1. Home
  2. Electronics Repair
  3. Problem with Tekpower PSU

Problem with Tekpower PSU

Problem with my Tekpower PSU:

Here's a teardown on YouTube:

Problem is it's developed a fault where the current meter always reads 0 when in constant-voltage mode. Everything else seems to work fine and it's putting out the right voltages, current limit works fine, when load current exceeds the set point it current limits OK and shows the correct value. Drop back and the display reads 0, again.

Here's the schematic (PDF), logic board only, unfortunately but the power board seems to be working OK.

Overcurrent detection seems to work by voltages IS- and IS+ coming in from the off-board current shunt and sent to pin 19 of the PIC, which I believe is a comparator-interrupt pin, and that seems to work ok, and the voltages out of that op-amp and across the current shunt look OK.

Constant current mode seems to work by using TL082 section "N18BB" as a comparator open-loop, and then the loop is closed around back from the ADJ line that goes off-board to the power regulator board, and the current-set DAC output from the PIC, comparing the two inputs against each other.

Constant-voltage mode works similarly the voltage-set DAC value from the PIC is compared against the voltage at the output and a line is asserted when they match. The output voltage in CV mode looks OK.

It's not clear to me though how what the current actually is, in constant-voltage mode, is sensed by the PIC, as I don't immediately see a pin on it where the current-shunt voltage is being read by the uP, so it's hard for me to know where to look for the problem as nothing obvious is amiss, and continuity on all the connections between the boards to the PCBs seems OK.

Reply to
bitrex
Loading thread data ...

Only other thing I notice is that the logic board ground and the PSU positive output seem to be connected, with the logic board fed by a separate isolated tap on the transformer. And it looks like one of the ADC inputs on the PIC is also connected to the logic board ground, so maybe some kind of differential ADC arrangement but how to diagnose still eludes me.

Reply to
bitrex

Have you replaced the OP07?

Could be one output is unable to pull in one direction.

Or N18A . . .

Biasing to V13 doesn't look right, in the schematic, so you may have to deal with some typos there.

RL

Reply to
legg

Yeah the diagnostic process is definitely not helped by the sloppy and confusing schematic. Before I jump into op-amp swapping I'm trying to understand how the current-sensing works in CV mode and is read by the processor so maybe I can trace it back, but I'm not very familiar with the PIC. Maybe it's reading the value from the "CCP" pin somehow that "VIS" connects to, pin 19, but the impression I got from the datasheet is those are some kind of comparator inputs for the over-current/over-voltage sensing, not ADCs. Not sure.

Like a lot of gear from China at this price point it's designed to meet a budget, but not badly designed to meet a budget and otherwise neatly constructed, but then often let down by some sketchy soldering on the connectors and through-hole parts. Often can "fix" this stuff by tapping the board to find the intermittent joint or just re-touching all the connectors but my first round of that didn't work, this time.

Reply to
bitrex

You can always measure IC output pin voltages during the discontinuity, before pulling them.

In a close-packed assembly, this isn't always much fun - but once exposed, they're easier to replace anyways.

V13 is just digital.

RL

Reply to
legg

Still no luck on figuring out this one. :( Tested the current op amp output with respect to the logic board ground, which is the output positive terminal, to avoid doing large voltage divisions to feed the

3.3V uP at the higher voltage settings I'm guessing. Output looks okay, and the constant current mode works correctly, it regulates down the output voltage if the load goes over the current set point. so the "ADJ" pin to the power board, transistor "V12" must be outputting signal OK. so the current sense amp from the shunt and DAC and all that must be able to enter closed-loop operation.

Still, the output indicator always reads 0 mA when in CV mode. Ugh. Where does the current sense amp voltage actually enter the PIC via an ADC? Can't find it anywhere. Maybe there's a processor fault or something, unfortunately.

Reply to
bitrex

DAC input current sense pin is VIS (pin19); so C22 short, R30 open, or contact trace problems.

?

RL

Reply to
legg

Thanks legg, I'll check those.

Reply to
bitrex

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.