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Author Topic: IC 746 no output on HF  (Read 870 times)

KG8JK

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IC 746 no output on HF
« on: May 24, 2022, 08:38:03 AM »

Hello,
I have a buddy who owns an IC 746 that puts out full power on 2 meters but has no output on HF. It receives fine on HF but no transmit. Any ideas?

Thanks and 73
KG8JK
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N4ATS

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Re: IC 746 no output on HF
« Reply #1 on: May 24, 2022, 09:01:42 AM »

There are three different versions of the 746 , I need to know what drivers are in his so I can walk you thru.
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KG8JK

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Re: IC 746 no output on HF
« Reply #2 on: May 24, 2022, 09:06:36 AM »

It is the 746 pro. Thanks for responding! He is hoping he can fix it otherwise will probably just use it as an all mode 2m rig.
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N4ATS

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Re: IC 746 no output on HF
« Reply #3 on: May 24, 2022, 10:37:14 AM »

He needs to pop the top and tell me what DRIVERS are installed , there are 3 board versions of the series of 746's (not talking Pro ProII or ProIII) but the actual drivers they used drove different board layouts...

N4ATS
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KG8JK

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Re: IC 746 no output on HF
« Reply #4 on: May 24, 2022, 10:46:11 AM »

I will let him know. I emailed you on your qrz.com address. I will put him in touch with you.Thanks for the help!
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K1VSK

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Re: IC 746 no output on HF
« Reply #5 on: May 24, 2022, 11:54:53 AM »

Hello,
I have a buddy who owns an IC 746 that puts out full power on 2 meters but has no output on HF. It receives fine on HF but no transmit. Any ideas?

Thanks and 73
KG8JK

Can we rule out that the radio is operating FM on 2 M and you didn’t simply key the mic on HF SSB?
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LA9XNA

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Re: IC 746 no output on HF
« Reply #6 on: May 24, 2022, 01:32:32 PM »

I did a repair on one of these many years ago.
As i remember the issue was that there where not enough cooling on the predrivers for the PA.
I have seen a mod somewhere that improves the coling of the drive transistors.

Repair procedure.
https://www.rudius.net/oz2m/ic746pro/ic746pro_biasing.pdf
https://docsbay.net/icom-ic-746-pro-modifications
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W9MT

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Re: IC 746 no output on HF
« Reply #7 on: September 20, 2022, 11:43:48 PM »

This doesn't sound like bad 2SK2975 transistors in the 746pro driver, but the dreaded IC151 IC has likely bitten the dust.

There was a rash of problems with these SOIC devices in the lower serial numbers of SOME of the IC-746pro's. I say some, because no all 746pro's will experience this problem, no matter what their S/N.

Icom attributed this to a static charge coming into the radio through the antenna connector, and came up with a fix that placed pin diodes at strategic spots in the board containing the transmit driver circuitry.

If you look at the reviews for the IC-746pro here on eHam.net, you will see a lot of loud complaining about this problem.

There's nothing wrong with the IC151 device's design. Icom runs it constantly when the radio is powered up and it does dissipate 250mW continuously. Some people switch the 8VDC going to the device to only be there when the PTT is keyed. I've owned a 746pro since 2005 (early S/N unit) and have never seen this problem in my radio. But I did buy 10 pieces of IC151 that are drop in for the board's footprint of solder tabs to keep the old girl going. So far I haven't needed them.
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K5LXP

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Re: IC 746 no output on HF
« Reply #8 on: September 21, 2022, 07:24:10 AM »

If TX is dead I'm not really sure it matters what version it is, you're opening it up and doing some surgery.  One can assess and effect repairs and updates to drivers and PA depending on what is discovered.

Mark K5LXP
Albuquerque, NM
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N4ATS

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Re: IC 746 no output on HF
« Reply #9 on: September 21, 2022, 07:57:48 AM »

I was trying to help him zoom in... if IC151 is the issue , its pretty easy to change, however if he has an RF Signal Generator , inject it after IC151 thru the small gray RF cable and see if the PA comes up prior to changing it out. With a generator , you can tell in seconds where the issue is...
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K5LXP

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Re: IC 746 no output on HF
« Reply #10 on: September 21, 2022, 09:42:03 AM »

Same with a basic o-scope, looking at the signal levels in and out of IC-151.  All versions have IC-151. 

Mark B.
Albuquerque, NM
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W9MT

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Re: IC 746 no output on HF
« Reply #11 on: September 25, 2022, 09:42:36 AM »

IC-151 is positioned under a shield, as I recall (it's been awhile since I opened my radio). In such case, one has to find a runner or part interconnection node in order to probe the inputs and output for signal in/bigger signal out. Since this IC works for all HF bands, I'd recommend doing the check on 160 or 80 to enable seeing a useful signal image on the o'scope instead of just a blur.

While being "100% duty cycle" powered up, dissipating 250mA will make IC-151 a bit warm when using the "fat finger test", but this is not outside of the device's operating parameters. As I said in my other reply, one can butcher the wiring on the pc board to only have the 8VDC line be a switched input to the device, only present when PTT is depressed/enabled. I don't believe this is going to have an increased "ruggedness payback", nor will trying to use a heatsink glued to the top of the device's PLASTIC case. That's an exercise in futility. Some hams will micro-solder "heat radiating wires to the IC's ground pins in attempting heat sinking. The small SOIC ground tabs of IC-151 don't allow for much radiant heat to escape. (Kinda like trying to run 20A through a #20 wire and removing the wire's getting blistering hot with a heat sink...another exercise in futility.)

Since Icom itself points a finger to static conduction onto the runners of IC-151 as the source of its early demise, one can do the PIN diode additions to the pads that already exist on the home board of IC-151. This begs the question of why they weren't there in the first place if the pads for them were!!!

There is another way to prevent this static invasion syndrome. Use an antenna whose feedpoint is already at DC ground to not let antenna static build up when storms are brewing. A good balun across the feedpoint of a dipole or beam will do this, as well as RF matched feed systems like a hairpin match or Gamma match. My Cushcraft R5 vertical uses a DC ground matching box at its feedpoint, too.

When I'm deploying a new dipole or other such doublet, I use an old trick...  I solder a 2W non-inductive 100K carbon comp resistor across the feedpoint / center insulator of the new antenna if I'm not employing a balun. I cover the resistor in heatshrink tubing to protect it from long term UV. It is simply soldered to each side of the center insulator, one side to the center conductor of the coax and the other to the shield side.

100K does nothing to your match and is "in-parallel" to the feedpoint. It also uses a tiny amount of your RF power applied to the feedpoint. The great thing it does, though, is two-fold. First, when electrical storms are brewing 100K equalizes the Voltage potential between both sides of the dipole connection on a continuous basis. This keeps the Voltage applied to your radio's coax input to a low level, much less than what it would take to blow a P-N junction inside of a semiconductor. This Voltage equalization across the antenna's two halves also reduces the propensity of a lightning strike. You cannot eliminate every strike, but a vastly reduced DC Voltage impressed by Mother Nature across your antenna's feedpoint reduces its "attractiveness" (pun intended) to violent static discharge to a cloud.

The 2nd benefit is that you have a "quick and dirty way" to do a crude health check of your coaxial cable's run to the feedpoint of your antenna. Put a VOM in DC resistance mode across your PL-259 in the shack. If it measures near 100K, you're good. If you get an open, you have a break somewhere in your coax run. If you have a measurement of zero Ohms or just something way lower than 100k Ohms, say up to 10k or so, you have a deterioration or some kind of shorting problem.

You get all of this plus IC-151 static protection at the same time.   --30--
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N2SLO

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Re: IC 746 no output on HF
« Reply #12 on: February 12, 2023, 12:18:52 PM »

Hello,
I have a buddy who owns an IC 746 that puts out full power on 2 meters but has no output on HF. It receives fine on HF but no transmit. Any ideas?

Thanks and 73
KG8JK

OK-- before I get this radio out for a fix, my regular 746 receives and transmits fine on HF, but on 2 meters, always transmits fine but after I power on for about 15 minutes I lose my receive. Only on 2M -- any thoughts?
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