Analysis and maintenance: LC-4A UV detector consists of optical system, liquid delivery system, UV receiving and amplifying circuit. The principle is that the D2 lamp emits ultraviolet light and is reflected by the lenses M1 and M2 to the grating, and the gratings separate light of different wavelengths. The ultraviolet light passes through the cuvette to the violet sensitive diodes D1 and D2, and D1 and D2 are measurement and reference diodes, respectively. Different concentrations of samples at the same wavelength absorb different ultraviolet light, and different currents are generated on the D1 violet sensitive tube. This current is amplified by A101 and A102 to generate different voltage signals. The baseline drift and interference are large, for the following reasons:

(1) The performance of the purple sensor tube is unstable

(2) The liquid in the cuvette is bubbled  

(3) Poor performance and asymmetry of the amplifier

(4) The power supply voltage of the amplifier and the power supply voltage of D2 are unstable, etc.

Symptom 2: When the 1mV calibration is performed, the peak value is negative.

Analysis and overhaul: There are two reasons for this fault. 1. UV detection circuit, ie current amplifier A101, A102, logarithmic amplifier A201, A202 fault; 2. Optical path part fault.

     Disconnect D2, close the optical path, adjust the zeroing resistance without signal, including coarse adjustment, fine adjustment knob, see the terminal display baseline adjustable zero, indicating that the circuit part works normally, the problem is in the optical path part. Clean the cuvette with absolute ethanol and adjust the manual wavelength adjustment knob to zero spectrum to see if the spot is in the center of the entrance in front of the cuvette. The vertical position offset affects the accuracy of the wavelength, and the left and right position offset affects the peak value. The position of the spot is up and down, but the left and right positions are not allowed, and it is biased toward the side of the reference pool. First adjust the front and rear position of the D2 lamp so that the spot is aligned with the positioning holes on both sides of the entrance slit, and then adjust the fixing screw A of the mirror M1 to align the upper and lower positions of the spot with the positioning hole. Adjust the M2 position, that is, adjust the fixed M2 screw so that the spot is in the center of the entrance in front of the cuvette. At this time, the optical path is adjusted. After correcting the wavelength, everything is normal after booting.

     When inspecting, first drain the liquid in the cuvette to eliminate interference caused by air bubbles. Open the lid of the sealed violet sensor tube and the cuvette. Under the illumination of natural light, the output of the current amplifier, Mo, Ro, is 14.8 VDC and stable. Re-close the sealing cover, disconnect the D2 lamp power supply, measure Mo, Ro under dark current, the voltage is 0V, so the purple sensitive tube is normal. The power supply voltage is measured. The D2 anode voltage is 90VDC and the filament voltage is 3V. The positive and negative power supplies of the amplifier are +15.12VDC and -15.09VDC, respectively, which are relatively stable, indicating that the fault is in the amplifier section. Replace the D1 and D2 with a 1MΩ resistor and short-circuit the A101 and A102 amplifier inputs. See the CRT display and there is still interference at the baseline.

It is measured that Mo and Ro respectively have less than 1V and unstable signals. The signal generator is used to input the signal of 10mV and 20Hz, and the signal of Mo and Ro output is measured by the oscilloscope. It can be seen that the signal is distorted and unstable. Check that the amplifier circuit components C101, R101, C102, and R102 are normal. Therefore, A101 and A102 are faulty. This component is a high precision, low drift integrated op amp, model OPA104CM. Replace the AD515J with the performance op amp. When the amplifier input is 0V, adjust the resistors R103 and R104, that is, the compensation resistors, so that the amplifier outputs Mo and Ro are 0V±0.2mV. After the replacement is commissioned, everything is turned on normally.