[AMPS] Re: resistance

[km1h @ juno.com]

On Wed, 26 Nov 1997 11:01:46 -0600 Richard Carroll
<w0ex@scan.missouri.org> writes:
>>
>> > A 2W carbon resistor will show a gradual and steady R increase 
>when run
>> > above 1W or so....this is at DC.
>> 
>> Amen Carl. That's why carbon resistors should NEVER be used in power
>> supplies across electrolytics.
>
>
>  Years ago I worked on a GE High band VHF base station that had blown
>the AC fuse in the plate supply. It was an old unit, in service about 
>15
>years. The trouble turned out to be a 100K ohm 2 watt carbon resistor
>across the electrolytic filter cap that had worked it's way *DOWN* in
>value to 30 ohms.  Quite out of character. The rig couldn't have 
>worked
>long at all with the lowered resistance. It must have happened rather
>suddenly.
>
>Dick W0EX

Wow, I've not experienced that Dick. I have seen 10K and the like go to
almost infinity particularly the old style pre-war dog bones and early
50's tubulars. 
I have some old NOS tubulars that went way out of tolerance just from
aging. 

Another thing that bears mentioning again is that those resistors DO have
a voltage rating. 
The SB-220 HV metering circuit is one example of dumb
engineering....three 4.7M 1W carbons across 3200V  !!  I have seen those
go all the way up in value including open.  

73   Carl  KM1H

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