[Collins] Help: Collins 75A1 Wiring don't Match Schematic

[Dr. Gerald N. Johnson]

There were a number of changes in the first thousand or so 75A-1. So 
many that made the revised receiver work better and that made the 
dealers charge a lower price for low serial numbers and a higher price 
for later factory revised radios. I don't know how well the revisions 
have been documented and whether the first edition of the manual matched 
the earliest radios. There might actually be some service letters about 
the changes so sellrs and owners of early radios could have made the 
modifications. I'd go look at the data collection on the Collins 
Collector's Association data site.

The pricing annoyed Art so after a year or two of S-line production he 
decreed that S-line serial numbers shall be issued in random order which 
they were. However model changes and trim changes showed differences but 
the S-line radios didn't have the history of so many effective revisions 
as the 75A-1.

73, Jerry, K0CQ, Technical Adviser to the Collins Radio Association.

On 7/19/2018 4:25 PM, forum4ll at verizon.net wrote:
>
> I've been repairing a Collins 75A1 for my boss this past month. It has a low serial number in the 400s. There are some serious discrepancies in the wiring when compared to the schematic (3rd edition Collins Instruction Book) and another 75A1 with a higher serial number. Here's a list so far:
> C87 minus side wiring is not attached to center wiper on MAN/AVC/CW switch
> MAN/AVC/CW switch has a wiper to wiper instead of a MAN to AVC jumper
> Extra wire coming from the wiper on Rf Gain pot. Has three instead of two wire connections
> Ouch, standby relay terminal has B+ connected from R43/R49 node instead of being grounded. Shocking!
>
> Missing B+ wire from R43/R49 to R48/C81C
> R51 is connected to #8 pin and ground instead of pins #8 and #6 where the latter is terminated at standby relay
> The Off/Fil. On/B+ On does not have one of its center wiper connected to ground. Have not finished its trace but there is impedance.
>
> Was there a revision to the wiring on the 75A1? Is it reflected in the 1st and 2nd edition of the Instruction Book? Some of the soldering looks fairly original like the wiring of the Off/Fil. On/B+ On and MAN/AVC/CW switches. Then others were definitely done by a hobbyist. Any suggestions are appreciated.
>
>
> Thanks,
>
>
> Lyman
>
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