Mastech HY3002D-3 troubleshooting

[OldTechie]

Today, a friend gave me a Mastech HY3003D-3 power supply to attempt to repair for him. It's a 3 channel linear supply, with 2 supplies 0-30V/3A and a 5V/3A supply.
My friend has four of these PSUs and all have the same problem. The issue is that one of the 0-30V supplies (the master) only goes down to approximately 5.5V. Other than that problem, the supply seems to be working properly. I haven't checked it for regulation at full load but all controls seem to be operating properly. A quick check at 1A load indicates that regulation is good.

Just wondering if anyone here has any troubleshooting experience with these supplies, or if this is a common issue with them. I have a schematic for a HY3005D-3, which is very similar, but it's kinda hard to follow and I haven't been able to locate any documentation having a theory of operation. I can figure it out eventually, but it would be much easier to draw from someone who has worked through this problem. Mastech says they only help customers who bought directly from them, and refused to send the correct schematics.

Since all of the units exhibit the same problem, I'm wondering if they have been modified for a specific purpose. Possible, I guess, but no idea of what the modification involved.

Thanks for any help.
Dave M

 

[MrAl]

Hi,

Maybe you can upload the schematic you have.

Where did your friend get his power supplies? Four that do the same thing do indicate a systematic error of some kind unless they all were damaged by the same test somehow.

The voltage regulation scheme used by these supplies are not that straight forward but i may be able to help if i can see the schematic you do have.

If there was a modification, then it would probably come in the form of a changed resistor value. It may be hard to figure out but i think it is very possible to do this as long as the schematics are not too different.

If someone can step forward with a schematic of the actual unit that would help too of course.

Also, what is the difference between the supply schematic you have and the actual unit you want to fix?
Such as different voltages or max current of the outputs?

 

[OldTechie]

Hi,

Maybe you can upload the schematic you have.

Where did your friend get his power supplies? Four that do the same thing do indicate a systematic error of some kind unless they all were damaged by the same test somehow.

The voltage regulation scheme used by these supplies are not that straight forward but i may be able to help if i can see the schematic you do have.

If there was a modification, then it would probably come in the form of a changed resistor value. It may be hard to figure out but i think it is very possible to do this as long as the schematics are not too different.

If someone can step forward with a schematic of the actual unit that would help too of course.

Also, what is the difference between the supply schematic you have and the actual unit you want to fix?
Such as different voltages or max current of the outputs?

I don't know the details of the differences between the HY3005D-3 or the HY3003D-3, other than the current rating of the two 0-30V supplies (the HY3005D-3 is 0-30V/5A unit, whereas the HY3003D-3 is 0-30V/3A output). I have the schematic for the HY3005F-3, sent to me by Mastech customer support. The support guy said that the only difference between the Hy3005D-3 and the HY3005F-3 is the type of display (LED vs. LCD).

Where did the power supplies come from? I don't know; didn't ask. He said that he had them for several years, and they all came from the same source (a local auction from a defunct Gov't contractor), and they all exhibited the issue when he bought them.

Thanks for any help/advice you can give,
Dave M

 

[MrAl]

Hello again,

Ok that might help.

As i said before, this could have happened due to a mod or that they all became defective due to a problem that caused them to exhibit this behavior, but let's proceed as if it was a mod because that is the most probable.

The purpose of a mod like this is to limit the range of the pot and/or get finer resolution on the adjustment range. The simplest way to get this to happen is to increase (or add) the resistor on one terminal of the pot so that adjusting it only allows the total resistance on that side to go only so low. For example if we had a 5k pot and got adjustment from 0 to 5v but then added a 5k fixed resistor to the bottom terminal we might only get an adjustment from 2.5v to 5v.

So the first thing to check for is the two resistors that connect to the voltage adjustment pot. That pot on your schematic is labeled "W5".
You can also note that the "top" resistor is W6A, and there is no "bottom" resistor. However, they may have added a bottom resistor so you should check for that.

To see how the top and bottom resistors are added you can look at the current adjustment pot which is W6 on this schematic.
Note the 'bottom' resistor is 1K adjustable, and the 'top' resistor is a 4.7k adjustable in series with a 3.9k fixed resistor. The top resistor usually limits the top adjustment range and the bottom resistor usually limits the bottom adjustment range but take note the functionality could be reversed.

So look for the pot W5 or equivalent and try to find the top and bottom resistors. On the schematic there is no bottom resistor, but they may have added one in series with the bottom terminal of the pot so look for that. On your supply they may actually have one anyway even if not shown on the schematic.

If you find a resistor addition modification then it becomes obvious that they added it and it has to be either shorted out or reduced in value. I would try reducing the value first and see what the effect is on the operation. To reduce the value by 1/2 just parallel it with another resistor of equal value, and that should be a pretty good test.

There are other ways to get a limit on the low end, but let's hope they did it the simplest and most common way first.

Even if the power supply is different than the schematic the idea is the same on almost all power supplies like this.

So to recap, first find the voltage adjust pot, then find the bottom resistor, parallel with the same value resistor, test to see if the bottom end range has moved down lower so you can adjust below 5v then. If you can, the test was a success, if not, you have to take that resistor off and parallel the top resistor and test again.