Allen,
1) I do wish you luck with this (and, I do have some wise troubleshooting tips below, please have a look!)....but, I'm wondering if by chance you're from Copenhagen? 'Cuz to quote from Hamlet, "Something is rotten in the state of Denmarkâ....
Now, maybe (like me) you've got a lot "going-on" and while intelligent / experienced, you're finding yourself feeling over-stressed / forgetful, etc.? If this is so, no worries here....take a breath, walk away from the radio, handle whatever you need to / what is more important....ham radio will still be there when you get back (just let us know that you're serious and that we haven't wasted our time).
2) But, when you write these things:
It has nothing to do with the coax or switch. It is in the radio itself and it isn't the SO-239 on the radio either.
If it was something that easy I wouldn't be posting here.
The problem with an ohm meter is it is designed for DC measurements and not RF. If you have a tuned circuit on the input than a tuned circuit can show a DC short but not be a short at a RF frequency. The 50 ohms at the antenna port is a RF impedance and not a DC measurement.
The receiver is a Drake SPR-4
<snip>
I have lots of test equipment and service ham gear on a regular basis.
<snip>
Other receivers work fine with the same connection etc.
<snip>
Access to make measurements is very difficult in the front end stages of this receiver
<snip>
I can't safely get a probe in there safely. I suppose I will have to do some disassembly and see what I can find. I do have another SPR-4 and perhaps I can do some a/b comparisons for resistances or voltages that may help. It does act like the center pin is shorting to ground though...
I have two of these receivers and so A/B comparisons are easy to make and when one is 40 db down from the other with the exact same test setup.
I don't know that I mentioned a SO-239 or PL-259 but if I did that isn't accurate but doesn't change
the issue. It does have the stock RCA connector.
These make me wonder if we are all just having our legs pulled here? I mean, the Drake SPR-4 manual is one of the most detailed that I've seen, in regards to circuit description, voltages, testing, alignment, etc.....and it is darn easy to "access" most of the components...
And, at first we are not told what radio it is....but, are told the radio has an SO-239....then told the radio is a Drake SPR-4....then, after I mention that the SPR-4 didn't come with a SO-239, we are then told there are now two Drake SPR-4's and it is implied that they both now have RCA jacks, or at least one has an RCA antenna connector? Or is it both of them?
Whew, I'm having trouble keeping all of this straight...Hi hi

And, if someone really does have
"lots of test equipment and service ham gear on a regular basis", they should know that there is more to the words "ohm meter" when written here, yes? I mean, we include VTVM, DMM, a scope, etc. when we say "looks like a job for an ohm meter"....(and, if someone does service ham gear on a regular basis, we should also assume they had sig gen, RF probe, freq counter, power meters, cap meter, etc. etc..)
But, most importantly they would have most probably already looked at the manual AND inside the rig, to see what's what before asking here-abouts, yes?
And, a quick look should allow an inspection of the inside of the female RCA jack on the rear panel of the SPR-4, to verify its mechanical and electrical condition and see what (if anything) changes as an RCA male plug is inserted....also it should be rather easy-peasy to inject a signal into the receiver input there inside the SPR-4, bypassing the RCA jack entirely? After verifying the RCA plugs/adapters, etc., those things (look at the manual, look at the inside of the SPR-4 chassis, and injecting a signal in / bypassing the RCA jack) would be the first steps I'd take.
3) Now again, while I do find this to be "fishy", it might be a case of being a bit distracted...so, if that is the case, here are a couple other tips:
a) Many times the center pin of a RCA plug is much longer than needed, and/or the "shell" is fairly short in comparison....and, this is especially true of adapters, like "RCA to BNC" and "RCA to UHF", etc...and, as such it is easy to push the RCA plug into the mating jack so far that you are unfortunately causing a short (either of the RCA jack itself, or other parts/components nearby).....please note that if this is the case, this is not about the "shell" making contact and causing the problem, but the center pin being too long and causing the problem, the fact that the "shell" makes contact at this same time may also allow this antenna connection to "short out", but is not the actual proximate cause....{also be aware that this is not necessarily due to a "defective" RCA jack on the receiver, but rather the "male" plug or adapter being used....but, if you do find this is your main issue....you may find a modification to the receiver's RCA jack, or replacing it with an SO-239, to be a better permanent solution?}
b) Your many postings here with only bits and pieces of info in them, are a bit confusing to put together....so, I'm not clear if BOTH of your SPR-4's have RCA jacks, or only one? Also, does the coax you're using to plug into your SPR-4's actually have an RCA plug soldered onto it? Or are you using an adapter of some sort? Are you using the same adapter for both (all) receivers? Does whatever RCA plug you're using have a center pin that extends too far?
Allen, I do wish you well in all of this....and, I do hope I've helped some? (but, I do still think this is all just you pulling our legs.)
73,
John, KA4WJA