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 Delco R-1131 with half an AM

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gregd



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PostSubject: Delco R-1131 with half an AM   Tue Oct 12, 2010 11:35 pm

After a recap on my delco it now has only the upper half of the scale getting reception, from about 880 to the upper end.....nothig but silence from 880 down.....Any ideas on what I'm doing wrong would be appreciated..............Thanks, Greg
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OldWireBender



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PostSubject: Re: Delco R-1131 with half an AM   Wed Oct 13, 2010 12:18 am

Hi Greg,

If the portion of the dial is correct (a station at 1230KHz comes in at 1230 or close to it), then the most likely cause is a tuning capacitor with a short caused by plates that touch or have some conductive debris caught between them. The short can be in either the RF or oscillator section of the tuning cap.

The easiest way to check for this is, with the power off, temporarily remove any connection(s) to either the RF or oscillator section stator (fixed plates). This will eliminate any false readings caused by the coils attached to that section. Connect an ohm meter to the capacitor, "-" to the cap frame and "+" to the stator terminal for the section that was disconected. Use the Rx1 ohms scale.

Now, tune the set (Power Off!) from one end of the dial to the other while watching the ohm meter reading. A good tuning cap section with no shorts will read infinity over the whol e range. A section with a short somewhere will usually read infinity over some of the tuning range, but will drop to near zero ohms at the position where the short occurs.

If you do find a short at this point you can now set about finding and fixing the cause. Sometimes a strong light shining through the plates will let you see if a plate is bent or there's something caught between the plates. Recheck with the ohm meter to make sure you've fixed the problem.

After you're satisfied that either the section has no shorts or that you have removed any shorts you've found, you can reconnect the wire(s) to the the stator for that section.

Problem fixed, right? Well, maybe. But to be safe, and as long as you've got the ohm meter right there and the soldering iron is still hot, why not go ahead and repeat the same procedure for the other tuning cap section? You see, I've found that if one section has bent plates of debris causing a short, chances are good that the other section may have suffered the same fate.

I mean, if it's a bent plate problem for one section, that means somebody's been stickin' their fingers (or something else) in there in the first place that most likely cause the 'bend'. And they may have not been satisfied with fooling around with one section of the cap. If it was some foreign debris in the first section causing the problem, the other section is likewise suspect. They are side-by-side after all.

Good luck! Very Happy

John
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gregd



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PostSubject: Re: Delco R-1131 with half an AM   Wed Oct 13, 2010 8:58 am

Thanks John for your help, I checked them out as you instructed and found no problems...no bent fins and every thing reads ok.....This radio didn't work when I got it and being rather new to radio repair I'm just a lot confused as to what or why only half the scale would work.......thanks, Greg
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pixellany



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PostSubject: Re: Delco R-1131 with half an AM   Wed Oct 13, 2010 10:37 am

If nothing is shorting, then I think the most likely cause is that the oscillator stops at some frequency.

You can check this with a scope---or: I have seen threads on how to use another radio to pick up the oscillator signal (I''ve never tried this.

If it IS the oscillator stopping, then the likely cause is a weak tube or some component that is way out of tolerance.

But, back to the shorting/dirt theory--it will not take much leakage across the tuning cap to cause a problem---did you check it at a high resistance setting? (I would think at least 1 Megohm--preferably more.)
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Dave McClellan



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PostSubject: Re: Delco R-1131 with half an AM   Wed Oct 13, 2010 11:41 am

I think I replied to this on the forum when it was active. But please check the GROUNDING FINGERS on the tuning capacitor. I had the exact same problem on an Atwater Kent 308. The oscillator only worked from about 1000kc on up. The brass fingers were NOT making good contact with the rotor of the tuning capacitor. I removed them, cleaned them, cleaned the grooves where they contact the rotor, and reinstalled them. They were originally held only by rivets to the frame of the capacitor. I soldered them to the frame.

Also check the grounding of the capacitor to the chassis or to wherever the frame is supposed to connect. Some radios have ground leads to the frame of the capacitor. Others have wires that connect to the fingers, and some have both. The problem case I mentioned had ground leads only connected to the fingers. The only connection between the grounding fingers and the capacitor was a rivet - notorious for poor connections.
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tubes4life



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PostSubject: Re: Delco R-1131 with half an AM   Wed Oct 13, 2010 1:52 pm

Do the correct stations line up with the dial pointer? If not, the oscillator trimmer probably needs to be adjusted.
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gregd



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PostSubject: Re: Delco R-1131 with half an AM   Wed Oct 13, 2010 11:42 pm

Thanks for your replies....Tubes4life,The stations line up exactly on the part of the scale thats working....Dave,,I see the brass fingers you are talking about I think, I will get to trying to clean those tomorrow.......Also, when I got this radio, a large resister between the oscillator and the 1st det-mixer on the schematic, was blown to pieces......By the schematic I took it to be 100ohm....Does that look right??.....I replaced it with a 100ohm ,10 w resister......Also, are these directional and if so might I have it in wrong???.........Thanks again for all your help.....Greg
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gregd



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PostSubject: Re: Delco R-1131 with half an AM   Thu Oct 14, 2010 11:33 pm

Uh-oh........I must have said something very wrong ......
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Tony Wells

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PostSubject: Re: Delco R-1131 with half an AM   Thu Oct 14, 2010 11:51 pm

For those playing along at home:

http://www.nostalgiaair.org/PagesByModel/577/M0022577.pdf


If you're talking about component #62, it is a 100 ohm resistor. The wattage is not specified, and there is no parts list on NA. Can you tell what size it was to physically determine the wattage? If there is any doubt on the wattage, you should go up a bit. There is no direction on resistors to follow, no polarity or anything.
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gregd



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PostSubject: Re: Delco R-1131 with half an AM   Fri Oct 15, 2010 6:59 pm

Thanks tony for the info......Yes that would be the resistor, the old one was about 1/4 inch in diameter and about 2 1/2 inches long and was black on the end brown then blown all to heck......the resister has 260 volts on it and does not get hot at all........thanks, greg
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smeezekitty

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PostSubject: Re: Delco R-1131 with half an AM   Fri Oct 15, 2010 9:18 pm

gregd wrote:
Thanks tony for the info......Yes that would be the resistor, the old one was about 1/4 inch in diameter and about 2 1/2 inches long and was black on the end brown then blown all to heck......the resister has 260 volts on it and does not get hot at all........thanks, greg
Probably 2w.
Best to use a 5w to replace it.
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pixellany



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PostSubject: Re: Delco R-1131 with half an AM   Fri Oct 15, 2010 10:52 pm

"260 volts on it"----If you are talking about the voltage at one end, then that is not relevant to sizing the resistor. If you were talking about the **drop**--ie the voltage difference across the resistor, then you have 260 volts across 100 ohms = 2.6 amps. That would be a **very** high power resistor... Smile
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