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Thought I'd tackle this one myself but:
Have gone through this Ebay win and fixed the display, then the IC151 plus the Q25 and D22 on the control board.
Rec good all bands, dial lit, all appears ok.
After all above replaced:
With rf power pwr set at min; 40mtrs, enable transmit, power is output, tuner starts, PO, SWR, ALC bars in motion, then all three trail off within seconds. Scoping 1678 shows input has rf on transmit, output dead. Replaced 1678 again same behavior. Removed output cable to PA, replaced 1678 again no problem on 1678 output for 5 minutes. Reinstalled cable to PA, 1678 gone in seconds.
HRX line, Q25 and D22 all correctly operating per schematic. Thought 0.1 coupling cap leaking but the 1678 output to the PA has 2 sets of 0.1 in the path.; unlikely.
Subbed a 75ohm for the 1678's 10ohm 5v supply limiter and replaced the 1678 again still blew it. No heat no sparks no burning chip just output and 2 seconds later no output.
What ya think?
Thanks
Jack KG6INX

Hello Jack,

This sounds like an interesting and frustrating problem the way it's happening. Unfortunately I'm sitting here without a schematic of the IC-746 but from what your describing about this one IC 151 contining to blow under an unusual load, makes me wonder what else in the transmit circuit could be presenting a surge / spike back to that one IC.

I hope you've installed a good grade IC socket into the PC board for as many times as you've replaced IC151. There are some questions I have about your general description that leaves me wonding. For instance, "power is output," what did you mean by that? You've stated that there are 2 sets of .1uFd coupling capacitors in the PA circuit but you doubt they're bad. Jack we're only talking about nickel / dime items opposed to the ICs that are blowing up. Are there any Silver Mica capacitors in the Power Amplifier circuitry because these will only fail under a voltage. They are generally used as RF Bypasses along side the RF transistors or on critical DC voltage lines in the PA circuitry.

What does the output of IC151 feed into the RF PA circuitry? DC? Or does it feed the internal antenna tuner circuitry? Can the transceiver transmit into a known good 50 OHM Dummy Load without using the internal antenna coupler?
Have you substituted an external DC power source in place of the output of IC151 and watched the DC current draw in the Tune mode? How does this transceiver act on 20 meters rather than 40 meters? Can you disconnect the RF Drive to the Finals and measure that Power Output and while your at it, vary the Drive level control to see if the P.O. varies up and down?

Dan

Hello Jack,

I spent a few monutes on Google researching ICOM 1678g IC and the first return provided the technical information that you were looking to find on your IC-746. It seems to be an ICOM inheirent design problem with a relatively easy fix.

http://64.233.167.104/search?q=cache:XZ ... =clnk&cd=1

73,

Dan

Ghostwriter wrote:Hello Jack,

I spent a few monutes on Google researching ICOM 1678g IC and the first return provided the technical information that you were looking to find on your IC-746. It seems to be an ICOM inheirent design problem with a relatively easy fix.

http://64.233.167.104/search?q=cache:XZ ... =clnk&cd=1

73,

Dan

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? ICOM I.C. ?

I'll not buy radios with proprietary , expensive
, inferior I.C.s , that "hook" me .

The PS125 has no protection and uses a prop' hybrid
I.C. . and "hype" MOSFETs .

----------------------

1) MOSFETS dont do high voltage as well as bipolars .

2) Bipolar's are better , run very cool and are fault tolerant .
I use NO fans .
3) We use only single bipolars , not the more complex
push pull circuitry . Thus no need for LM494 .

4) Class D ( saturating core) allows parallel bipolars without
ballasting/balancing .

My hobby is ARM cpu , as in NDS Lite game box .
i also have many Eval Boards using the ARM CPU .
Im working on a free Ham s/w for the ARM .
It will obsolete all the ARM s/w , because its "built-in"

Because it will be so easy to program , i guess
it will be used to run radios , and repeaters and Packet
radio and even replace the IBM PC .

It wont require any loadable s/w , everything needed
will be "built-in" , and it will not use a computer "language"
nor ASCII text , nor 101 keyboards , nor large HDD's nor

It will be free and open to all . It wont be left brained ,
so there will be no need for a operators manual .
You will learn it by inputting thru a track ball and
"softkeys" , then see only images ( NO ! TEXT ! ) .
It then interogates what you want to do , but with
images and voice clips , NOT TEXT !
S/W is my hobby . KC7CC .