[MRCA] VRC-12 update

[Ray Fantini]

An update on the RT-246/VRC-12 project, after much downloading I now have the complete set of manuals for the entire family of the VRC-12 series, interesting to find several sites that will allow you to download but don't include the foldouts and being that the foldouts have all the block diagrams and schematics the book is just about useless without them.  Found that my issues with low sensitivity were due to a switch being out of alignment on one of the two RF amplifiers in the front end of the receiver. Now have plenty of sensitivity with the squelch being set to open at 0.33 microvolts and a relatively quiet signal around 0.6 micro volts. Not bad for a transceiver that's over forty years old and still uses tubes in its front end. The squelch problem turned out to be a bad transistor in the A5200A can, now know that if you have cans that end in "A" they are the newer silicone cans and the non A are older germanium transistors and I have the newer A series in my audio squelch section. The little bit of the manual that I have digested says that the silicone cans were not intended to be repaired and were supposed to be thrown away but they did include schematics of them so repairing one was no big deal. I had a bad transistor in the squelch relay driver circuit, they used a 2N2222 to pull the relay in and although it's a small relay that looked kind of week to me so I used a slightly larger transistor that has a higher current rating and its working well now. Having gone from never having any of the VRC-12 stuff open a week ago to now having  striped down all the cards and sub-assemblies and doing lots of work on this one have to say it's not a bad radio for its time. Lots of mechanics and motors and switches to clean, lubricate and aligned but when it's finished not that bad at all of a level of performance for something that was produced so long ago. Over the holiday installed the mount, antenna assembly and all that stuff on the M151A1 so now have the AN/GRC-106 on the driver's side and the RT-246 and all its associated stuff on the passenger side. Had thought of trying to wedge in a VIC-1 intercom system but cannot find any open space in the back to mount it. The next question is what's next for the M151 project, do have to admit that I am happier with the VRC-12 family of components installed on the mutt then with the newer stuff that I was thinking of installing and its turning into a theme of trying to have equipment form the seventies and eighties. Thanks again for everyone who provided links and other assistance.

Ray F/KA3EKH



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