For the Lockheed L-1011 project I picked a number of VHF COMM heads made by Gables Engineering, Inc. The unit that we are taking apart in this posting was last inspected in 1989. You can see a more detailed posting about the VHF COMM head in one of my other blog postings.
The unit has a rather simple aluminum enclosure that hast two parts, a top plate and a "U" shaped casing. The top plate comes off by removing 4 screws on top.
After removing the top plate, we can see the the mechanical elements for the left and the right frequency selector as well as the diode board on the back of the unit and the frequency selector switch.
The "U" shaped case can be removed by removing four screws; two on each side of the unit. Once the screws are removed, the case will simply pull off.
Now the VHF COMM radio head can be accessed from all four sides. The next few images illustrate the various component elements:
Top View
Right Hand Side View
Bottom View
Left Hand Side View
The digit drums have their own lighting elements, there are 4 lamps for each drum. Which frequency is being sent back to the actual radio is controlled by the selector switch. The switch not only powers an indicator light above the frequency window, it also powers the respective encoder mechanism on the backside of the frequency selector.
When the toggle switch on the front of the instrument is moved to the right, the right selector is energized, and with the toggle switch thrown over to the left, the left frequency selector is energized.
All encoder signals are being fed through a number of diodes and then to the back connector of the instrument. The diodes keep the left and the right frequency selector isolated. Again, the choice which of the two frequency selectors is being used is made with the toggle switch.
Despite there being 21 diodes for each selector, the frequency of each selector is actually only encoded with 16 bits. 8 bits for the numbers on the left hand side of the decimal point and 8 bits for the digits to the right of the decimal point. For example the frequency of 118.15 and 118.17 are encoded as follows:
- 118.15 010110000011010010011
- 118.17 101100000011010010011
What is the binary encoding, I can't make it out? Which ones of the 21 bits are the extra ones considering the only 16 needed?
ReplyDeleteThe instrument made by Gables Engineering in Florida uses the old ARINC 2-of-5 encoding scheme. If you take a meter to the diodes in the instrument you will see that the digits are encoded in BCD. The bits are actually for most parts sorted correctly in a MSB order going from left to right.
ReplyDeleteThe data output on the connector is parallel, so I guess that would be ARINC 419 ... well, it is specified in one of the 400s. If you need the exact details for the protocol, you can go to https://www.arinc.com/cf/store/catalog.cfm?prod_group_id=1&category_group_id=1
Curd.