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Author Topic: so... what fails in old radios  (Read 3734 times)
WB4SPT
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Posts: 132




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« on: March 08, 2012, 02:37:05 PM »

What fails, just with age?   My mental list is in this order:  electrolytic caps, paper/foil caps, then what???  Are disc ceramic caps low risk at 40 years of age?  I read that dipped mica caps are stable with age.

Also,  I was doing surgery on a 4 section "electrolytic" cap and I discovered that the cap uses aluminum foil that appears etched, but not oxide covered.  That is, the cap uses paper as the insulator, not AL oxide like modern caps.  I suppose this is how modern caps are so much smaller.
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KE3WD
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Posts: 5694




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« Reply #1 on: March 08, 2012, 04:51:33 PM »


Add Carbon Resistors to your list. 

73
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WW3QB
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Posts: 692




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« Reply #2 on: March 08, 2012, 05:07:04 PM »

Silver migration in silver-mica capacitors and silver migration in general. Also tin whiskers.
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K1ZJH
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Posts: 884




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« Reply #3 on: March 09, 2012, 05:48:10 AM »

Domino type micas are going bad. I'm finding bad padder caps in some of the Hallicrafters receivers in my
shack. A SX-42 that was working great several years ago needed to have the mica caps in the IF cans
replaced.
Sometimes just disturbing the leads on a mica domino (postaga stamp) can cause it to become noisy.

Pete
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KAPT4560
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Posts: 76




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« Reply #4 on: March 09, 2012, 06:16:43 AM »

 Ditto on the dominos. I have some early FM radios that use them in parallel with a resistor in the RF sections. You can't tell if they are leaky until you lift one side of the resistor.
 Cheap (esp. grainy finish) resistors can drift very high. I just did some 100Ks that measured 24 meg.
 Mouse piss can corrode and open choke windings. Beware cleaning variable air and mica caps with solvents, as that can drift them to another frequency threshold far from spec.
 Spread and/or dirty tube pins/socket terminals can be noisy or intermittant.
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WW3QB
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« Reply #5 on: March 09, 2012, 06:38:31 AM »

Let's not forget wafer switches. Difficult to clean, impossible to replace.
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G3RZP
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Posts: 4313




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« Reply #6 on: March 09, 2012, 09:44:36 AM »

Some cement coated wirewound resistors.

The trouble I've had with postage stamp micas is that they develop a tendency to change value if you squeeze them, and squeeze hard enough, they go back  to the correct value - but don't stay there!

But I also have perfectly good electrolytics still working that were made in the 1950s.
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AE6RV
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Posts: 146




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« Reply #7 on: March 11, 2012, 10:28:36 PM »

Hi voltage diodes in transmitter/amp power supplies are another common point of failure.
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KC2VDM
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« Reply #8 on: March 13, 2012, 02:14:56 PM »

The lightbulbs.  Grin
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KE4DRN
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« Reply #9 on: March 14, 2012, 05:30:39 PM »

hi,

band switch shaft couplers made of plastic and dial cords.

73 james
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WX7G
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« Reply #10 on: March 15, 2012, 06:39:13 AM »

In order of failure rate:

vacuum tubes
electrolytic capacitors
dial strings
power cords and plugs
resistors
other than electrolytic capacitors
switches
wafer switches
power transformers
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G3RZP
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Posts: 4313




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« Reply #11 on: March 15, 2012, 08:22:18 AM »

Components most likely to fail are those made of unobtainium, those that are very expensive if obtainable, and those that are the most physically difficult to replace.
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AC5UP
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Posts: 3812




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« Reply #12 on: March 15, 2012, 10:12:51 AM »

Observe the handiwork of Satan beneath this chassis.......

http://farm3.static.flickr.com/2120/2428607084_ccb1434dbb.jpg

The only way it could be any fuglier is if the word 'Philco' appeared on the rear apron.
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Never change a password on a Friday                
WB4SPT
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Posts: 132




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« Reply #13 on: March 15, 2012, 12:09:26 PM »

Square nuts and ceramic tube sockets do date it.  But a bit of krud kutter and it will at least LOOK like new!
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KC2VDM
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Posts: 145




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« Reply #14 on: March 15, 2012, 12:35:44 PM »

One thing I forgot. The wires almost always become brittle and the insulation cracks. Not Good! Especially on B+ lines!
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