Re-calibrating bench PSU...

B

bitrex

Guest
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

<https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/>

Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

<https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0>

sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
<https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?
 
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0

sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?
Click to expand...

Here\'s a schematic of the display/current and voltage sense logic board:

<https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image>
 
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0

sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?
Click to expand...

Here\'s a schematic of the display/current and voltage sense logic board:

<https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image>
 
On 7/17/2020 11:05 PM, sea moss wrote:
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
Click to expand...

Meh, can\'t find anywhere on it \"active.\" Retouched the solder joints
that didn\'t help. Have to pull it and see what\'s what.
 
On 7/17/2020 11:05 PM, sea moss wrote:
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
Click to expand...

Meh, can\'t find anywhere on it \"active.\" Retouched the solder joints
that didn\'t help. Have to pull it and see what\'s what.
 
On 7/17/2020 11:05 PM, sea moss wrote:
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
Click to expand...

Intermittent pot, looks like. Resistance drops to 1.5 ohms as you turn
it at various points. I\'ll try bodging in a better 1k trimmer this and
weekend see how it do.
 
On 7/17/2020 11:05 PM, sea moss wrote:
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
Click to expand...

Intermittent pot, looks like. Resistance drops to 1.5 ohms as you turn
it at various points. I\'ll try bodging in a better 1k trimmer this and
weekend see how it do.
 
On 7/17/2020 11:05 PM, sea moss wrote:
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
Click to expand...

Intermittent pot, looks like. Resistance drops to 1.5 ohms as you turn
it at various points. I\'ll try bodging in a better 1k trimmer this and
weekend see how it do.
 
On Fri, 17 Jul 2020 23:11:22 -0400, bitrex <user@example.net> wrote:

tweak it at the low
end, turn the voltage up to max and it\'ll read high there, tweak it down
a bit, turn it back down to the low end and adjust it again there, and
just kinda do the best you can
Click to expand...

I can\'t read your schematic, but normally, there should be two pots
for calibration. One for gain and one for offset.

You adjust the offset at a low voltage, and then turn the voltage to
high. If it reads high, you reduce the gain and then go back to low
and readjust the offset. Keep going back and forth until you are
happy.
--
RoRo
 
On Fri, 17 Jul 2020 23:11:22 -0400, bitrex <user@example.net> wrote:

tweak it at the low
end, turn the voltage up to max and it\'ll read high there, tweak it down
a bit, turn it back down to the low end and adjust it again there, and
just kinda do the best you can
Click to expand...

I can\'t read your schematic, but normally, there should be two pots
for calibration. One for gain and one for offset.

You adjust the offset at a low voltage, and then turn the voltage to
high. If it reads high, you reduce the gain and then go back to low
and readjust the offset. Keep going back and forth until you are
happy.
--
RoRo
 
On Fri, 17 Jul 2020 23:11:22 -0400, bitrex <user@example.net> wrote:

tweak it at the low
end, turn the voltage up to max and it\'ll read high there, tweak it down
a bit, turn it back down to the low end and adjust it again there, and
just kinda do the best you can
Click to expand...

I can\'t read your schematic, but normally, there should be two pots
for calibration. One for gain and one for offset.

You adjust the offset at a low voltage, and then turn the voltage to
high. If it reads high, you reduce the gain and then go back to low
and readjust the offset. Keep going back and forth until you are
happy.
--
RoRo
 
On 7/18/2020 8:22 AM, Robert Roland wrote:
On Fri, 17 Jul 2020 23:11:22 -0400, bitrex <user@example.net> wrote:

tweak it at the low
end, turn the voltage up to max and it\'ll read high there, tweak it down
a bit, turn it back down to the low end and adjust it again there, and
just kinda do the best you can

I can\'t read your schematic, but normally, there should be two pots
for calibration. One for gain and one for offset.

You adjust the offset at a low voltage, and then turn the voltage to
high. If it reads high, you reduce the gain and then go back to low
and readjust the offset. Keep going back and forth until you are
happy.
Click to expand...

\'Fraid there are only two adjustment pots on the sense amp board total,
for both current and voltage signal input to the uP ADC.

Looks like the output voltage and voltage across the current sense shunt
are both fed to a LM358 gain/buffers and then the trimpots are the
series component of a voltage divider hung off the output of the \'358
buffers, prior to going into the ADC.

The trimpot in the current sense line was bad. Replaced that and it\'s
looking better, I can tweak it to adjust the current display value.
Problem is now it won\'t go high enough, 10 volts across 50 ohms reads
100 mA and I can use the trimpot to push that up to 120 mA and down to
0, but not up to 200mA where it needs to be.

There\'s a button on the PCB that zeros out the current display so now I
guess I have to figure out where the trimpot and other controls need to
be set so I can hopefully zero it appropriately and push that up to the
correct value
 
On Friday, July 17, 2020 at 2:59:31 PM UTC-7, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/

Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0

sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?
Click to expand...

My guess is the second trimpot is used to calibrate the current limit knob. Try applying a short to the output through an ammeter and see if turning the trimpot changes the display.
 
On Friday, July 17, 2020 at 2:59:31 PM UTC-7, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/

Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0

sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?
Click to expand...

My guess is the second trimpot is used to calibrate the current limit knob. Try applying a short to the output through an ammeter and see if turning the trimpot changes the display.
 
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image
Click to expand...

I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(
 
On 7/17/2020 10:56 PM, sea moss wrote:
On Friday, July 17, 2020 at 2:59:31 PM UTC-7, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/

Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0

sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?

My guess is the second trimpot is used to calibrate the current limit knob. Try applying a short to the output through an ammeter and see if turning the trimpot changes the display.
Click to expand...

Yep that is right. Schem:

<https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image>

Display reads the correct voltage but wrong current. The voltage across
current sense output cap C10 never budges much no matter how long I spin
that trimpot so looks like I\'ve also got a shorted pot, there.

There are two more pots on the control board to adjust the current and
output voltage limits of the knobs so I guess you could \"mod\" this into
a higher voltage supply straightforwardly, so long as you stayed within
the limits of the associated components and tweaked the current limit
down to keep max output power limit the same.
 
On 7/17/2020 10:56 PM, sea moss wrote:
On Friday, July 17, 2020 at 2:59:31 PM UTC-7, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/

Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0

sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?

My guess is the second trimpot is used to calibrate the current limit knob. Try applying a short to the output through an ammeter and see if turning the trimpot changes the display.
Click to expand...

Yep that is right. Schem:

<https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image>

Display reads the correct voltage but wrong current. The voltage across
current sense output cap C10 never budges much no matter how long I spin
that trimpot so looks like I\'ve also got a shorted pot, there.

There are two more pots on the control board to adjust the current and
output voltage limits of the knobs so I guess you could \"mod\" this into
a higher voltage supply straightforwardly, so long as you stayed within
the limits of the associated components and tweaked the current limit
down to keep max output power limit the same.
 
On 7/17/2020 10:56 PM, sea moss wrote:
On Friday, July 17, 2020 at 2:59:31 PM UTC-7, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/

Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0

sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part and
the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is the
display is off-calibration, and I doubt I\'ll ever find a service manual
for the thing. On the controller PCB there\'s a button which seems to
reset the current display to zero, it reads about 150 mA on all voltage
settings with nothing connected otherwise. There are also two smt pots
labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage display value,
and RP2 does...not very much as far as I can tell. They seem to interact
with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?

My guess is the second trimpot is used to calibrate the current limit knob. Try applying a short to the output through an ammeter and see if turning the trimpot changes the display.
Click to expand...

Yep that is right. Schem:

<https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image>

Display reads the correct voltage but wrong current. The voltage across
current sense output cap C10 never budges much no matter how long I spin
that trimpot so looks like I\'ve also got a shorted pot, there.

There are two more pots on the control board to adjust the current and
output voltage limits of the knobs so I guess you could \"mod\" this into
a higher voltage supply straightforwardly, so long as you stayed within
the limits of the associated components and tweaked the current limit
down to keep max output power limit the same.
 
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(
Click to expand...

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
 
On Friday, July 17, 2020 at 7:58:36 PM UTC-7, bitrex wrote:
On 7/17/2020 10:22 PM, bitrex wrote:
On 7/17/2020 5:59 PM, bitrex wrote:
I need another PSU on my bench and I remembered I had this
thing in my junk closet, a SMPS-based supply from www.mpje.com.

https://www.summet.com/blog/2012/01/22/review-mpja-9615-0-30-volt-0-3-amp-bench-power-supply/


Something was wrong with it I couldn\'t remember but all I recalled was
this is what the power PCB looked like, from the \"factory\" mind you:

https://www.dropbox.com/s/65km6pbq8wzch54/IMG_20171005_104357775.jpg?dl=0


sweet baby Jesus I don\'t know what they did to it. Looks like a return
someone hacked back together in a shed in China. I plugged it in again
and the NTC immediately smoked and cracked.

I cleaned up the board real good, retouched the solder joints and
replaced the diodes (smaller TO-220, MURF1620CTG) with the same part
and the switch (2SK1358) with a STW9NK90Z:
https://www.mouser.com/ProductDetail/511-STW9NK90Z>, and the NTC with
an appropriate part.

Seems to work OK now and looks a lot better inside. Only problem is
the display is off-calibration, and I doubt I\'ll ever find a service
manual for the thing. On the controller PCB there\'s a button which
seems to reset the current display to zero, it reads about 150 mA on
all voltage settings with nothing connected otherwise. There are also
two smt pots labeled \"RP1\" and \"RP2\". RP1 seems to adjust the voltage
display value, and RP2 does...not very much as far as I can tell. They
seem to interact with each other somewhat.

Anyone got any ideas on what combination of voltage, and current limit
settings, load, and pot tweaks I should try to calibrate this thing?



Here\'s a schematic of the display/current and voltage sense logic board:

https://www.eevblog.com/forum/projects/cheap-benchtop-psu-upgrade/?action=dlattach;attach=398874;image


I can wrangle current sense trimpot \"RV1\" for days and the voltage
across C10 never changes so shorted pot, looks like. :(
Click to expand...

Does the trimpot have a mechanical start and stop point, beyond which you can\'t turn it? Some of them will allow you to keep turning forever, and once it hits the end of the resistance range the value doesn\'t change anymore as you continue to turn. So you might have to back up by like 20 revolutions to find the active range.
 
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