Chinese Hiland 0-30V/Audioguru PSU build (down the rabbit hole)

[Audioguru]

On your Rev 6 the bars on the diodes are on R3 side ?

Yes, because they limit the negative voltage to two series forward voltage drops. When I replaced a 5.6V zener diode that was in the original circuit with the diodes then I did not know their current so I guessed a few mA but since you have 0.75V each then their current is about 15mA which is fine since their maximum allowed continuous current is 300mA.

 

[bushtech]

Well, I'm so relieved something is correct. Thank you

 

[bushtech]

I was hoping that would sort U3. I think pin 6 on U3 s/b 24-28V but I am only seeing 1.7V

 

[Audioguru]

The U3 output pin 6 will be 1.7V when its (-) input pin 2 is higher than its (+) input pin 3. But pin 2 connects to 0V though 0.47 ohms R7 so it is 0V when the project has no load and pin 3 has a voltage usually positive for the current regulation setting. How can its pin 2 be +3.2V? Then the project's output current is 3.2V/0.47 ohms= 6.8A (!) or U3 is fried. Maybe R21 feeding pin 2 is disconnected.

 

[bushtech]

Just checked R21, connected to U3 pin 2 o/s and connection to C4 other side. I have 2 x spare TLl2141 but I am too scared to try them for fear of blowing them.

Oh, and led still lit

 

[bushtech]

and U3 getting hot still

Starting to despair

 

[Audioguru]

Simply remove U3 and post the voltages of its pin pads. If pin 2 drops to 0V and its pin 4 is at -1.5V then its replacement might survive. Maybe it was mounted backwards so that its pin 6 was on the pin 2 pad?

 

[bushtech]

Pulled U3 and all voltages at 0 except pins 6 and 7 at 32.4V

Busy checking to see why that is

 

[Audioguru]

With U3 removed then isn't its pin 4 pad at -1.5V?
Your voltage on pin 7 is normal. Your voltage on pin 6 makes the IC hot and might destroy it. U3 is destroyed anyway because its pin 2 input had a voltage that is gone when the IC is removed.

How can the 10V zener diode D3 survive if U3 was shorted and causing its pin 2 to be +3.2V then was producing 6.8A though R7 and D13?? Maybe instead you had a solder short. Are you using 63/37 rosin-core electronics solder or are you using plumbers solder that makes whiskers of solder all over the place?

 

[bushtech]

With U3 removed pin at .3V or -.3V depending on probe orientation.
Been hunting that voltage on pin 6 all day. Can only be coming from base of Q3 as far as I can see.
If 10V zener D13's job is to drop voltage by 10V then it's still working.
I am using resin core solder, don't know mix. Solder short unlikely, have inspected board till my eyes cross.

 

[bushtech]

I don't get this. 42V gets to D13, 32V leaves D13, then passes through 2 resistors totalling 22.2kΩ and arrives at pin 6 as 32V???

How is pin 6 going to end up not having some sort of voltage?

 

[Audioguru]

Of course, I forgot that R20 will lift the pin 6 pad to +32V when the U3 is removed and is not holding it down. Before, pin 6 voltage was higher than it should have been but it was later found to be caused by the missing negative voltage on pin 4.
Your U3 pin 2 voltage problem was fixed somehow by removing U3.

Before, the D7 diodes were backwards causing the negative supply to be -3.9V then you corrected their polarity and measured the correct -1.5V. But now on U3 pin 4 pad you measure only +0.3 or -0.3V?? What changed, a broken trace on the pcb??

 

[bushtech]

Thanks Audioguru, I wish I knew, will check

 

[bushtech]

Apologies, not 0.3 but 0.03V

 

[bushtech]

Just noticed that R1 the 2.2k resistor gets hot. Don't know if that's new, it's hidden under big R7, so probably have never felt it

What next?

 

[Audioguru]

Since your -1.5V supply is missing then something is broken or disconnected. Before I listed its 8 parts to be checked.
Your supply is 42V so the 2.2k for R1 dissipates (42V squared)/2200 ohms= 0.8W. My parts list says it should be a 2W resistor (and it should not be near R7). What size is yours?

 

[bushtech]

Thanks Audioguru

Yes, my R1 is 2W. Well, on the pcb it's just next to R7 so I guess I'll just have to live with it.
If I put U3 back I get my -1.5V on pin 4, It just disappears with U3 removed so I guess that's ok.

U3 pad voltages with U3 removed: (U2 in situ)
1 0V
2 0V
3 0V
4 0.3V
5 0V
6 32V
7 32V
8 0V

U2 pad voltages with U2 removed (U3 in situ)
1 0V
2 0V
3 2.3V
4 0V
5 0V
6 0V
7 40V
8 0V

Base of BD139 at 0V
LED still lit
U3 still getting hot (don't know if it's excessive)
R1 gets hotter than U3 ( got spares so might replace)

Not sure where this leaves me
Should I do the R12/C6/10K resistor exercise again?

 

[bushtech]

Fortunately my new 2N3055's arrived:

 

[Audioguru]

The LED that indicates current regulation is turned on and off by Q3 that is controlled by U3 that is activated by its comparison of the voltage from the current sensing resistor R7 with the voltage set by the current setting pot P2. With no output current then pin 2 input of U3 is 0V and if P2 is turned down then it produces a small positive voltage on pin 3 then the (+) input is higher than the (-) input causing the output of U3 to go as high as it can which turns off Q3 and the LED. Whatever is wrong is probably causing too much heat in U3. Measure the voltages to see what is wrong.

U2 pad voltages are normal so try using a new TLE2141 for U2 and connect R12 and C6 to its output pin 6 instead of to the output of the project to see if P1 causes its output pin 6 to go from 0V to about +30V.

I never heard of ISCSemi so I looked it up. It is a Chinese company and I do not know if they make junk or good stuff. Before adding the output transistors wait until U2 works properly then try connecting a new BD139 to U2 and to R16, R12 and C6.

 

[bushtech]

Audioguru Thanks
P2 turned right down
R7 voltage on one of the pins = 0.002V
In - (Pin 2) = 0.94V
In + (Pin 3) = 0.73V
Any other Voltage I can check?
P1 turned halfway