Lorentz Force Apparatus Current Adjustment Stuck at Full

[Rjc987]

Hi guys! I have some Lorentz Force apparatuses (basically an electron tube and focusing coils connected to a Chinese adjustable voltage and current power supply that looks pretty off the shelf) that have failed in the power supply somewhere. The current adjustment is stuck at full essentially. The current adjustment has a 1k pot that goes to a 202 trimmer, an lm324n op amp and an L7809cv voltage regulator, then from the op amp to a c3175 NPN through a reverse biased zener to a L7812CV voltage regulator, and over to a cdd102c power transistor. If the trimmer is maxed out, turning the knob all the way up does increase the current from 2.5a to 3A but as you turn it down, it stops decreasing at 2.5a and there’s no more movement as the knob goes to zero. I’ve swapped all the transistors other than the power transistor from a working unit as well as the op amp and the problem remains. I’ve also tested all the diodes pots and trimmers (all good). What could be going on here? I assumed it was a bad transistor letting full current through but now I’m at a loss. I’ve added board pics for reference (back flipped to mirror front)

[Rjc987]

I just managed to track down the schematic. See attached. Sorry for the bad quality. I don’t get what’s going on. I swapped out the 7809, the LM304 and the C3175 (labeled as c2601 in the schematic) and the problem persists. It’s working somewhat but won’t adjust downward. I don’t see what else it could be. Any advice on tracking things down?

[Rjc987]

Here’s a translation of the operating principle of the current adjustment:

“The secondary voltage of TI transformer No. 9 and 10 is 20 volts, which is bridge rectified by V1-4, filtered by CI capacitor, and supplied to the excitation coil LI, L2, and R3 after being stabilized by the current stabilizing tube VS. They are the sampling resistors of the stabilizing circuit, and their sampling signals It is sent to the feedback end of the operational amplifier N2, and after amplification, it drives the VII transistor to control the base of the current regulator to keep the current stable. Its size is determined by the potential of the N2 input end. By adjusting the RP2 potentiometer, the stable current can be between 0- 2.5A changes. Terminals X3 and 7 of 15 Change the direction through switch Stb.Sle and connect to the excitation coil. The current can be connected clockwise or counterclockwise, and the LED voltage can be connected through the Sta switch to indicate the direction of the excitation current. The V10 light-emitting diode is used as a power-on signal light, installed on the instrument panel, and powered through R2. TI transformer No. 7 and 8 secondary 9V AC, bridge rectified by V6-9, NI three-terminal voltage regulator block outputs 9 volt DC voltage, specially designed for regulator control circuit op amp N2 and tertiary tube V11. On the instrument control and power supply combination chassis, fix a pair of excitation coils L1 and L2. In the center of the excitation coil, install the Lorentz force tube V20. After adding appropriate voltage to each electrode of the Lorentz force tube, a beam of electrons is emitted, and the linear trace of the movement of the electron beam can be seen.”

The op amp and associated circuitry seems to be working fine, when the current supply is off, it reads in+ = 0, in- = 0, out = 7.6, when the current is on but the pot is at zero, it reads 0v, 1.2v, 188mv out, and at full, 1.4v, 1.4v and 6.47v out. But it’s not controlling the current out to the coils, they just stay at full. Why??