The PHILCO Phorum

Hello everyone,

I'm getting very close to victory on my 48-482! I have the FM aligned pretty well, and the AM and SW are about as sensitive as they can be (I spent a couple of hours monitoring late night AM radio to see how good the reception was--I was able to get all the full-power stations from around the country).

But I think the AVC is malfunctioning: the distant stations are very low on volume, though when I turn it up (3/4 to full), they sound fine without excessive noise, but when I roll across a local station, it just blasts me out of my chair. I'm sure the BC antenna is working fine, and the IF chain is continuous and zeroed in on 455 KC, but it seems like the AVC should be working better to attenuate the locals, and bring up the distant stations.

I tested R507 at 3.1 MOhms (should be 4.7 MOhms), but I'm having a real problem finding R506, the other branch of the AVC voltage divider. It's supposed to be on the pushbutton board, right next to C511, the coupling cap for the AM band. It isn't there. I've measured every terminal on that board, but I'm not able to find R506, another 4.7 meg resistor to ground.

Has anyone else had this trouble? How critical is the actual resistance of the AVC voltage divider? Would a bad reading like this cause the lack of apparent AVC on my AM band?

Thanks in advance to you all!

Mike

Mike

You do not have to locate the R506 as long as you are not set on replacing it.
Now that you know where the R507 is, just buzz its pins to the Ground (chassis). One should read very high (or open) and another - 4.7M (or whatever R506 became over time), this value will be the value of the R506. If it is not there or too high, then you do not have it.

I have yet to lift one leg of R507 (the one on the bandswitch) to see what the actual value of that resistor is, and maybe that should be my next move.

What I'm asking is that in this voltage divider scheme, do both of the resistors on either side have to be the same value? Does that matter?

Thanks, morzh for your reply.

They aren't super critical, but the closer they are to in balance, the wider the range of AVC will be.

From this schematic (and let's face it, not the best print I ever saw, talking of the one on the NA.org) I did not see any DC path from either end of the R507. So unless I miss something your de-soldered value should be the same as in-circuit one. There may be some slow RC there, but still.....although it is indeed interesting why it is then lower than 4.7M and not higher as might be expected with old resistors.

The easy thing to do would be to take a 4.7M resistor and run it between either end (unless you can locate which end should connect to R506) of R507 to ground and see if it makes a difference on you AVC. As noted above, one end of R507 should read ~4.7M and the other should read ~9.4M to ground. These will of course vary some due to value drift of the resistors.

Thanks for the replies. I brought my set and my problem to the Vintage Radio Service class at the Pavek Museum today (great place, by the way, don't miss it when you're in the Twin Cities!), and with some help from the experts there, we zeroed in on the junction of C417 and R412 as the AVC test point. From there, we were able to determine it's working just fine, and that the actual problem is poor contact on the band switch. In fact most of the toughest problems I've had with this set have been because of that freakin' band switch! Distorted fingers making loose contact with the sliding conductor--that's what's to blame!

I did, by the way replace R507 with a fresh 4.7M resistor, which I think helped, because measuring the removed old resistor, it came it at barely 2.7M. Strange--I thought these old resistors tended to drift up, not down.

In any case, all is well, and the 48-482 is just about finished! Thanks to you all for your thoughtful comments!

Mike

Hi Mike,

I finally finished my 48-482 and you're right about that switch. The problem with mine was the set screw that holds the lever from the FM pushbutton. It loosened up so the switch wasn't lined up properly. Now that it's tight the radio sounds great on AM/SW and FM.

Eric

That compositon board link between the band switch and the pushbutton broke on mine, so I made a replacement from plastic of the same thickness. So far, so good!

I still need to tweak those contacts so they make consistently good contact, but at least it's better than it was.

You're right, Eric, this radio sounds great on all bands, but especially FM! I'm amazed at how good it sounds, properly adjusted. I would love to hear the push-pull version of this radio, the console with the record player ... anyone have one?