[Hallicrafters] update SX-101 resistors

Sat May 31 19:22:07 EDT 2014



-----Original Message-----
>From: Richard Knoppow <1oldlens1 at ix.netcom.com>
>Sent: May 31, 2014 4:20 PM
>To: Jim Wilhite <w5jo at brightok.net>
>Cc: hallicrafters at mailman.qth.ne
>Subject: Re: [Hallicrafters] update SX-101 resistors
>
>
>FWIW,  I've tested a large number of resistors using a Boonton 250A RX meter. This instrument measures resistance and reactance up to 250 Mhz.  I found in general that film resistors do very well up to perhaps 50 mhz, maybe higher depending on the value.  Curiously, the handbook for the 250 shows comparisons of carbon composition, carbon film and wire wound resistors as an illustration.  The film resistors have the best RF characteristics.  Carbon comp resistors are useful for a few things but in general are inferior to film types.  Composition resistors are made from a slurry of an insulating material, usually a ceramic, and carbon particles.  The problems come from the contact area of the carbon changing over time. Film resistors are stable with time and can be made very temperature stable.  They also do not have a voltage coefficient of resistance, which is typical of carbon comp. Metal film resistors can be made with either positive or negative TC or even zero TC over some range. Also, the numerous contacts in a carbon comp resistor make them noisy.  This is noise in excess of the normal thermal agitation noise found in all resistors.  Metal film are the quietest. 
>     IRC made what they called "metalized" comp resistors, I never found out what they did. IRC's have a finish that looks like tree bark. It was supposed to help dissipate heat.  Ohmite and Allen-Bradley look so nearly identical that I've wondered if they were not from the same factory. They have a smooth Bakelite case without seams.  At least the Ohmite seem to be about the most stable of the bunch.  I have seen other types which have longitudinal seams in the cases, whoever made them too many seem to be bad. 
>    I had some experience in my mis-spent youth working for a company who made established reliability metal film parts and mil spec carbon parts and had a liberal education on how they were made. Of course we didn't have laser cutters then, we used diamond saws in the trimming lathes.  
>
>-----Original Message-----
>>From: Jim Wilhite <w5jo at brightok.net>
>>Sent: May 31, 2014 3:47 PM
>>To: Hallicrafters <hallicrafters at mailman.qth.net>
>>Subject: Re: [Hallicrafters] update SX-101 resistors
>>
>>
>>
>>-----Original Message----- 
>>
>>
>>
>>
>>
>>>The moisture gets driven out.
>>
>>In some cases you are correct but the accuracy of construction played a 
>>part.  A 10% 1 K resistor could be anywhere from 900 to 1.1 K, so if one 
>>came off the line at 1.09 K it passed QC but any later factors or moisture 
>>could drive it out of spec.  Then using it could drive it further out.
>>
>>
>>>> Most of those who exhibit that property were made by IRC.
>>
>>
>>>I presume there is a way to tell the difference, but I can’t remember - can 
>>>someone elucidate?  (Some IRC reistors were marked “IRC” and some military 
>>>manual parts list show the supplier.)
>>
>>Most of the IRC version were a bit smaller than the Allen Bradley and their 
>>surface looked as if it had bumps all over it.  Some were also marked IRC.
>>
>>
>>>> ... You can still buy carbon composition resistors from some suppliers 
>>>> but those I have bought, many are way out on value.
>>
>>>justradios.com sends carbon comp resistors in sealable poly envelopes for 
>>>this reason.  They also suggest you store them in the bags.
>>
>>Mouser sells them as does a number of other suppliers, but how long were 
>>they exposed to the environment before being placed in the sealed bag?  I am 
>>not questioning the suppliers integrity here, but I don't want to purchase 
>>those resistors and have to replace the in just a year or so.  In other 
>>words I don't know their history.
>>
>>>> Also be careful to not use metal film in places that deal with RF.   If 
>>>> they couple RF from one stage to another they can foul up the works.  In 
>>>> those places it is best to use ceramic.
>>
>>>I invite more information please:
>>
>>Some of the metal film are a ceramic body covered by metal then a laser 
>>cutter is used to cut excess metal from the body to meet the resistance 
>>requirements.  While very accurate, the cut is in a spiral making a small 
>>inductor.  In some cases you don't want additional inductance in sensitive 
>>circuits.  This is especially true above 30 megacycles but can have an 
>>effect at 20 Mc.  While it may not apply to all circuits, I suggest this 
>>because I don't know the specific application and I do know that ceramic 
>>resistors behave in all circuits.
>>
>>>In the R-390/URR and R-390A, there are Q-spoiler resistors across the IF 
>>>can coils (and possibly the variable RF coils).  These resistors operate at 
>>>IF/RF frequencies.  I KNOW OF VERY FEW OTHER PLACES WHERE RESISTORS 
>>>ACTUALLY OPERATE WITH RF FREQUENCIES.
>>
>>>Here is a beginning list:
>>- Input RF attenuators in modern solid state receivers .
>>- Any other RF attenuators we have from long ago that use desecrate 
>>resistors not disk types
>>- Quite possibly biasing and load resistors in solid state systems.
>>- RF detector probe resistor possibly.
>>- RF noise bridge and SWR bridge components.
>>- Antenna directional coupler parts.
>>- The load resistor in the HP VHF and UHF signal generator “waveguide beyond 
>>cutoff” attenuator, and similar GR and Boonton attenuators.
>>- URM-25 output load devices and impedance adapters, including the “standard 
>>dummy antenna” circuit.
>>
>>>Can any one provide additional examples?  Please?
>>
>>I will not argue with your list, however, I have and will use metal film at 
>>voltage points in the circuits you list below.  I just don't use them in 
>>places you suggested above.
>>
>>So to avoid confusion for those who are not aware or highly technical I make 
>>the blanket statement about using them.  Catalogs will specify the 
>>construction and another thing that many people lack awareness of, the 
>>voltage specification.  I have seen instances of carbon comp resistors used 
>>in 1 KV circuits and this invites problems  Simplifying  the process for 
>>many is what I try to do, maybe not you because of your technical knowledge.
>>
>>>Here are commonly found parts that do NOT operate at RF:
>>
>>- IF and RF amplifier screen dropping resistors are bypassed at the screen 
>>and sometimes before that.
>>- RF and IF amplifier plate dropping resistors are often at the “cold” end 
>>of transformers and are bypassed.
>>- AGC string resistors operate at DC and are bypassed for the IF 
>>frequencies.
>>- Resistors past the detector and in the audio section should not have any 
>>RF on them
>>
>>>- I once encountered a McIntosh ST-75 amplifier that was oscillating at 
>>>ultrasonic or RF frequencies.  It was not healthy!  Audio amplifier plate 
>>>load and other resistors should not be operating at such frequencies, 
>>>although audio folks do report differences in sound because of different 
>>>type resistors being used.
>>
>>Roy
>>
>>
>>Roy Morgan
>>RoyMorgan at alum.mit.edu
>>K1LKY Since 1958
>>
>>Jim
>>W5JO
>>
>>P.S. you have been around almost as long as I Roy.  It has been a fun ride 
>>hasn't it?
>>