I am not certain why exactly ...but it is believed that the reason carbon composition drifts high is because the originally compressed carbon granules dry out maybe becoming less densely packed together causing an increase in the overall resistance.
This WILL continue to degrade by drifting further upwards with time.
Vinny A.
::I am in the midst of restoring an EICO 221 VTVM and all the pots are high. Four 1Ks read around 3.4K the 2K was 3.01K and the 2Meg,3.5Meg. Funny the 1Ks all went triple. My question is, since I have tested then for continuous contact and fairly linear resistance for their new values should I just relabel them and use them when I need a 3K pot or will they continue to deteriorate or what? PL
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:Most likely they are carbon composition.
:I am not certain why exactly ...but it is believed that the reason carbon composition drifts high is because the originally compressed carbon granules dry out maybe becoming less densely packed together causing an increase in the overall resistance.
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:This WILL continue to degrade by drifting further upwards with time.
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:Vinny A.
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Some carbon resistors seem to hold up a little better than others.
Whenever I measure the old dogbone style.. they are way way high.. triple.
Only a couple are ever still in spec... so I always check and replace those.
I start checking every resistor.. in any new set for restoration that hits my bench...and if most have drifted high I replace all.
.. but if most are still quite close like 10% - 15%.. I still check 'em all but leave the good ones.
Hey.. it's only a few cents.. right?
As far as old pots. Those do change value due to a few things. The end connections where the metal is making contact with the carbon wafer, gets a resistance there. After years of rotation, the carbon gets worn. That too makes a change in value. The use of wrong type cleaners also can brake the carbon down.
Are we sure of the meter testing the pots? Sometimes a DVM will not check resistors properly if the resistor is still connected in the circuit. Just a thought.
Radiodoc
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:I am in the midst of restoring an EICO 221 VTVM and all the pots are high. Four 1Ks read around 3.4K the 2K was 3.01K and the 2Meg,3.5Meg. Funny the 1Ks all went triple. My question is, since I have tested then for continuous contact and fairly linear resistance for their new values should I just relabel them and use them when I need a 3K pot or will they continue to deteriorate or what? PL
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:Planigan,
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:Are we sure of the meter testing the pots? Sometimes a DVM will not check resistors properly if the resistor is still connected in the circuit. Just a thought.
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:Radiodoc
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::I am in the midst of restoring an EICO 221 VTVM and all the pots are high. Four 1Ks read around 3.4K the 2K was 3.01K and the 2Meg,3.5Meg. Funny the 1Ks all went triple. My question is, since I have tested then for continuous contact and fairly linear resistance for their new values should I just relabel them and use them when I need a 3K pot or will they continue to deteriorate or what? PL
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1.) If the value seems low them it may be in parallel with something else and should be tested with one lead free.
2.) If it reads higher than expected it is an accurate reading and if that high reading is out of tolerance .....the reading can be trusted and the resistor needs replacing.
BTW talking about drifting carbon resistors:
I was just re-checking my work on the Arvin 544 I just restored (posted here above) because it has an annoying hum now.
While re-dressing leads and also checking resistors I just found the 4.7k resistor to the plate of the 12SQ7 has DRIFTED up to 1.5 megs!!!
Can't trust those old carbon resistor!
:I think that it's safe to measure a resistor "in circuit" as long as you consider this:
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:1.) If the value seems low them it may be in parallel with something else and should be tested with one lead free.
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:2.) If it reads higher than expected it is an accurate reading and if that high reading is out of tolerance .....the reading can be trusted and the resistor needs replacing.
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:BTW talking about drifting carbon resistors:
:I was just re-checking my work on the Arvin 544 I just restored (posted here above) because it has an annoying hum now.
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:While re-dressing leads and also checking resistors I just found the 4.7meg resistor to the plate of the 12SQ7 has DRIFTED up to 1.5 megs!!!
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:Can't trust those old carbon resistors!
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You are EXACTLY right sir!... That would have been a big load.
But right after I posted that... a moment or so later I realized my TYPO and posted my correction in the very next post..
You must have over looked it...
Yeah... that was just a typo... lol.
But as far as hum goes... that was only item #1) in my debugging process,
....but not the exact source of my hum problem to be sure.... though exactly what level of problem contribution it has at 1.5 megs I'm not sure.
There were lots of other minor things I discovered too.
#2)The coupling cap from the 12sq7 to the 50L6 was dressed improperly and next to the heater wires.
#3)The .03 uf cap (tone cap) that was supposed to be across the primary of the output trans was instead simply wired from pin 3 to pin 4 on the 50L6 (plate to screen). (B++ to B+)
For this Arvin 544 the plate has B++ right from the rectifier...and the screen has B+ after the pi filter....so that cap was basically across the 1500 ohm pi-filter resistor instead of across the transformer primary.
Not a big problem just not per print.
#4) A big nasty cold solder joint in the B- ground path seems like it was the major contributor however.
I was probing wires with a plastic stylus when it suddenly started buzzing like an old saw!
The old solid cloth wire was so oxidized even scrapping it wasn't much help.
I pulled the WHOLE entire B- wires out and re-ran new ones. And THAT brought the hum down to normal!
....A very low ...mostly acceptable state.
There was way less than 1v of ripple on B+ but there is 12v of ripple on B++ going to the 50L6 plate.
So I decided to improve the power supply.
I have NOW added a 3rd stage which really did the trick..lol
The original pi filter had 40uf then 1500 ohms then 20uf.
The B++ comes off the 40uf right at the rectifier cathode.
BUT... I now have 40uf 100 ohms - 33uf 2k ohms - 20uf
So now B++ comes from the 33uf and B+ is taken off the 20uf.
B++ now has less than 5v ripple and B+ is almost flat.
That did it now beautifully.
I feel that for a drop-dead beauty like this Arvin 544... I want whoever else should own it one day... to have the performance match the look.... y'know?
I said 4.7 meg and meant .47 meg or 470k
geeze I hope this is finally right...lol