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Author Topic: IC-746 PRO Reduced power can you point me in the right direction?  (Read 779 times)
KK6RHB
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Posts: 4




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« on: November 08, 2017, 06:30:52 PM »

OK I'll start by saying i'm sure I did all the wrong things the whole time. And I'm a new general.

IC-746 PRO Reduced power,
Overview: Purchased used radio. Worked fine for 3 weeks all bands all modes. good power all bands. good signal reports, US, Canada and Hawaii. took it camping the other weekend, got home and it worked for 10 min then output power dropped to 0-20% of normal. I was operating out of band, but with the balun in place I was only between 2-3:1. currently I have reduced or no xmit power on all bands. antennas and feed lines look good. tested all antennas with either a TS-520 or my yeasu mobile rig. all have good reports, and consistant SWR with previous readings.

Specifics: input power 13.5v RX 12.9 TX power supply, 12.6 rx 12.1 tx 12V batterey.
Output All output figures measured in RTTY 20 Meters 10W used to be well over 60Wmaybe even 80 can't remember 1.3SWR measured on external meter on 20Mhalf wave dipole 20' off the ground (I know it needs wo be higher) added current balun swr now down to 1.2 with the balun.

I have 3 antennas: 20M half wave tuned dipole 1:1 current balun ferrite core 1.2SWR on the bottom of the band where I chat.
10M full wave rough tuned, reading around 2.0-1.6 SWR coax aircore balun
2M Jpole air core balun 1.1-1.4 swr

I'm fairly decent at electrical, so the question is can I resolve this? I don't have an oscilloscope, can I diag this thing with a good fluke meter? I'd love some direction. The other posts I've read seem to point to the drivers or the drive tune circuit. But i'm still getting 10W out. And it seems that the tuner motors are tuning and turning. Ive tried all the settings I can think of. I figure I blew something.

How do I begin testing?

thanks for the direction
-KK6RHB
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KK6RHB
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Posts: 4




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« Reply #1 on: November 08, 2017, 07:07:26 PM »

I'm planning on buying a scope.... is this the time?
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K3LI
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Posts: 44




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« Reply #2 on: November 08, 2017, 07:13:21 PM »

A couple of things.  One "operating out of band"?  Unless it was MARS? Out of band ops are not legal per FCC.

The other, unless you know how and where to test and have the skills, don't even attempt it unless you  have a skilled person supervise.  Better to send the 746 off to the shop.
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KK6RHB
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« Reply #3 on: November 08, 2017, 07:51:12 PM »

Sorry, misunderstanding. I was operating on 7.173.00 LSB using my 20 Meter tuned antenna. I'm thinking the first step is a oscilloscope and a signal generator and digging into the service manual. I'm not afraid of electric or circuit boards.
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KC1BMD
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Posts: 608




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« Reply #4 on: November 09, 2017, 03:41:11 AM »

A couple of resources:

General resource with links:
https://www.dxzone.com/catalog/Radio_Equipment/HF_Transceivers/Icom_IC-746/
     e.g.
     Dead TX (IC151 RF Unit)
     http://www.rudius.net/oz2m/ic746pro/index.htm

Check 746Pro Yahoo Group:
https://groups.yahoo.com/neo/groups/icom746pro/info

Join "mods->dk" website and search articles (e.g. "Low tx power on all bands IC-746 PRO")
https://www.mods.dk/index.php?ModelId=291&RadioId=1-icom&ModelRec=ic-746pro
« Last Edit: November 09, 2017, 03:51:28 AM by KC1BMD » Logged
K5LXP
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« Reply #5 on: November 09, 2017, 03:43:11 AM »

I have reduced or no xmit power on all bands.

Did you test it into  a dummy load?  What are you using for a power meter?

Quote
I don't have an oscilloscope, can I diag this thing with a good fluke meter?

Probably not.  Maybe not even with a scope, but would depend on the scope and where troubleshooting led you. 

If the problem is as you describe, often it's the driver transistors.  There was an issue with inadequate heatsink compound leading to an overtemp failure and there was a TSB on biasing.  A scope or RF voltmeter would narrow that down.

Quote
it seems that the tuner motors are tuning and turning.  How do I begin testing?

All testing should be done into a dummy load with the tuner bypassed.

Mark K5LXP
Albuquerque, NM
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AC2EU
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« Reply #6 on: November 09, 2017, 06:15:04 AM »

You have high SWR 2-3:1 ? Perhaps its the just the radio's power rollback system doing it's job.
If you have excessively high SWR , modern radios will cut the power below the danger threshold.
Running into a dummy load as mentioned, will tell you if it's the radio or the antenna system.

If you are asking questions about fundamental things such as this, I don't think a scope will help you. In fact, you may make things worse.  You have to crawl before you walk!
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KC1BMD
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Posts: 608




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« Reply #7 on: November 09, 2017, 09:07:51 AM »

With "1.2SWR" -> power fold-back is not likely to be the issue. Try testing into a dummy load as AC2EU suggested.
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G8JNJ
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« Reply #8 on: November 09, 2017, 11:57:44 AM »

HI,

One other thing - you mentioned the DC supply volts.

Were these values measured at the DC source or actually at the radio ?

Lots of rigs have problems with fuses in the DC power leads going high resistance, so it's always worth checking first before investigating anything more complicated.

Regards,

Martin - G8JNJ

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K4HB
Member

Posts: 232




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« Reply #9 on: November 09, 2017, 05:55:56 PM »

Can't tell you what's wrong with your 746Pro, but those radios do have a problem with the driver board (B5703B) that will cause reduced or no power. Q1 and Q2 (2SK2975) are power FETs on the board that causes problems. I tried to get the boards from Icom, but was told they aren't available, and that no boards are available for that radio. Those FETs are SMD devices and the source solders directly to a heat sink. It's not an easy job to solder those little boogers in. But this may not be the problem with your radio, I'm just letting you know about a possibility. That's what caused the same problem with both my 746Pros.

I say take the advice of others who have posted and make sure everything is set right and you are using a dummy load. If it's definitely a problem with your radio, don't try it at home unless you are skilled in electronic repair. Being fairly decent at electrical and skilled with electronic devices are two different things. I'm in my seventies and have done electronic repair and soldered practically my whole life, but had to learn to solder SMD devices a couple of years ago to fix my 746Pros. Bought a hot air soldering station and a microscope for soldering SMDs.

You said you were operating on 7.173. That's out of band for General Class. For lower sideband, you need to be no lower than 3 KCs from your band edge of 7.175, which is 7.178. For USB, you should be 3 KCs from the upper edge. Here's the ARRL frequency band chart

http://www.arrl.org/graphical-frequency-allocations
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VE7REN
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Posts: 557




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« Reply #10 on: November 09, 2017, 07:50:36 PM »

http://wa3wsj.homestead.com/IC-746Pro_Sudeden__Death_Fixes.pdf

ive had 3 746. 2 did ther same thing. I ordered the parts,and did the repairs myself. not everly difficult. read into the ic151 issue. once repaired. very solid radio
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KK6RHB
Member

Posts: 4




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« Reply #11 on: November 09, 2017, 08:55:36 PM »

Thank you all. I've got a long way to go with my ham shills. Contacted the seller and there's a 30 day return if it failed. I'm inside that window. Looking for another one now..... and apparently a dummy load, and a rf volt meter. Thank you for all the advise. Nice to get kind advise. Not like most of the inter webs.
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K4HB
Member

Posts: 232




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« Reply #12 on: November 10, 2017, 03:38:57 AM »

Later production models of the 746Pro had factory upgrades to solve the IC151 issues. This started somewhere with serial numbers beginning with 0202 I believe.
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W5CBO
Member

Posts: 97




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« Reply #13 on: Yesterday at 11:40:25 AM »

Being a new General, you might want to check where you can and can't operate. You state you were on 7.173 and Generals are limited to 1.175 and above.

Be careful.
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