On Sat, 15 Aug 2020 01:38:58 -0400, bitrex wrote:

On 8/11/2020 3:38 PM, legg wrote:
On Tue, 11 Aug 2020 12:12:46 -0400, bitrex wrote:

On 8/11/2020 8:36 AM, legg wrote:
On Tue, 11 Aug 2020 03:54:01 -0400, bitrex wrote:

On 8/11/2020 3:41 AM, bitrex wrote:
Problem with my Tekpower PSU:

https://tekpower.us/downloadable/download/sample/sample_id/1/

Here's a teardown on YouTube:

https://youtu.be/Nfe28vGW6Ik

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.
snip

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


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.

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


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.

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

?

RL