I've read a couple helpful threads here about antenna couplers. In my case, I'm restoring a 42-390 console. According to a service bulletin, it calls for a 42-1361 antenna coupler, which I don't have. I live in the boonies, so I have to use my outside long wire antenna to pickup AM stations. I know these couplers were used mainly to match the impedance of an outside antenna to the radio.
Anyway, my question is what impedance do these 1940 Philcos expect? 50 ohms? I know my long wire is much higher, but I do run it into a 9:1 unun, which feeds a RG-8x coax coming into my house. My radio does pickup AM ok if I connect the center wire of my coax to either pin 3 or 4 of the FM jack. That's with the cabinet FM antenna (pins 1 & 2) unplugged. It does sound a little distorted, but that may be a capacitor that needs to be replaced.
I'm just trying to understand if my antenna hookup is working as well as it can, before I do more troubleshooting.
Hi and welcome to the Phorum,
In general terms most prewar receivers have a high impedance input for the antenna if it's the bc,sw, or fm band. Coax wasn't used much in the public sector till after the war. Typically your looking at something between 300-600 ohms for the input impedance.
If you were using a dipole antenna 300 ohm twin lead would be a good choice or 450 ohm ladder line. I don't know of any diagrams of the 45-1361 but there is a diagram for the three pin antenna coupler used in the 1940-42 sets.
As for weak and distortion. Check all the tubes especially the XXL's. Measure all carbon resistors to check their 20% tolerance against their marked value. Replace all foil/paper caps. Do a complete alignment and then you'll be in a good position to do the trouble shooting without guessing what small parts are bad.
GL
I ruled out my antenna hookup when I noticed the radio's sound was getting worse and worse the longer I had it on. I used my sign gen to put an AF signal into the final audio amp and that seemed ok. But when I put it into the 1st audio amp, it was weak and erratic. I was afraid the XXFM tube was bad but didn't have one, so I replaced 6 of the paper caps in that area. Luckily one of them was the culprit. At that point I replaced the rest of the paper caps.
Then it was time to see how it played. First tried it with no ext antenna. Wow - I'm totally impressed with this Philco radio and its built-in antennas. I don't think I have any radio that works so well in the basement with no outside antenna. No noticeable RF noise either.
When I do connect my outside antenna, it definitely improves SW reception but seems to introduce some RF noise on the AM band. I'm sure back in the day, an external FM dipole would have been needed here in rural NH for that band.
After downsizing a couple of years ago, I don't have a good resistor inventory, or I would have checked and replaced some of those. I was up half the night as it was! Are there any particular resistors in these 1940 Philcos that go bad? Of course, right now It's working so well I might be better off leaving it alone.
Without the coupler I've tried connecting a long antenna to it and it worked BUT the image rejection was poor on my 41-300. As for the resistors I would say that higher values are more likely to drift but that doesn't preclude checking the lower one's values. If you have some time to kill you can use these:
https://philcoradio.com/library/index.ph...-envelope/ to put your resistor in.
Now you do realize that this set was never designed to receive signals from what is known as the fm 100 band just the fm 50 band. It tune from 42-50mc. This means that ant and rf circuits are not going to be resonant at the fm 100 band. Nor is the local oscillator. So what is happening is that your incoming signal @ 84-100mc has to have enough power to plow thru ant and rf stages because they are passing signal @ 50mc or so. Then the mixer has to try to mix with the 2nd harmonic of the oscillator (this is why the sensitivity sucks) so the it can tune the lower half of the fm band. So unless you have a second mixer that converts the 42 to 88mc. With that the set only tunes 8mc it would tune 88-94mc.
I had done some checking and discovered several towns here in NH still have active FCC licenses for the public utility band, somewhere right around 49 MHz. Those towns are all 50 miles or so away, but I thought I might pick something up but no luck. Of course, they may have switched to a modern encrypted digital band and are just letting those licenses run out.
I do remember picking up various transmissions on the PBL band on my Midland back in the 1970s and 1980s but not in the past 30 years.
This Philco is the same model a neighbor gave me when I was a teenager in the late 1950s. No idea what happened to that one. I was cursed with the affliction of always taking everything apart. At the time I wasn't aware of the of history of the FM broadcasting bands. But looking back on it, they had no doubt upgraded to an FM radio that could play the new frequencies. For me that radio was my first exposure to SW.
Thanks for the feedback
Marv