[Hallicrafters] update SX-101 resistors

[Roy Morgan]

On May 31, 2014, at 9:35 AM, Jim Wilhite <w5jo at brightok.net> wrote:

> One last thing on the resistors Bob.  I would use the receiver for about 3-6 months turning it off and on daily and being sure it heats up and cools off many times.  Then check the resistors again.  I have found that they have a habit of checking good then as they heat and cool, drift.  I am not saying they will just that I have seen them do that.

The moisture gets driven out.

> Most of those who exhibit that property were made by IRC.  The later versions of the Allen Bradley do not seem to have that problem as much.

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.)

> ... 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.

> 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 fowl up the works.  In those places it is best to use ceramic.

I invite more information please:  

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?  

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