Below is an attachment with the main schematic for a Mastech HY6003D power supply. It was working great and abruptly quit putting out voltage. Using a second HY 6003D, I checked voltages in the circuit. It appears that the TL 431 is not operating correctly. There is 12 volts DC at the cathode and 1.1 volts DC at the Ref pin and .68 volts DC at the anode where there should be zero volts as it is ground. According to Texas Instruments datasheet, the ref voltage should be 2.5 to turn on the internal op amp and transistors. The attachment is labeled for a HY 5003, but the same schematic is used for 5 different voltage/amperage output power supplies.

What I am looking for is a more positive way to check the operations of the TL 431 short of just getting a new one and soldering it in. Thanks for the help.
DickW

 

and .68 volts DC at the anode where there should be zero volts as it is ground

If ground is not at 0V potential, then that's a problem right there.

 

V5A certainly isn't zenering, but the cathode and the ref pin are wired together in the schematic. They aren't wired together now! The chip could be perfectly good. Go looking for why those 2 points are not connected to each other.

 

sounds like a busted tl431. I would check for continuity between the anode and ground first to make sure that the device is actually in circuit.

after that, put a forward biased led in its place and the led should light up and depending on the type of led picked should provide 1.9v - 3v and you can then test to see if the rest of the circuit works.

TL431 typically fails open.

BTW, the design is a little bit probamatic as the TL431 is draining a lot more current than it should. dial up the value of R1A.

 

Your Tl431 is wired as a 2.5V zener, there should be the same voltage on the gate as the Cathode as they are shorted together,

I would use pin8 of the transformer as the ground point, and take readings of the Anode and Cathode to decide.

 

Thank you all for the replies. On Monday, I will put some more time on the problem.
Dick W

 

Below is an attachment with the main schematic for a Mastech HY6003D power supply. It was working great and abruptly quit putting out voltage. Using a second HY 6003D, I checked voltages in the circuit. It appears that the TL 431 is not operating correctly. There is 12 volts DC at the cathode and 1.1 volts DC at the Ref pin and .68 volts DC at the anode where there should be zero volts as it is ground. According to Texas Instruments datasheet, the ref voltage should be 2.5 to turn on the internal op amp and transistors. The attachment is labeled for a HY 5003, but the same schematic is used for 5 different voltage/amperage output power supplies.

What I am looking for is a more positive way to check the operations of the TL 431 short of just getting a new one and soldering it in. Thanks for the help.
DickW

It sounds like you are on the right track. It may be the TL431 or the capacitor next to it. Of the two I would check the capacitor first.

 

I am back trying to get the Mastech HY6003D power supply running. The first thread in this post has the schematic for the power supply. I have changed the TL431 shunt regulator with no improvement. I have two further questions that can be seen on the schematic.
1. On the main transformer there are numerous outputs. I am concerned with the bottom six as shown on the schematic. All six of these outputs show up next to the three relays K2A, K1A, and K3A. These relays click on and off depending on how much voltage you adjust the voltage output meter to when setting up for the 6003D to be either a battery charger or as a power supply. My question is how can a DC relay turn on and off with AC voltage coming directly off the transformer. There is 12 volts to the relay and the diodes on all three relays are functional, but I can't see how the relays turn on and off with varying voltages or how the relays do anything. I do not see where the outputs of the relay KA2-3, KA1-3 and KA 3-3 go.

2. I may have been reading the DC volts wrong during my troubleshooting. I have been using x4 as ground for voltage readings, but I am now seeing I should have been using the case ground which seems separate from the x4 (the black output lead) with x2 being the red out lead. X3 is labeled GND on the schematic and is attached to the case.

There are three schematics in the attachment below. The middle schematic is for a power supply that is not incorporated in this power supply. The bottom schematic is the readout for the HY6003D.

Any help would be appreciated. Dick W

 

You're reading the circuit wrong, the relays are fed from the 12V dc supply by transistors, the relays have Changeover contacts, and are wired directly to the transformer outputs to switch different AC voltages.

 

You're reading the circuit wrong, the relays are fed from the 12V dc supply by transistors, the relays have Changeover contacts, and are wired directly to the transformer outputs to switch different AC voltages.

DodgyDave, I finally got the power supply repaired. With your comments on the switching relays, you pointed me to an area of the circuit board I had neglected. The problem child was the Constant Current 741 op amp which was upsetting the bias to the output transistors. Also the schematic is wrong compared to the actual traces on the circuit board. The center tap on the N5 portion of the transformer does not go to ground, but terminates in a 10K resistor; this is a mystery how they could produce this schematic wrong. Thanks again for the help.
Dick W