Spectra: RF Connector Swap

[Splat]

Had a bad mini-uhf connector on my Spectra. I dissembled the PA board and cut the wire at the back of the mini uhf connector. I'm probably having a brain fart here but I tested for short/continuity from the ground braiding layer of the cable to its center wire and it's shorting. When I throw a meter on my other antenna ports on the other radios I don't get continuity. What am I missing here? Is the Specrtra RF wire going from the PA board to the RF connector supposed to be shorted? Methinks no but I think I'm in slow mode today. Thanks.

PS, gonna put a TNC in there unless someone's got a better idea. Not a fan of the mini-uhf's.

[Tom in D.C.]

Most final amplifier tank circuits go directly to ground through the coil. At RF it's a completely
different story. Check basic transmitter amplifier diagrams and you'll see how it's laid out.

[Will]

You need the replacement chassis connector and cable assembly. The coax solders to the PA board at the antenna switch.

[Splat]

Thanks guys. My revolt against the mini uhf is complete. I replaced it with a bulkhead SO239 and a new run of RG174 to the board. It works and all is well.

[GlennD]

Blah, A mini-uhf is a good RF connector at least through 800Mhz. A UHF connector was UHF when 30Mhz was considered UHF. It barely works at VHF and is bad ay UHF. I realize hams worship the connector but it is a very bad design!

[WB6DGN]

A mini-uhf is a good RF connector at least through 800Mhz.

I agree, ELECTRICALLY it is a good connector; I believe its spec'd to about 2 GHz (It really surprised me when I read that). However, MECHANICALLY, well, that's another story. It seems the newer ones are better than the first generation, though. I remember being out in the field and going through every antenna cable I had in the truck to find one that would retain the threaded collar on the connector body. Like most techs., I suspect, I developed a few additions to my vocabulary over that connector. NOW, the connector seems to have improved but, sadly, my vocabulary remains the same!
Tom

[Tom in D.C.]

FWIW, some of you may recall that years ago Collins Radio used an RCA jack for RF output on their HF transceivers.
The story is that when questioned about the use of that connector Collins said that they'd found that it worked
fine (whatever that means) all the way up well into the UHF frequencies.

[Bill_G]

Thanks guys. My revolt against the mini uhf is complete. I replaced it with a bulkhead SO239 and a new run of RG174 to the board. It works and all is well.

How did you do that? There is no physical space to mount it. It must be on a short stinger hanging out the back of the radio.

[Splat]

How did you do that? There is no physical space to mount it. It must be on a short stinger hanging out the back of the radio.

Nope, no stinger. You're thinking of the panel mount SO239 with the square flange and 4 holes. I used a bulkhead connector. I did have to enlarge the hole in the Spectra. Here's the connector. I no longer have the proper crimp tool for mini-uhf connectors and all my antennas have PL259's on them these days.

[Bill_G]

You got that to fit *and* you can tighten the sleeve? Good job.

[Splat]

You got that to fit *and* you can tighten the sleeve? Good job.

Thanks. Honestly, the swap was not hard at all. It looks like it's taking me longer to find the damn short for the antenna on the pcb!

[racerman1cars]

Hi, Friend of mine mounts a flange mount so239 on the side where the dead space is.
Steve

[Tom in D.C.]

You got that to fit *and* you can tighten the sleeve? Good job.

Thanks. Honestly, the swap was not hard at all. It looks like it's taking me longer to find the damn short for the antenna on the pcb!

Splat,

Did you read my post to you about DC continuity on an RF tank circuit? If you did, did you understand it? I don't
think you have a "short" on your antenna connection to ground.

Regards,

Tom, W2NJS

[Splat]

Did you read my post to you about DC continuity on an RF tank circuit? If you did, did you understand it? I don't
think you have a "short" on your antenna connection to ground.

I appreciate your help, Tom. It's my understanding that there are some transceivers that will ground the antenna on Rx, and that's what I thought you meant. The RF solder pads are clean. I just got done with checking them all out again. What I don't understand is the radio worked fine before I did the connector change. When I removed the RF's center conductor off the board the radio booted up fine. Put it back onto the board and it fails 01/90. That's with the other end removed from the antenna jack and the center and braid are not crushed/touching. I just started going around checking the other components and am taking a dinner break right now.

[Jim202]

Did you read my post to you about DC continuity on an RF tank circuit? If you did, did you understand it? I don't
think you have a "short" on your antenna connection to ground.

I appreciate your help, Tom. It's my understanding that there are some transceivers that will ground the antenna on Rx, and that's what I thought you meant. The RF solder pads are clean. I just got done with checking them all out again. What I don't understand is the radio worked fine before I did the connector change. When I removed the RF's center conductor off the board the radio booted up fine. Put it back onto the board and it fails 01/90. That's with the other end removed from the antenna jack and the center and braid are not crushed/touching. I just started going around checking the other components and am taking a dinner break right now.

The 01/90 is a serial bus failure. Should have nothing to do with the PA section.

The reason you see a short on the antenna is that in the PA filter section there is a coil from the center coax conductor to ground. It is L3914 on the VHF 25 watt PA. I believe the 100 watt PA is the same for what it's worth.

Jim

[Splat]

The 01/90 is a serial bus failure. Should have nothing to do with the PA section.

I know, but if something's shorted then that might be the general hardware fail that is also attributed to that 01/90.

The reason you see a short on the antenna is that in the PA filter section there is a coil from the center coax conductor to ground. It is L3914 on the VHF 25 watt PA. I believe the 100 watt PA is the same for what it's worth.

Got it. Thanks Jim for clarifying that. I saw the coil and thought that could be it but I'm usually wrong about things.