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Old 21st September 2013, 04:44 PM   #11
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Quote:
Originally Posted by kevinkr View Post
Don't use metal films in ANY of the RF, IF or detector circuitry. (&Replace only those that are bad)
I'm not disputing you - but WHY.
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Old 21st September 2013, 04:51 PM   #12
SY is offline SY  United States
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In an RF circuit, you want to maintain the original parasitic reactances to the extent possible. If you change them, the performance will degrade unless you're prepared to realign.

In the HT rail, there *may* be some difference in failure mechanism, but not general performance. Change away!
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Old 21st September 2013, 04:57 PM   #13
kevinkr is offline kevinkr  United States
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Quote:
Originally Posted by KatieandDad View Post
I'm not disputing you - but WHY.
Because they are quite a bit more inductive than the carbon resistors originally used and may affect circuit operation in unpredictable and irremediable ways (short of using the proper part).

All passives have parasitics some of which are relevant to certain applications and not to others.
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Old 21st September 2013, 05:02 PM   #14
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It makes sense. If metal resistors are more inductive than carbon one, now I understand. And I thank moderators and their knowledge !
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Old 21st September 2013, 05:04 PM   #15
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I accept that film resistors will have more inductance, I stated that in my earlier thread. Carbon resistors are becoming unobtainium.
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Old 21st September 2013, 05:11 PM   #16
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It's unusual for resistors to fail unless there is an underlying failure.
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Old 21st September 2013, 05:18 PM   #17
kevinkr is offline kevinkr  United States
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Vintage carbons unfortunately had a propensity to shift values drastically over time, more modern types tend to be stable unless overstressed or subjected to a circuit failure.

When I was restoring vintage tube tuners I checked all carbon resistors in the front end and IF, anything still within the nominal tolerance range was not touched - if bad I replaced the part with a comparable NOS part from my inventory.

All those parts are long gone and I no longer do this sort of work except to keep a couple of my old tuners alive.
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Old 21st September 2013, 05:18 PM   #18
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You're right, but without failing, they can see their value changing a lot with years, that's why quadmods advices to replace them. And their value can change especially in a tube gear, I think carbon resistors don't like to much heat. That's why I wanted to change for metal film. It wasn't a good idea. Finally,as wrote Kevin, changing only the bad is better.
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Old 22nd September 2013, 02:04 PM   #19
DF96 is offline DF96  England
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In most cases a change from carbon comp to film resistors should only need a tweak to the alignment. It will depend on exactly what role the resistor plays in the circuit. High value resistors tend to have capacitive reactance rather than inductive.

The best rule with VHF circuits is: if you know what you are doing, try it, but be prepared to undo the change; if not, don't try it. The IF stages will be less critical.
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Old 23rd September 2013, 08:26 AM   #20
multi is offline multi  Australia
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Carbon resistors; the fact that they change value was used to stop drift in Rf circuits, as the other parts in the Radio warmed up. I made the mistake of using Metal film and the radio drifted badly.
Not sure if I explained it very well.
Phil
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