30v Power supply does not go over 7.9 volts

[jansjobe]

Hi all!

I have a chinese power supply that chokes over 7.9 volts. As i turn the voltage adjust pot further, it shorts and no voltage output. I have removed the diode on the output, but it makes no difference.
I have attached the schematic. Can somebody help?

[FIXITNOW2003]

if you search "power supply K305D" you will find loads off post on these units, lots off Chinese makers but all the same unit lots off faults mostly poor soldering and duff diodes

[jansjobe]

Well, thanks, but I would like to know how it works and what I should look for. Search for K305 gives only this thread as a hit...

[xavier60]

It's a complicated design. The wiper pin on the VR pot should adjust smoothly over a range of 0 to 5 volts.
Check the voltage on pin 2 of U4. Check the supply pin voltages on U1.

[ArdWar]

The abrupt nature looks like U4 pin 9 is crowbar-ed by one of, uhh, things that also connected to pin 8.

Fuck you whoever drew schematics like that...

[xavier60]

The operation of the control circuitry is mainly based on current signals. The circuitry at the CT is unusually weird.

[jansjobe]

I suddenly realize that the schematics is not the same as my actual device. Mine says LW-K305D-1, Rev 20191108.
There is no D4 at all, and U1 is a TL084 amongst other things. Do anyone have a newer (supposedly) schematics?

I do not get 0-5 V on pin 2 of U4, rather 2.19V when VR is at zero. Turning VR pin 2 of U4 rises to 2.35V when output maxes out at 7.8V. Continuing to turn VR makes pin 2 slowly fall again, until it is 1.25V when VR is at maximum.

[xavier60]

I suddenly realize that the schematics is not the same as my actual device. Mine says LW-K305D-1, Rev 20191108.
There is no D4 at all, and U1 is a TL084 amongst other things. Do anyone have a newer (supposedly) schematics?

I do not get 0-5 V on pin 2 of U4, rather 2.19V when VR is at zero. Turning VR pin 2 of U4 rises to 2.35V when output maxes out at 7.8V. Continuing to turn VR makes pin 2 slowly fall again, until it is 1.25V when VR is at maximum.

Not much can be made of that without seeing the surrounding circuitry. The pot should be tested out of circuit. Look for a large change of resistance across the outer pins while turning it. Do the test in circuit first, while power is off of course.

[jansjobe]

I have already tested the VR pot out of circuit, and it works as it should.