

As soon as I get back home (on travel) I will upload a video to Youtube showing you how to get them out safely.

Fantastic !
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