Mass Flow Controller repair - STEC SEC-4500

[tech_builder]

Hi,
I am trying to reverse engineer a board on a mass flow controller I purchased on eBay so that I can hopefully fix it. The control input is sinking a lot of current (I noticed about 400mA at about 2V), but is specified to have an impedance of over 1Meg (signal is 0-5V). The device does not respond to any input signal functionally. I found the section of the board that appears to deal with the input signal. There are numerous scorched components, indicating I am looking in the right spot

I do not have a schematic, but I have managed to trace the circuit and I am now putting it together again. However, there are some mystery components that I am hoping somebody may know more about than me.

The first is a transistor looking device with the identification “10H” on it. I have seen references to this being a diode, or a bandgap voltage reference. I’ve even found a datasheet, but it didn’t match the pinout I deduced (ZXRE4041). The circuit has the designation D5 next to it, so that implies to me that it is a diode, but what specifications? It is connected to one of the scorched parts, so it may be damaged as well.

Next is a similar device with the marking “A7” on it, with a bar above the A and designated on the PCB with D1-4. I think these are diodes, with the bar indicating direction. However, I still don’t know how to find the specifications for these if they need to be replaced.

The next one is a little brown tube. It is labelled “D6”, so I guess it might be a diode. If it is, then it is blown because it conducts both ways in the milliohms range. If it’s not a diode, then I have no idea what it is. If it is a diode, I don’t know what direction it should be either.

The next device is an 8pin soic package by National Semiconductor (it has the little N in the corner). It has the markings 258 614 on it, and nothing else. I’m guessing that it might be some kind of opamp. This one has some scorch marks under it, so I will likely need to know what it is to be able to replace it.

Finally is a device that looks like a potentiometer, but is labeled with SW1, so it is a switch? This also has some scorch marks on it, so a more specific part number would be good to know for replacement. It has a number and a letter written on it “6” and “H”. There are also two dots on it that might represent position, however, the device was positioned with the switch in the middle of these dots when I got it, so that may not be their function.

Here is the gobbledygook I came up with for tracing the connections. It's a bit much to look at, but I find it handy for tacing things around the board. It's just difficult to make it into to something recognizable.

I’ve also attached pictures of the board for reference if needed. I will put together the schematic as best I can and probably come back with more questions.

Thanks for the help.

[carl_lab]

Try these:

10: BAT18 diode
A7: BAV99 diode
brown MELF: standard Si diode (1N4148/LL4148 or equivalent)
258: LM258
SW1: probably a switch. Have you tried to turn the rotor? Measure resistance between the contacts.

http://www.marsport.org.uk/smd/mainframe.htm

[tech_builder]

Thanks so much for the help. That web site link is great!

[tech_builder]

I have another question about some other components on the board. At first I thought the components marked Z were zener diodes, but when I do a search for component designators I see that Z may mean miscellaneous.

https://blogs.mentor.com/tom-hausherr/blog/tag/reference-designator/

Now I'm not too sure what these components are? Even if they were zeners I wouldn't know what voltage they were.

[carl_lab]

I don't know about the Z marking...
The component maybe a NTC/PTC, inductor or capacitor - definately not a zener diode.
Have you tried to measure resistance or capacitance between the contacts?

[tech_builder]

Hi,

I did measure them with a UNI-T UT612 LCR meter. Z1 measures as a resistor at 20k, and Z2 measures as a capacitor at 730pF. One thing to note about Z2 is that it looks like it got burned, and I wonder if it was a resistor that opened and now shows with some capacitance? I know the resistor beside it also got burned and is now open (was 100 ohm) and reads at about 12nF. I will try heating up Z1 and measuring its resistance to see if it is an NTC/PTC.

Thanks again for the help, I really appreciate it. These smd parts are difficult for me to determine.