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48-482- Weird Problem - Printable Version

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RE: 48-482- Weird Problem - jerryhawthorne - 03-07-2013

Your right BrendaAnnD, may I refer to you as BAD?Icon_biggrin I would rather attack a Scott 800B than that radio. None the less that is a real challenge. You might update your profile with a location, from previous posts I sense you are quite remote from the US. Could be wrong. I would certainly start with some good switch cleaning. That radio is a real nightmare of wiring.
Jerry

RE: 48-482- Weird Problem - Eric Adams - 03-08-2013

Now you know how I feel!

RE: 48-482- Weird Problem - Eric Adams - 03-08-2013

I'm contemplating a small explosive charge under one of those switches. But I guess I'll work on cleaning them first.

Thanks everybody,
Eric

RE: 48-482- Weird Problem - Raleigh - 03-08-2013

You probably won't be able to make a good measurement on the mica caps, unless you have the kind of meter that you plug the leads directly into. Otherwise, your test leads have so much capacitance that you can't get a good reading of the micas, certainly not with the cap in the radio. Sometimes you can connect the test leads together and subtract the number you get from the one you get when you measure the caps, but it isn't very accurate with caps in the picofarad range.

I don't like to do it, but when I remove a mica cap from a radio for testing, I just replace it with a modern one. They are OK more often than not, but it is as much trouble to test them as to replace them. And if you don't have the right kind of meter it is really not possible to test them accurately.

I'm not sure, reading this thread, that you have injected your 455 KHz modulated signal into the mixer output. It sounds as if your IF stages are all OK, but I'd like to hear that you've successfully injected a modulated 455 KHz signal through a (.1 uf capacitor to protect your signal generator from the high voltage) at the input to the first IF coil. That would be point "E" in section 5 of the schematic.

Some of these 40s Philcos used a type of mica cap that looks like a large, thin, wax covered rectangle. I've mostly seen these are in the oscillator/converter circuit. Sometimes these were dual caps, two caps in one package. I don't know if your radio has any of these caps or not, but if you see any, they might be a likely culprit. I've heard that these are more prone to failure than the "domino" style of mica caps that you see elsewhere in the radio.

I do like the suggestion that you clean the band switch exhaustively. I've seen an otherwise restorable band switch make a radio silent. There are lots of switches shown in the schematic. You might try measuring the voltage on both sides of each switch if you can get to them, to see if the switch has continuity or not. Measuring resistance across the switches would tell you much the same thing.

I have a couple of these radios on my "to do" list, but I haven't opened them up to work on them yet.

RE: 48-482- Weird Problem - Eric Adams - 03-08-2013

Thanks John,

I'll check into those ideas and let you know how it turns out,
Eric

RE: 48-482- Weird Problem - Eric Adams - 03-08-2013

John,
I sent a modulated 455Khz signal into the mixer output and got a loud and clear tone out of the speaker. Because of all the guidance that I've been receiving, by the time I actually get this to work, you guys will be asking me for help! ;-)

RE: 48-482- Weird Problem - codefox1 - 03-08-2013

Replace the mica caps with current or near current silver micas. No foolproof cost effective method that I know of will rout out the problems other than complete shorts or opens. I also believe that all elderly carbon resistors be replaced at the same time as the recap job is done. Once you eliminate these possible points of failure, the restoration becomes less complex. Others may disagree.

RE: 48-482- Weird Problem - Eric Adams - 03-08-2013

Hey everybody! I found it! Since we knew it was in the oscillator circuit, I just started testing every component connected to that tube. Sure enough a 22k resistor on pin 5 was open. I replaced it and now have AM and SW. Now to tackle the FM circuit.

THANKS to all who jumped in!

Eric

RE: 48-482- Weird Problem - jerryhawthorne - 03-08-2013

YES Eric!!!!!!!!! Glad you got there. Earlier I suggested voltage measurements on that tube. Might have spotted it that way. It has been a long road. Now that your on a high, on to the FM, it's always worse if not working. Do not start with the golden screwdriver on the IF and second detector until such time as you have something working. A real PIA adjusting that part. Hook up an antenna as see what ya got.
Jerry

RE: 48-482- Weird Problem - Eric Adams - 03-08-2013

Thanks Jerry. I'm going to start component checking. I also have a squeal when touching the treble control. Well, one thing at a time.

Eric

RE: 48-482- Weird Problem - codefox1 - 03-08-2013

So glad you are getting there. Shut down and shoot that control with a little deoxit and work it back and forth a few times. Might as well do the rest of them as well. Let it ll dry out overnight.

"Note well" N.B. keep the golden screwdriver away from the set until all the usual suspects such as out of range resistors, of course all capacitors, corroded tube sockets and pins, and switches (did I miss any?) are ruled out. Tubes do not usually go out all of a sudden, but are the next in line as suspects.

RE: 48-482- Weird Problem - jerryhawthorne - 03-08-2013

Eric, the treble control is a fairly simple circuit, only 3 parts in it. The control, a .01 cap and a 220pf cap. Looks like perhaps a shielded cable from the high side of the pot to the .01. Does it squeal when you touch the bare shaft or when you adjust it? Check the resistance to ground from the center tap and from the low end. Also did you change out the .01 cap?
Jerry
http://www.nostalgiaair.org/PagesByModel/637/M0013637.pdf

RE: 48-482- Weird Problem - Eric Adams - 03-08-2013

Jerry,
I saw in the schematic where the 220pf cap is supposed to be but this radio doesn't have one! Believe it or not I have a 2nd 48-482 that is all original (I got it to compare with the one I'm working on. You know, correct parts placement) and it doesn't have one either. Anyway, the.01 was replaced whith all the others and the tone control seems to ohm out OK. I did notice that some of the push buttons are dead, so I think that is what I have to look at for the FM.

RE: 48-482- Weird Problem - jerryhawthorne - 03-08-2013

Eric, please supply a little more information.
Is the AM working properly and does the treble control work alright in this mode?

Are the push buttons you are worried about presets for the AM?

What are you getting on FM at this point, any thing?

When and how do you get the "squeel" when touching the treble control?

If your worried about dead push buttons on the AM preset, don't worry about them now. If a few work, great. You have better things to focus on at this point. If the AM is working on manual tuning it is time to sort out the FM. The little details of preset AM buttons can be sorted later.
Jerry

RE: 48-482- Weird Problem - codefox1 - 03-08-2013

OK, we need to go back to basics, start with power supply and voltage regulator, and if that looks good start backwards from the speaker to the antenna, and somewhere in between you will find a problem. Then do a complete alignment and then further eliminate stages. This is how I do it, anyone else have a better plan, please tell us.