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Author Topic: Help Identify Collins 75s-3c Mystery Modification  (Read 1009 times)
VE3XKD
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Posts: 51




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« on: August 30, 2009, 02:46:16 PM »

Hi

I recently bought a very dirty non-working Collins 75S 3C at an estate sale, Round Emblum, serial number in 30k range, and am in the process of cleaning it up. I noticed a rather unusual circuit board, 2 inches by 4 inches mounted below underneath the chassis. It has 2 coaxes going to VFO output phono jack, L3 and 1 going to a small pot mounted in the pre-selector bar assembly that moves the preselector coils up and down. There are 2 ICs on the board, an M 7720 and M 7707.  there is one part number on the board, 85-1412-3.

Can anyone help identify what this is?
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KA5N
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Posts: 4380




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« Reply #1 on: August 30, 2009, 04:35:59 PM »

A wild guess would be circuitry to allow the 75S-3C to operate transceiver fashion with a non-Collins transmitter.  The only reference I could find for the two IC\'s alleged them to be DEC "flip-chips".
The mod is probably useless unless you have the transmitter the mod was designed for.  That the PCB has a part number might mean this was a manufactured item rather than a homebrew mod.
Or who knows.
Allen
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WA9FZB
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Posts: 171




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« Reply #2 on: August 31, 2009, 09:08:34 AM »

I may be way off target here, but if you Google the board\'s part number, it brings up references to a Heath noise blanker.  Could someone have fitted the blanker to the receiver?
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KA5N
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« Reply #3 on: August 31, 2009, 10:41:21 AM »

I think WB9FBZ has hit the mark.  I found  a Google reference to such a mod for "older 75S Collins" which gave a magazine year of 1979 for the article, but for some reason didn\'t include the name of the mag.  Probably 73 Magazine as they had dozens of construction and mod articles in that era.
The SBA-104-1 noise blanker was designed as an acessory for the SB-104 transceiver, the first solidstate transceiver Heath built.
Allen
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WA6HDZ
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Posts: 52




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« Reply #4 on: August 31, 2009, 11:48:31 AM »

The numbers on the IC\'s may be date codes for when they were manufactured- for example "7720" meaning 20th week of 1977.  Are there any other numbers there?

That time frame looks like the SB104 era.  Heath used private numbers on a lot of IC\'s, typically "442-" or "443-" followed by a one to four digit number.  There\'s a cross reference at http://www.d8apro.com/HCross.htm
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VE3XKD
Member

Posts: 51




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« Reply #5 on: August 31, 2009, 11:53:35 AM »

Hi

Thanks for all of the information and research everyone has done. It seems that it is a Heathkit Noise Blanker, and at one time it was a mod for the Collins receiver, though not a popular mod, and there is no documentation of the mod that I can find on the web.

I hope to turn the radio on in after a few more days of cleaning, and will see what the mod does.
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WA4PRR
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Posts: 1




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« Reply #6 on: September 04, 2009, 09:55:57 AM »

I believe this mod was described in QST quite a few years ago.  I got a noise blanker board to add to my SB303, but never got around to it.  Good Luck.

Ron  WA4PRR
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KA5N
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« Reply #7 on: September 04, 2009, 10:36:27 AM »

The mod was described in QST Hints and Kinks in the
December 1979 issue.  The article can be downloaded for members only in the publications search area of the ARRL web site.  
Shows how to do the mod but doesn't say much about how well it works.  
Allen
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W5DC
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« Reply #8 on: September 04, 2009, 12:44:47 PM »

I had one in a 75S-1 and also tried it in  Heathkit receiver and it didn't do anything for my power line noise.
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W6OU
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Posts: 191




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« Reply #9 on: September 04, 2009, 02:20:20 PM »

There is a YouTube video showing the effectiveness of the Heathkit noise blanker installed in an old SB-300 receiver at:

http://www.youtube.com/watch?v=CDX_O0ZyB-A
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