Hello to all,
1) Of course, my first thoughts here was as others mentioned (a shorted piece of coax, shorted connector, blown balun, bad antenna switch, blown grounding block / lightning arrestor, etc. etc.)....but, then Allen actually tells us the radio in question is an old Drake SPR-4....and, my mind wonders who has worked on this radio before, 'cuz an SO-239 isn't supposed to be there.

Without getting into the middle of the "argument"....has it not occurred to anyone that the Drake SPR-4 didn't come from the boys in Miamisburg with an SO-239?
It came from the factory with what was de rigueur in those days, an RCA jack / phono jack for the "antenna connector".


So, while someone may have done a good job modifying it, by professionally changing out the RCA jack with a SO-239....it is also possible there is an issue with this connector....
And, of course it is also possible that something else is amiss inside this receiver, as it is obvious that someone has done at least one modification to it. And, there could be other mods done, other "repairs" attempted, etc., and/or plenty of things that need looking at and measuring.
Looks like a job for an ohm meter, to me too!

And, Allen forgive me, but if I came up with this in less than 10 seconds, I assumed that you already realized the SO-239 is not "factory"....but, now I'm wondering?
The receiver is a Drake SPR-4 which is a solid state older receiver.
I have lots of test equipment and service ham gear on a regular basis. It is a strange problem that sounds very simple and I mistakenly thought it would be an easy fix. I believe I have covered the simple possibilities already.
2) Then, as I started to write the above answer, I had another thought...
Some of these older receivers also had various inputs...some low-imp (~ 50-ohm) and some hi-z....and, I wondered if the SPR-4 was one of these....
Now, I am a Drake guy, but even I wasn't sure, so I spent another 5 seconds downloading the SPR-4 manual.....and then spent about 60 seconds scanning the doc...and, what-da-ya-know, it is...

Perhaps Allen, KL7AM, needs to have a more detailed look at the SPR-4's manual?
www.wb4hfn.com/DRAKE/DrakeManuals/PDFDOCS/SPR-4_Manual.pdf'Cuz, when I took just a quick look at it, it mentions that while for best single-band performance a 50-ohm coax-fed 1/2-wave dipole is good, it also mentions that "Bands A and B require a long wire antenna."....
So, who knows what "bands" Allen has selected when he is testing this radio...
For optimum performance the SPR-4 should be used with a good antenna. For general broadcast and
short wave listening, a long wire antenna from 50to 100feet long will give good results. An antenna kit (Model AN-5) is available.
For best single band performance, a half wave dipole or other resonant antenna, fed with unbalanced
50 ohm coaxial cable, maybe used.
How ever, Bands A and B require a long wire antenna.
For directional reception on 150 kHz to 1600 kHz, the accessory AL-4 shielded loop antenna may be
used. All antennas other than the AL-4 loop antenna, connect to the Ant. jack on the rear of the
chassis.
So...
So, in addition to picking up the ohm meter probes, maybe a better read of the manual, switching a switch, etc. will find the "issue" in short order?

Hey, I could be all wrong on this....but, as I wrote, this was just my first thought....and, I thought I'd spend a few minutes pecking away on the keyboard to see if this helps.
I do hope this helps?
73,
John, KA4WJA
P.S. I was recently accused of "talking down" to a fellow ham here, who was asking for advice / recommendations....when all was actually doing was attempting to pull some further info FROM him, attain more details, that was needed in order to better answer him in anything but generalities...
And, here in this thread, I see the same issue....there is little detail to make accurate recommendations / give detailed advice....so, I'd usually ask for more...
But, because of this recent episode, I decided to actually do some of the work for the questioner.....
This is, of course, not the best way to teach....but, it seems that these days it isn't acceptable to most to actually question them....so...
So....