The PHILCO Phorum
48-482 Performance Second Opinion - Printable Version

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48-482 Performance Second Opinion - DCASAZZA - 11-02-2009

OK folks I have restored a 48-482 radio and carefully aligned it. All the IF's demonstrated a sharp sweet spot during alignment so the cans should be OK. The radio works fairly well, even the FM. I am some what disapointed at the performance of the AM and SW sensitivity. Am I expecting too much? This thing has 3 IF's and I somewhat expect more because of it. I must disclose that the detector and 1st audio ( 7B6 ) does test weak on a tester. All the rest test really, really good on the tester and my benchmark 6V6 tube tests as usual. Like most in this hobby I have seen weak tubes work good and "good / tested " tubes not.

SW band is fairly quiet but works and AM picks up all local and some stations 50 miles away but the volume has to be up. Is this just what it is for this radio. Will an antenna setup of some type pluged in to the 4 pin scocket on the rear improve it? Any wire clipped to it just hurts the performance at this time. Do I need a tuned loop? Again that seems like over-kill for a 3 IF stage radio. I do realize it's a converter front end.

So what do ya think? My ears' to the ground at the best gathering of OT (original technology) radio techs I know of so somebody....throw me a bone.

Re: 48-482 Performance Second Opinion - Fred R - 11-02-2009

Dave,

I have had one of these sets in the bedroom for almost 20 years and the AM and shortwave should be hot. This the only Philco table model with inverse feedback and has a sound that no other 1948 table set has. I have the set below it, the 48-475 which sounds rather plain. The biggest problem I had found with most early post-war FM sets is the oscillator radiates terribly on FM. Philco uses a rather odd IF frequency on FM. Before the days of cable, when tuning the set to 93.3, one of our local rock stations, channel 6 TV was totally unwatchable. Cable and now digital broadcast has eliminated that problem.

I find that most of the 40's Philcos with the "low-noise" front ends with 7F8s or similar setups with XXLs or XXDs, with the extra IF stages and low impedance loops, work extremely well. Is your loop hooked correctly and is the matching transformer that couples it to the 7F8 OK? The 7W7 RF stage is not used on AM and shortwave. The loop is coupled to the 7F8 mixer section through the loop transformer inside the RF deck.

Fred

Re: 48-482 Performance Second Opinion UPDATE - DCASAZZA - 11-08-2009

Made it better
While mucking around one last time before mounting it back in the cabinet I noticed the reception got better if I touched the mica cap lead between the ant coil and the converter tube with a small screw driver so I replaced it. Wow is this front end HOT. Even SW got a shot in the arm. I'm DXing the eastern US and don't know what else (at night of course) with the 3 turn loop ant it brung to this dance. Even though the RF peaked as I adjusted the ant trimmer; still, it got better if I touched the 22pf's lead. I figure it should get worse if I touched it by throwing it out of tune.
I mis-read the schematic and replaced a 22pf with a 220pf cap. The Ant Trimmer range on BC and SW is the same as with the old 22 pf mica cap
A 27pf cap took me back to the old disapointment. The 220pf is back in service!

Now here's the question.
Did I discover a new mod?
Is there a replacement of a slowly failing ant coil in my future? This coil ohm'd out OK and looks very good. As much as I want the radio to be original I also realize that success is measured by results.

Thoughts?