Philco 48-482

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Philco 48-482

Post by ea327 on Wed Dec 19, 2012 4:10 pm

I'm working on what I've read is a really great sounding radio but I have a few problems. It has AM,FM and S/W. FM and S/W are silent. AM, with my long wire antenna gets a lot of static and 1 local station all over the dial. I replaced all caps, all tubes have been checked. I've gone back and checked all wiring and anything I replaced four times. One other thing, if I give it any treble I get a loud squeal.

Any ideas where to look?

Thanks,
Eric


Last edited by ea327 on Wed Dec 19, 2012 4:53 pm; edited 1 time in total
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Re: Philco 48-482

Post by N7ZAL on Wed Dec 19, 2012 4:49 pm

It has AM,FM and S/W. FM and S/W are silent. AM, with my long wire antenna gets a lot of static and 1 local station all over the dial.

There multiple frequencies that pick up the station or do you only hear the one station and it doesn't change when you move the dial?
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Re: Philco 48-482

Post by ea327 on Wed Dec 19, 2012 4:54 pm

I hear the same station in about three different places across the dial.
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Re: Philco 48-482

Post by N7ZAL on Wed Dec 19, 2012 7:02 pm

Hard to quantify what is going on, but if the local station has a very strong signal, then it could be just a front end overload...or it could be alignment.

Why the tone control causes an oscillation may not be related to the multiple frequencies from the AM station. How does the radio work on other bands??
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Re: Philco 48-482

Post by ea327 on Wed Dec 19, 2012 7:26 pm

It's not an alignment problem. FM and S/W are both silent.
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Re: Philco 48-482

Post by N7ZAL on Wed Dec 19, 2012 8:06 pm

FM and S/W are both silent.

Sorry that was in your first post, my error. I'd pick a mode, short wave for instance, and troubleshoot from there. You used a signal generator for the alignment? Is the band switch clean? Things are OK up to the audio section?
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Re: Philco 48-482

Post by ea327 on Wed Dec 19, 2012 8:22 pm

I used a signal generator. I'm not sure about things up to the audio section. I can get sound when I touch an audio source to the center lug on the vol control.
This is where I'm stuck. I'm not really sure how to go about troubleshooting this problem.
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Re: Philco 48-482

Post by N7ZAL on Wed Dec 19, 2012 9:30 pm

I'm not sure about things up to the audio section.

Hopefully we can have someone with knowledge of the particular set will chime in. I don't have the schematic and trying to analyze from general experience. As with all troubleshooting it is "divide and conquer." Wink Since you get noise touching the pot, and you get audio in AM, if is probably before the audio section. I'd try to then determine if it is in the RF or IF stage...or the mixer/oscillator. Did you use a scope with the signal generator or a meter?
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Re: Philco 48-482

Post by ea327 on Wed Dec 19, 2012 10:03 pm

Ok, at this point we have passed my experience level affraid
I used a meter as I don't have a scope yet. I think it's time for me to do some more reading. I'll do some looking in those stages and get back to you.

Thanks,
Eric
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Re: Philco 48-482

Post by Bill Cahill on Wed Dec 19, 2012 10:08 pm

What is the tube line up? What size of I F transformers? What brand of radio?
Do you know age of set?
I have a feeling, but, like a computer, I need more input.
Bill Cahill

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more info

Post by ea327 on Thu Dec 20, 2012 12:18 am

Bill,

It is a Philco model 48-482.
This will take you to the schematic: http://www.nostalgiaair.org/Resources/637/M0013637.htm
Any input would be appreciated.
Thanks,
Eric
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Re: Philco 48-482

Post by Resistance is Futile on Thu Dec 20, 2012 1:22 am

That is a very good schematic with trouble shooting suggestions.
What you first need to do is test all the tubes and do voltage checks per riders instructions. Since you have a VOM you should be able to follow for voltage checks without any trouble. Make your self a chart of readings. You should be able to solve a lot that way.

Since you don't have a generator, noting that you get one station is a good sign. Sounds like your oscillator is working, and your audio is working. What you can do is make a signal tracer out of a transistor radio and a crystal diode for a probe, be sure to put in a blocking capacitor to block DC voltage from going into the transistor radio and causing smoke. Hook up a piece of wire with the diode and connect to the center tap of the transistor volume control. If you don't want to do that then just get a high impedance set of headphones and a diode and a cap and use it the same way. Keep one hand in your pocket for safety at all times while testing this way.

Start at the antenna and see if you get any signal at all. If you do then just follow the signal path to the audio section. If the signal gets weaker then you have found a defective stage.

That should keep you busy for a while just make sure to make notes at every step of what you have done so you can review and not keep having to go back over testing steps.
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Re: Philco 48-482

Post by Bill Cahill on Thu Dec 20, 2012 3:19 am

If you connecyt the am antenna wrong, you can also get this symptom. I have this model, but, haven't done much with it yet.
I think it uses an XXFM tube ? If so, that may be bad, causing dead FM.
Bill Cahill

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Re: Philco 48-482

Post by ea327 on Thu Dec 20, 2012 11:05 am

I'm pretty sure I have the antenna connected correctly. The tube is an FM1000-Philco's own design so they didn't have to pay royalties on existing FM technology.
I'm going to build a signal tracer this morning and dig into it.
BTW, I do have a signal generator.

Eric
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Re: Philco 48-482

Post by Dr. Radio on Thu Dec 20, 2012 11:56 am

I second the bandswitch cleaning. Any resistance in the contacts can cause you grief.

The voltage readings on the diagram will be most helpful. This is a good guide. It appears Riders just reprinted Philco's own troubleshooting guide on this one.

Bear in mind resistors can go out of tolerance with heat and age. I would check to see if any in the oscillator section have drifted high.

I also see that the IF cans (Philco proprietary units) have a sort of highbred design, both variable and fixed capacitors inside the cans, sometimes the fixed value ones can give you grief depending on how they are constructed--i.e. the smaller units in smaller AC/DC radios in which many of us have battled "silver mica disease".
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Re: Philco 48-482

Post by Resistance is Futile on Thu Dec 20, 2012 12:33 pm

I read so many posts I get muddeled in the details sometimes. Sorry I didn't remember you saying any thing about a signal generator. Anyway you could could also use your generator and go theough each stage, starting at the audio stage and work back to the RF section. Start with a 1000Hz audio signal or 600Hz at the grid and listen at the speaker. Since you have a signal generator I don't think I need to elaborate further.
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48-482

Post by ea327 on Fri Dec 21, 2012 7:42 pm

Thanks, the signal generator sounds like the best idea. I'll let you know how it turns out.
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Re: Philco 48-482

Post by ea327 on Tue Mar 19, 2013 9:05 pm

Well, I finally got back to it. I found a bad resistor in the oscillator circuit. The AM and SW play great. (That sure felt great)! I got the presets adjusted and they work great. Now...
No sound from FM. Not even static. Since I have been using this as a fill in project it's back on the shelf for now, although I keep thinking about it.
Thanks to everyone for all the ideas!
Eric
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Re: Philco 48-482

Post by Wildcat445 on Tue Mar 19, 2013 9:43 pm

No sound from FM may be caused by not having an antenna hooked up. I would hook up some tv twin lead to the FM antenna terminals and then see what happens before I would dig too deeply. Since AM and SW work, most of the circuits in the radio are working. The bandswitch being dirty is still a possibility. I may have missed this, but I don't remember seeing anything about your having recapped this radio. If that has not been done, this is a good time to do that.

EDIT.......you have recapped the set. Please disregard this comment. Sorry.

Regards

WC

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Re: Philco 48-482

Post by ea327 on Tue Mar 19, 2013 10:07 pm

I had one wire plugged into the one place that is supposed to be for FM (it is supposed to wind around the power cord) but I'll try a better antenna and let you know.

Thanks,
Eric
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Re: Philco 48-482

Post by mr_ed01 on Wed Mar 20, 2013 6:56 pm

Clean the band switch first. Get the AM / SW working before working on the FM. FM alignment with the FM1000 tube is not easy. FM IF is not 10.7MHz as in other FM receivers. I have the Philco Service Manual if you need it.

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Re: Philco 48-482

Post by ea327 on Wed Mar 20, 2013 8:28 pm

Thanks, I have a copy of the Philco Service Manual showing 9.1Mhz for FM IF. The band switch has been cleaned. AM and SW work great.
A strange thing happens when I switch to FM: I get no static but if I tap anywhere on the chassis I can hear the tapping through the speaker just like I was tapping on a microphone. Ever heard of that?
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Re: Philco 48-482

Post by Dr. Radio on Wed Mar 20, 2013 9:46 pm

ea327 wrote:Thanks, I have a copy of the Philco Service Manual showing 9.1Mhz for FM IF. The band switch has been cleaned. AM and SW work great.
A strange thing happens when I switch to FM: I get no static but if I tap anywhere on the chassis I can hear the tapping through the speaker just like I was tapping on a microphone. Ever heard of that?

Yes, it's called "Microphonics". You can have a microphonic tube or connection. Sometimes elements inside the tubes get loose, other times you may have a bad solder joint or a capacitor or resistor that has a microscopic break or knick in a lead or internally causing this. Best thing to do is carefully tap like you are a surgeon--little by little to home-in on the trouble area--been there done that--not fun, but you will find the offending area by process of elimination.
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Re: Philco 48-482

Post by ea327 on Wed Mar 20, 2013 11:02 pm

Wow, Thanks Doc! I'm going to dig into it now. I'll probably be up most of the night. It sounds like a good time.

Eric
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Re: Philco 48-482

Post by ea327 on Thu Mar 21, 2013 1:59 am

I got it! After retesting the tubes (My tube tester has a place to put headphones to listen for any sounds the tube might be making) I found one disc cap, .01 that made some noise when I touched it. I replaced it and now have FM! I also put another .01 cap in the treble circuit. It still gives me some treble but the squeal is gone! One final alignment and I'm done.

Thanks to all for your help,
Eric
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Re: Philco 48-482

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