EEVblog Electronics Community Forum

Electronics => Repair => Topic started by: czesio666 on October 25, 2019, 01:34:50 pm

Title: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on October 25, 2019, 01:34:50 pm
Hi everyone
I have a problem with Longwei K3010D power supply. It shows permanent 0V and 0A and CC led is lit. No reaction to V and A knobs.
I have found similar topic, read it and tried to follow suggestions.
https://www.eevblog.com/forum/testgear/problem-power-supply-tech-star-k3010d/ (https://www.eevblog.com/forum/testgear/problem-power-supply-tech-star-k3010d/)

What i have checked:
- there is no short on output, the resistance is ~160Ohm (two 330Ohm RA, RB in parallel for discharging output cap)
- power mosfets and schottky are ok
- 12V supply is ok
- VR and AR pots are ok, they are correctly wired to the mainboard
- U4 supply voltage is ok, reference voltage is ok
- TL084 (LM324) seems to be ok, input voltages changes with V and A knobs
- there is no output on SG3525, permanent zero with repetitive glitches, therefore the is no switching on power mosfets
- U4 pin 1 is 0.6 to 2.5V sensitive to VR, pin 2 1.2 to 2.2V sensitive to VR, pin 9 is 0.7V,
And I have no more ideas what to check.
After switching on the fan starts spinning for a while (less than 1 sec). And then power supply is dead. One can hear the coil(s) sound and its frequency changing with current knob.

Thanks for any ideas

The mainboard is K305D revision from 2019 so it is a little bit different than the one from the schematic. I observed that U1-B seems to be unused and there is no Q8, Q9, Q10, W2. U4_2 connected to R44 and D4_C
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on November 05, 2019, 01:40:02 pm
Should SG3525 always generate complementary signals on output? I have 0V on both output and I don't know if this indicates SG3525 failure.
I can't figure out why CC led is on. Q6 is turned on which is controlled with U1-A output. How the current limit works?

Edit: Ok, I get it. Outputs are shifted by period time. Max. 50% pwm can be generate on each output. Until inverting input of pwm is smaller than noninverting (sawtooth) the output is high.
If I have 0% pwm on both outputs it means that inverting signal must be smaller than 1V (sawtooth signal on #5 is correct). I have 1.2vdc on #1 (inverting input) and 1.75vdc on #2 (noninverting input). Error amp gives permanent high output. Therefore PWM amp gives permanent low output.
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: Ordinaryman1971 on November 13, 2019, 03:49:53 am
Check if you have short on the output. There is a capacitor there and diode for reverse protection, check those two..
Did you also check the wiring for cold solder joints. Check the ones that are there for voltage and for current sensing and for output. I would just try to resolder them and see if that will change anything.
Some of those power supplies have something like a crowbar circuit that basically opens up if you overload it. You can't reset it by powering the power supply down, there are actual parts that brake down in the process...
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on November 19, 2019, 10:25:07 am
Thanks for reply

I have already checked it. There is no short on output. Measured resistance is 170 Ohm, it means two discharging resistors in parallel, each 330 Ohm.

Look at the schematic:
(https://obrazki.elektroda.pl/9098897800_1572015395.png)

I can't see any crowbar circuit in output section. There are only converter diodes D22 (are ok, checked after desoldering), discharging resistors RA RB, caps and sensing resistor (RL1 - two copper wires in circuit ). There is also reverse protection diode D30 (not in schematic). Working properly. When voltage is applied to output in correct polarity, the current is V / ( RA|RB). In reverse polarity voltage drops to 0,33V with current limit (diofe forward voltage).

After powering on, current display shows 0,01 A and current limit turns on. I think there is something wrong with current control part that gives wrong information. Pots are ok.
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: BradC on November 19, 2019, 11:26:53 am
What is the voltage on pin 8? If pin 9 is 0.7V that'd stop the PWM.
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: xavier60 on November 19, 2019, 12:16:52 pm
With the CC set to 10A, pin 2 of IC U1-A should be 100mV. 1.47mA sourced by Q1 through R4 is supposed to produce the 100mV
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on November 19, 2019, 12:27:15 pm
Pin #8    0.2 VDC
Pin #9    0.8 VDC
Pin #2    1.75 VDC
Pin #1    1.2 VDC

Pin #16    5.1 VDC
Pin #4    60kHz 10% PWM
Pin #5    60kHz sawtooth 1 to 3.5V
This means U4 is correct

Pin #11 and #14     0.2 VDC

There is no reaction on U4 pins to pots position.
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: xavier60 on November 19, 2019, 12:35:00 pm
With a + voltage on an op-amp's Inverting input, the output should be saturated negative.
Is the negative rail OK? Maybe U1 is faulty.
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on November 19, 2019, 12:47:31 pm
U1 supply correct 12V

Current pot set to max:
U1-A #2    0.18V
U1-A #3    0V
U1-A #1     10.7V


0.12V drop on R4
Q1 open, 0.63V on BE, negative 6.4V CE
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on November 19, 2019, 12:59:43 pm
U1 Vcc- is not grounded.

Corrected values

Current pot set to max:
U1-A #2    0.18V
U1-A #3    0V
U1-A #1     10.7V
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: xavier60 on November 19, 2019, 01:01:53 pm
U1 supply correct 12V

Current pot set to max:
U1-A #2    0.18V
U1-A #3    0V
U1-A #1     10.7V


0.12V drop on R4
Q1 open, 0.63V on BE, negative 6.4V CE
With 0.18V on pin 2, pin 1 should be saturated negative.
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: xavier60 on November 19, 2019, 01:05:06 pm
Does U1 have a negative rail?
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on November 19, 2019, 01:06:43 pm
Yes, the same thing is on U1-B.
U1-C and U1-D seems to be ok.

Ok, so I will replace TL084BC (LM324 is on schematic).

U1 Vcc+ is 12V, Vcc- is 0.8V. Something wrong with supply?
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: xavier60 on November 19, 2019, 01:12:30 pm
Yes, the same thing is on U1-B.
U1-C and U1-D seems to be ok.

Ok, so I will replace TL048BC (LM324 is on schematic).

U1 Vcc+ is 12V, Vcc- is 0.8V. Something wrong with supply?
You will need to trace it, the schematic isn't clear about what supplies negative rail to U1.
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: Chris56000 on November 19, 2019, 01:52:10 pm
Hi!

The Chinese might be good at designing cheap electronics (& counterfeiting it as well!!!) but their diagram & documentation skills are bloody atrocious!!

The Op Amp Negative Supply is provided by D6 (SR1100), with reservoir capacitor C14 (100uF), with decoupling filter R30, the –12V for the op–amps @ pin 4 is across C8 at the junction of C8, R01 (2M2) and R30 (4R7) – this can be inferred from the low value of the –12V line decoupling–filter resistor!)

I've got some nice Drawing Office Lettering now, so I'll get started on a new drawing in due course!

Chris Williams
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on November 19, 2019, 01:57:55 pm
Vcc- goes to connector solder point named -5V. I think it is test point (the second one is 12V). Next it goes to R30/C8-. I found also that C6- is connected to the same node (the schematic is probably for older board revision).

It looks like T1 with D5 D6 should produce positive 12V (this one is ok) and negative 5V? (this one is not ok). D6 is ok. Vcc- is 0,86V. But could it be due to U1 failure?
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: xavier60 on November 19, 2019, 02:06:23 pm
Vcc- goes to connector solder point named -5V. I think it is test point (the second one is 12V). Next it goes to R30/C8-. I found also that C6- is connected to the same node (the schematic is probably for older board revision).

It looks like T1 with D5 D6 should produce positive 12V (this one is ok) and negative 5V? (this one is not ok). D6 is ok. Vcc- is 0,86V. But could it be due to U1 failure?
Is the winding OK?
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on November 19, 2019, 02:21:08 pm
Ok, i found it. Negative winding was not connected. Faulty joint. Resoldered and current limit disappeared. Now I will check if output is ok.
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: czesio666 on November 19, 2019, 02:37:54 pm
Tested with some load resistors and everything seems to be ok.
The Vcc- is 4,8V.

Thanks to all for help. regards
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: SirVer on January 18, 2020, 11:34:43 am
Hello czesio,

I seem to be having the exact same issues as you - knobs unresponsive, fans spin up for a second, coil hum is audible. You mentioned that it turned out to be your negative winding, and I'd like to check it on mine, but I'm kind of a noob and I don't know which winding that is - could you tell me which one it is? They all look fine visually, but I can't get at all of them with a multimeter without further disassembly, so I'd like to know which one I should go at first.

Thank you in advance.
Title: Re: Permanent CC and no output [Longwei K3010D]
Post by: faza1111 on December 28, 2020, 12:47:54 am
I had 3 damaged smd capacitors, I changed all of them. There was probably a bad series of all capacitors