[Hallicrafters] update SX-101 resistors

[Jim Wilhite]

-----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?