[ARC5] RAK-5 and RAL-5 Receivers

[Richard Knoppow]

    The specs without the balast tube is 60 Watts. Not a lot but 
keep in mind that the overall heat dissipation vs: volume does 
not tell you about hot spots. The resistors in the mod order are 
more than doubled in size and are changed from composition to 
wire wound. Composition resistors are notorious for changing 
value when heated. Depending on the stability of the particular 
resistor it may not take much heat. Modern carbon film and metal 
film resistors are far more stable and generally will stay within 
spec indefinitely, especially metal film. This is also true of 
wire-wound resistors which never drift unless mechanically 
damaged. Wire wound resistors have the main disadvantage of being 
reactive. Although there are designs to minimize parasitic 
inductance they are used mainly for resistance standards and 
special purpose resistors and are still not generally 
satisfactory for AC. Carbon composition resistors were generally 
supposed to relatively non-reactive but in fact are more reactive 
than film type resistors (can be proven with an RX Meter or other 
means of measuring low Q reactances).
    Carbon composition resistors are made from a material which 
is not continuous. Rather, it begins as a slurry of carbon 
particles in a non-conductive binder. The resistance value 
depends on the amount of conductive material (carbon particles) 
in the mix and probably on their size. After a time the particles 
begin to loose contact with each other which increases the 
resistance of the unit. Heat is the primary cause. In fact, the 
reason carbon comp resistors are never better than 5% spec is 
that even the heat of soldering can change them more. While they 
are superior for a very few uses (some pulse circuits) other 
types are better for general use. Carbon film and metal film 
resistors used to be quite expensive and were considered only 
where precision parts were necessary. Modern manufacturing 
methods have reduced the cost so that they are quite cheap now 
and, in general, have better characteristics than composition 
resistors. Where the ultimate in stability is desired metal film 
resistors are best, where somewhat less stable resistors are 
acceptable but some cost saving is desirable carbon film is 
usually satisfactory. There are some other types such as metal 
oxide film resistors but they have the same benefit of stability 
as the deposited film types.
    Wire wound resistors are still the type of choice where 
considerable power must be dissipated but are primarily for DC.
    My only consideration about these guys is that there is no 
way I could ever lift one.
On 4/28/2019 2:53 PM, Roy Morgan wrote:
> EXCESSIVE HEAT?  I wonder.  The whole receiver consumes only a few watts, and it’s quite big with lots of surface area to dissipate that “heat”.
> 
> It has been a long time since the RAL and RAK here have been running, but I expect a short time till they run again.  I will notice both the heat and the values of those resistors.  MAYbe those resistors themselves are running up toward their rated dissipation.  I think unlikely though.
> 
> Thanks for the info.


-- 
Richard Knoppow
1oldlens1 at ix.netcom.com
WB6KBL