Introduction and two Philcos
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Speaking of resistors, how do I tell the resistance of a dogbone resistor? It doesn't follow the standard stripe system that I'm familiar with. I was just watching a video on YT of someone replacing some dogbones, and one that was supposed to be 25000 ohms ended up being twice that. Now I'm SURE that those things have something to do with this Philco mystery. Unfortunately, my multimeter is being stupid and refuses to measure anything. It's just a generic one I got from Hamfest 2 years ago.
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<Speaking of resistors, how do I tell the resistance of a dogbone resistor?
Well those resistors are called Body End Dot resistors or BED resistors.The first band if you will is color of the body. The second band is the color of the end. And the last band is the color of the dot. They use the standard RMA color as the newer resistors with bands
Hope this helps
Terry
(This post was last modified: 03-10-2015, 12:31 AM by Radioroslyn.)
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Just read your other post. Sound like your osc coil is good too. Now there is a way to check the osc to see if it is working. Here what you do. get a transistor radio and tune it to 1460kc or khz Turn your Philco on and let it warm up. Tune it to 1000kc. Take the transistor and turn it on, place it near 77 tube closest to the 80 tube. Slowly tune the Philco's main tuning knob back and forth about a 1/2 " either side of 1000 kc. Listen to the transistor set as you tune the Philco. If all is well you will hear loud hissing sound at one point as you tune. Just like a radio station with no audio/sound.
GUD LK
Terry
ps OOPS! I think I misread your meter I believe your osc coil is open. You can try the above it won't hurt anything.
(This post was last modified: 03-10-2015, 12:48 AM by Radioroslyn.)
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How do I fix an open oscillator?
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Check and see if the wire is broken at the connection. If so resolder it. If it is broken in the coil then rewind the coil.
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Ok hear we go. Draw a diagram of the bottom of the osc coil so once get the coil out you will be able to reconnect it properly.
Unsolder the coil connections
Unscrew the screw holding the coil bracket.
Remove the oscillator coil.
TA DA!!
Now what you are looking at is a bakelite tube with 2 coils wound on it.
The one we are going to be rewinding is the small coil at the bottom of tube (known is a coil form)
Careful inspect the small leads of that bottom coil. Very Important note the direction that is wound.
Break the bottom connection of that coil.
Carefully unwind it from the form counting the turns. The wire that you are removing will fall apart in places as it is rotten.
Under that winding there is a piece celluloid (1930's plastic) Remove it and disguard.
"RR speaking here"" Take the form and bake it in a toaster oven @ 150 deg for 20min. Don't bake over 200 deg.
Replace the celluloid with a thin strip of masking tape.
Now it's time to rewind. You will need to get some small gauge magnet wire. The original is 38ga but size isn't very critical. If you can find 32ga or smaller it will be fine.
Remember you were counting the turns you removed? Well you need wind them back on with your new magnet wire.
Scrape the coating off of the end of the wire and solder it to the lug on the coil form.
Best I remember it's about 22 turns.
Wind them nice and neat side by side. You MUST wind it in the same direction as the original.
When you finish winding it clean the wire and solder the the other lug.
Spray the new winding with some clear lacquer.
Put it back in the chassis. Wire it back up and your good to go!!
Terry
(This post was last modified: 03-10-2015, 05:52 PM by Radioroslyn.)
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Last night I learned that old dogbone resistors are read using the BED method. I found a small dogbone in my radio, and does not have a dot. The body is yellow and the end is white.
The schematic calls for a 4 MFD electrolytic and an 8 MFD electrolytic. The guy that worked on this prior to me just slapped 2 10 MFD's in there and called it done. Is that too much of a difference?
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Your resistor maybe yellow yellow white. Check the chassis layout diagram.
The two 10mfd caps are fine.
Terry
ps Just looked it up pn# sez it's a 490K The diagram sez it's 500k. Either value is fine not very critical.
(This post was last modified: 03-10-2015, 11:32 PM by Radioroslyn.)
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When a dogbone resistor has the same color body as the dot, then they don't use it, as the dot would not show anyway on the same color background. So in your case, the resistor is yellow body, white end, yellow dot, or yellow-white-yellow which gives you 490,000 or 490K ohms.
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I don't know about that. That resistor is awful small for being 490,000 ohms. The 16,000 ohm resistor is 4 times the size.
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The 16k resistor is a 2watt job the 490k is 1/2watt. The 16k is in the high volt circuit ( it's part of the voltage divider circuit) and has the dissipate much more power than the 490k.
Terry
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(03-11-2015, 12:33 AM)TheUniversalDave1 Wrote: I don't know about that. That resistor is awful small for being 490,000 ohms. The 16,000 ohm resistor is 4 times the size.
Yes, it is the wattage that makes a resistor bigger or smaller.
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I printed the model 84 schematic from the Knowledge Base, and I can't find a 490K resistor anywhere in there.
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Resistor #23 is listed in the parts list as a 490000 ohm resistor yellow-white-yellow. On the schematic Philco shows it as 0.5meg and that's what Ron has on the schematic in the Knowledge Base. Close enough was probably the thought.
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Oh yeah. Now I found it.
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