shitty chinese psu kit

[m3vuv]

would 2x buw49 transitors work ok as the pass elements in this?,s look beefy items 30a a pop.?

[m3vuv]

would r4 deliver enough current to drive the driver tip 42c enough to drive all the current pass transistors (6x 2sd1047) to full power then or need to be a lower value or higher wattage?

[m3vuv]

So anyone should r4 be changed for a bigger wattage to drive the extra pass transistors,be good to know before i put it back together.Tia

[magic]

Dunno, it all depends on output current and β of the transistors. Q1 has to provide base current to Q2,Q3 and R4 needs to provide enough base current to Q1. It should be simple calculation using the datasheets.

[m3vuv]

Well having another go at this,found out i had the 7812 and 7912 regs mixed up,now output goes 21v max but sinks to about 8v halfway tru the volts pot halfway point then rises againe,cc light is lit all the time,getting volts out but no current,u1a output is vpot fully ccw=10.5v,turns negative halfway thru its travel then rises up agine,u1b  sits at -10.9 and doesent change,any help welcome! TIA.

[m3vuv]

forgot to mention i replaced both regs with new ones.

[xavier60]

Well having another go at this,found out i had the 7812 and 7912 regs mixed up,now output goes 21v max but sinks to about 8v halfway tru the volts pot halfway point then rises againe,cc light is lit all the time,getting volts out but no current,u1a output is vpot fully ccw=10.5v,turns negative halfway thru its travel then rises up agine,u1b  sits at -10.9 and doesent change,any help welcome! TIA.

Just considering the CC LED being always on, it shouldn't when the CC pot is at a high setting with no PSU load.
The input difference should such that the opamp's output is fully high, close to 12V.
I assume that the control rail voltages have been  checked and are correct?

Edit: Im referring to the CC opamp's inputs and output.

[m3vuv]

well just checked voltage over pins 1 and 8 on both opamps,u1=24v u2=11.9v.

[xavier60]

well just checked voltage over pins 1 and 8 on both opamps,u1=24v u2=11.9v.

Best to refer to them as CV and CC opamps. Are you measuring WRT to the PSU's positive output terminal?

[m3vuv]

was measureing over pins 4+ 8 on both
Heres some more readings both pots fully cw ie full volts+ current opamp power rail=24v,u1 a out=1.1v u1b out= -22.8v
u2 power rail=11.9v out a=0v out b=0v
this is mesuered with ref to both opamp pin 4 ground pins.

[xavier60]

That's difficult to follow. Take all measurement With Respect To the PSU's positive terminal.
But there seems to be something wrong with the CC opamp. Its output should be close to 12V.
Has the opamp been replaced recently? What voltages are on the CC opamp inputs.

[m3vuv]

ok so are you saying +meter lead to + psu output and neg lead on opamp outputs?,also where do the pots need to be?,i swapped one opamp as it was getting hot,cant recal what one tho i think it was u2,cant be sure the replacement is ok tho.

[xavier60]

ok so are you saying +meter lead to + psu output and neg lead on opamp outputs?,also where do the pots need to be?,i swapped one opamp as it was getting hot,cant recal what one tho i think it was u2,cant be sure the replacement is ok tho.

Black meter lead to the PSU's positive terminal. Take measurements with the red, noting the sign if minus. Both pots at half way.
The CC loop can be disabled by removing the LED to check if the CV loop is ok, but you must be careful to not short circuit the output.

[m3vuv]

will do it later as its 3.50am here.

[m3vuv]

Ok this is what i get(black meter lead to psu +ve terminal),opamp 1A ie pin1 measures +5.93volts
1B ie pin 7 measures 10.9v ,that is with both pots at halfway point.

[xavier60]

Ok this is what i get(black meter lead to psu +ve terminal),opamp 1A ie pin1 measures +5.93volts
1B ie pin 7 measures 10.9v ,that is with both pots at halfway point.

That indicates multiple faults. The opamp with the lowest output voltage has control of the output stage which is opamp 1A which should be the CC opamp. And the +5.93V is abnormally high.
Tell us what happens to the output voltage with a small load, 100mA or so,
Then measure voltages starting at the anode of D17 and every point along the drive path.
 

[m3vuv]

is that with meter -ve lead on the psu+ve output againe ?

[xavier60]

is that with meter -ve lead on the psu+ve output againe ?

Yes, always. You can measure the PSU output voltage with the red lead probing the negative output terminal. If you don't have a 2nd meter. 

[m3vuv]

I have a few avo mk8 meters but they take up too much bench space so use my fluke 83,heres some readings D17 anode =11.36v,D17 cathode=11.83v r4=12.14v, one leg,other leg is 11.83v,7812 out is 11.76v.,thats with a load on the psu output,black meter lead to psu +ve out,one thing that i notice is with the load connected the led lights,goes out with no load.

[xavier60]

I have a few avo mk8 meters but they take up too much bench space so use my fluke 83,heres some readings D17 anode =11.36v,D17 cathode=11.83v r4=12.14v, one leg,other leg is 11.83v,7812 out is 11.76v.,thats with a load on the psu output,black meter lead to psu +ve out,one thing that i notice is with the load connected the led lights,goes out with no load.

There is too much information missing.

I need the voltages at every point along the drive path on the way to the output.

[m3vuv]

what info do you need?,thats all points on the path from d17 to the 7812 output,what else do i measure?

[m3vuv]

just tried measuring the psu output with the load etc,no load =15v,loaded 0v,same with led disconected is the same.ie goes to zero with a load,thats the psu output.

[xavier60]

Starting at the Base of Q1, work your way out to the output, measuring every point.
After the Base of Q1, the next point will be the Emitter of Q1.

While it's loaded.

[m3vuv]

did i say the led goes off with a load,its on without,sort of working backwards.

[magic]

https://www.eevblog.com/2014/04/06/eevblog-600-opamps-explained/