[ICOM] 751A Main RF Board

Thu Oct 23 13:12:34 EDT 2008

Eddy,

PBT means Pass Band Tuning. Its a nice method of applying a variable 
bandpass filter to the incoming receiver signal. Its done in the RF stages.

On the now older removed Main Board with J-17 removed the PBT function 
was restored, the IF shift control became the PBT control. Icom in later 
years restored the PBT in the IC-751A. During the unsettled patent  
issue all PBT was removed from Icom products except for the IC-735 and 
the IC-781. The IC-781 was Icom's own PBT design. Some say the patent 
issue was with the Collins Radio company, I never actually found out though.

I have never tried a different main board in the NON PBT 751A versions 
but I think it should work.
You might give a local repair person Scott Malcom   mts at trip.net   an 
email.  He is very responsive and you can tell him I asked you to write 
if you want to.
I worked with Scott for several years and he is knowledgeable on the 
IC-751A and may have actually swapped main units like your situation.
Kuni Okuma is also a good tech,  w7jv at aol.com   You might email him too 
and see if he has done a board swap.

Kuni,  Scott and I worked for Icom America, Inc. I was in the Technical 
Customer Service group and  Kuni and Scott were bench techs. They may 
tell you I always grabbed the hot end of the soldering iron ;)  I had 
some limited time on the bench too but we almost never swapped entire 
circuit boards, they went to componet level repairs there.

Gary Fiber K8IZ

EP Swynar wrote:
> Hi Gary,
>
> Bingo...! Yes...!
>
> There is, indeed, a "J17" marked on the pc board, right next to the pin
> terminal...
>
> What does it all mean, then...? Can I safely install the board into my
> existing rig --- minus the black wire / J17 connection --- and expect it to
> work OK...?
>
> I assume PBT refers to "push button tuning"...? If I hook-up the board LESS
> the wire, will some button on the face of the rig allow me this option, vs.
> my simply spinning the dial...?
>
> ...And how'd you get so smart about these things, anyway, Gary...?! Hi Hi.
> Seriously, it's great to have the knowledge & experience of guys like you
> available to aid & abet dunderheads like me...I sincerely do appreciate it.
>
> ~73!~ Eddy VE3CUI - VE3XZ
>
> ****************************************
>
>
> ----- Original Message ----- 
> From: "Gary P. Fiber" <gfiber at comcast.net>
> To: "ICOM Reflector" <icom at mailman.qth.net>
> Sent: Thursday, October 23, 2008 12:04 PM
> Subject: Re: [ICOM] 751A Main RF Board
>
>
>   
>> On the original board was that post labeled J-17? If do that was the
>> defeat for the PBT. You may have a board that PBT is enabled on where
>> your original one PBT was disabled from the factory due to a patent
>> infringement that was settled years ago now.
>>
>> Gary K8IZ
>>
>> EP Swynar wrote:
>>     
>>> Hi Guys,
>>>
>>> Here's a good one...
>>>
>>> A year ago, my venerable back-up rig --- the Icom 751A --- suddenly went
>>>       
> belly-up in the transmitting department. A tekky friend of mine, who is in
> the business of consumer electronics repairs, had a good long look at it, &
> determined that a 9-MHz crystal-controlled stage on the main RF circuit
> board wouldn't oscillate in transmit...no idea why, either (all the parts,
> voltages, etc. therein apparently checked A-OK).
>   
>>> Anyway, last summer I bought a complete 751A board (less filters) from a
>>>       
> fellow who was parting-out his rig. Great! I'll just swap out the "bad"
> board, & replace it with the new one...
>   
>>> Well, to-day I proceeded to carefully dismantle my board from the rig,
>>>       
> recording which connector goes where, etc. etc. etc. ... and all went well,
> too, until the end, when I finished-up with a black wire that pulls out of a
> post at the juncture of diodes D-161 & D162 in my 751A...
>   
>>> The problem is, there is no such post on the board that I
>>>       
> purchased...and no diodes D161 or D162, either.
>   
>>> I examined my 751A service manual schematics, circuit board outlines,
>>>       
> and parts list --- and learned, to my complete & utter dismay, that these
> diodes aren't listed in the manual, either!!!
>   
>>> I must have a newer-vintage transceiver which was manufactured AFTER the
>>>       
> manual was printed, or an older transceiver & a new manual --- with a brief
> history of any & all engineering revisions made to the rig being NOT duly
> noted therein...(sigh!).
>   
>>> The black wire goes from the afforementioned post, and plugs into the
>>>       
> Matrix board.
>   
>>> I am debating whether, or not, I should simply proceed with the swap &
>>>       
> tape the wire off to one side (the same way automobile manufacturers do with
> redundant underhood wires in the face of upgrades) & hope for the best, or
> do as my friend did last summer, & simply part this sucker out to the world,
> piece by miserable piece...
>   
>>> ~73!~ Eddy VE3CUI - VE3XZ
>>>
>>> PS: How come I never have this problem fixing-up my old Drake &
>>>       
> Hallicrafters rigs...?!
>   
>>> ----
>>> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
>>> Icom Users Net: Sundays, 1700Z, 14.316 MHz
>>> Icom FAQ: http://www.qsl.net/icom/
>>>
>>>
>>>       
>> ----
>> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
>> Icom Users Net: Sundays, 1700Z, 14.316 MHz
>> Icom FAQ: http://www.qsl.net/icom/
>>     
>
> ----
> Your Moderator: Dick Flanagan K7VC: icom-owner at mailman.qth.net
> Icom Users Net: Sundays, 1700Z, 14.316 MHz
> Icom FAQ: http://www.qsl.net/icom/
>
>   