What causes the inaccurate exposure time of individual gears in medical X-ray photography?

In clinic, KB500 X-ray machine often breaks down. Two cases of troubleshooting of KB500 X-ray machine are reported as follows.

1 fault 1

The fluoroscopy is normal. When taking photos with Class III tube, sometimes it is not exposed, sometimes the exposure time is normal, and sometimes the exposure time is prolonged, but there is no phenomenon that the exposure time is shorter than the preset time. Moreover, the above phenomenon is irregular, so the fault still exists when taking pictures with I-II tubes.

1. 1 During the overhaul, remove P 1P2 first, and observe the preview of JX relay when taking photos. It is found that JX preview is as normal as exposure, sometimes it doesn't work, and sometimes the preview time is prolonged. Based on the circuit diagram analysis, the poor performance of controllable SeR 103 is considered first. No damage was found after the removal of SeR 103, and the failure of replacing the new tube with 3CT5KC still exists. Analyzing the oscillator circuit, it is found that the failure will not be caused by damaged components or poor performance. It is also important to consider whether the time-limited resistor group and resistor are damaged and the measured resistance is normal. Finally, when the contact on the turntable of the time selector turns to a certain position, sometimes it has poor contact, a certain resistance, sometimes it has no contact, and sometimes it has good contact. When contact is good, the exposure time is normal. When there is no contact, it cannot be exposed. When the contact is poor and the resistance shield is greater than the original resistance, the exposure time is prolonged. The above phenomenon is not fixed at a certain time, and the gear is irregular.

After inspection, it was found that individual contacts on the time selector turntable were worn, resulting in poor contact or no contact.

1.2 Cause analysis The above failures are caused by cumulative exposure times of KB500 X-ray machine and long-term wear of contacts on the turntable of time selector.

2 Fault 2

Perspective is working properly. When taking pictures with Class III tube, there will be underexposure occasionally, which is manifested as: the pointer of the milliammeter does not reach the required number, and the indicator of the milliammeter is normal. X-ray was taken under the condition of insufficient induced light, and the image was very bright but the outline was clear, and the kilovolt value was normal. The fault is the same as before, and the initial inspection found no reason. Continue to use the machine and observe the fault carefully. After several months of use, faults frequently occur, 7 ~ 8 times a day.

2. 1 Remove P 1P2 during maintenance, and observe the preview of TX relay. Nothing unusual. Connect a light bulb of 150W at P 1P2 as a load, select different exposure time, and observe the light emission of the light bulb. Sometimes it's normal, sometimes the light-emitting time is very short, much less than the preset time. The brightness of the light bulb is adjusted by photographic kilovolts. Analyze the circuit diagram and previous maintenance experience, and consider the fault of UJ 10 1 single crystal diode in oscillator circuit. The electrode voltages of UJ 10 1 b.c.e are all normal. No damage was found when the lamp tube was removed, and a new BT35C single crystal diode was replaced. The machine is normal.

2.2 Cause analysis Although the BT35C single crystal diode is not damaged, the lamp tube is aging after long-term use, and the change of its performance parameters leads to the shortening of the lamp tube conduction time, which affects the short conduction time after the SCR is triggered and shortens the exposure time.