Technical Manual

MX Series - 20 Watt UHF AMPLIFIER
TSM 20-311 rev 0: December 1, 1999 7 20 W UHF Amplifier
GENERAL SERVICE INFORMATION
2. Interpreting LARCAN Drawing Numbers (continued).
Other assemblies may be shown as a number of separate assemblies (Parts 1, 2, 3, 4, or 5), on several
sheets of a single drawing. The drawing "parts" all are assembled the same way, but vary in quantity and
type of components.
For vendor components such as resistors and capacitors, often a generic drawing describing basic
specifications, but having many "parts", will be used.
As an example, Drawing 3R152: Resistor, composition, ¼ watt. Drawing 3R152 describes a ¼ W resistor,
but the drawing PART number calls for the value and tolerance of the resistor wanted. The first two digits
after the "P" are the first two significant figures of the resistance, the third digit is the number of following
zeroes in the resistance value, and a J is 5% tolerance, or K is 10% tolerance. Some example part
numbers are: 47 Ω, ±10%, ¼W, is "3R152P470K"; and "3R152P243J" specifies 24 kΩ, ±5%, ¼W. For
resistance values between 1 Ω and 10 Ω, a letter "R" will appear in the part number to indicate the decimal
place; for instance a 5.6 Ω, ±5%, ¼W resistor will be designated as "3R152P5R6J".
Often the letter "R" appears on schematics or in parts lists where it specifies the decimal place when
referring to resistances, such as "51R" or "75R" or "5R6", and sometimes the letter "k" will appear in the
same context, such as "3k3". This practice is deplored by old-timers in our midst who attended North
American tech schools and therefore learned about component values that are specified with the use of
decimal notation, but schematics drawn this way are common in other parts of the world. In a parts list,
moreover, an R followed by a dash and a number indicates a revision, but you will be able to recognize
the difference from the layout framework of the list.
3. The LARCAN Assembly Prefix Numbering System
Because a transmitter is a complex device, a referencing system for unique identification of component
parts reduces the chaotic situation that would otherwise result from the natural numbering system found in
every parts list, in which the first capacitor is designated C1 and the first resistor is likewise designated R1,
but when several assemblies using capacitors or resistors appear together, the entire question then
becomes "which C1 or R1 are we talking about?"
The LARCAN prefixing system and its "undocumented features" represent a beginning and evolving
solution to some of these problems. It begins by assigning a distinct assembly prefix number to each
subassembly.
Prefix 2, the amplifier heatsink assembly, has other subassemblies, which are uniquely prefixed such as
the metering board, which is prefixed 5. This sub assembly could have a C1, an R1, etc. Identification of
each component in full is done by simply adding the prefix number to the component designator, thus 5C1,
5R1, etc.
Prefix designation is great for written communications such as letter or FAX, but when talking about it
during a phone conversation, it is probably more natural to simply say "C1 in the preamp board" or "R7 in
the output amplifier" than to go through the routine of looking up the prefix number.