SAFETY FIRST! DANGEROUS VOLTAGES ARE PRESENT DURING THIS TEST. EXTREME CAUTION MUST BE TAKEN!

Question: How do I troubleshoot the SCR Power control system?

Answer: The SCR Power control system consists of an SCR, gating control card, and drive transformer.

A dc control voltage drives the gating control cards. This dc voltage is provided by the Power Monitor/Control card in the 816R-C series and by the A8 Control Card in 816R-B and older Continental Electronics and Collins transmitters. A normally functioning transmitter should have from 4 to 6 volts at normal power output. The manual power control will vary this voltage from about 0 to 8 volts dc. Locate this voltage on 816R-C models at Test Point 3 on the Power Monitor/Control card and on the A8 Control Card orange test point on remaining models.

Under conditions of plate breaker tripping there may be a problem with the SCR gating cards. First, locate the gating cards. On 816R-C they are in the front card cage on the left. On the remaining 816R's they are located in a card cage assembly located in the center rear bay. On Collins transmitters they are accessible from the front by removing the Control Panel.

Next, remove two of the three gating cards, restore power, and turn filaments and plates on. In Manual Power Control operate the Power Control Adjust and observe that the plate voltage, plate current, and power output change smoothly. Note the maximum plate voltage. If the gating card, drive transformer and SCR under test are good there should be no breaker trips, no unusual noises from the transformers and no building lights blinking. Repeat the process for the second and third cards using this position. Repeat the above process in the second and the third position.

Each card and each position should result in the same plate voltage, plate current and power out. If all cards work in one or two positions then cards are good and problem is a bad drive transformer and or SCR. Experience indicates that the SCR's are the least likely to fail.