I am new to this group and the forum. I am trying to help out a friend of mine who has a Hitachi EX120-2 circa 93 or 94. Hes been having some issues with the machine which he has traced back to the two modules (pictured below) that my current search seems to be pump control modules. When he opened these modules there are a number of the diodes that have completely disintegrated. I have electronic repair experience which is why he brought them to me. In looking at the diodes, none of them have any numbers on them to help source their specifications or part numbers. I was able to get a test reading on a couple of them which showed a forward voltage of around .53 - .56 volts. All the other components on the boards look quite good. There are a couple of zener diodes that look like they are starting to leak, but I am hesitant to do anything with these without being able to determine the zener voltage. Does anyone have a schematic for the two circuit boards and/or the specifications for the various diodes. I have attached photos of the two boards that I am working on.
If you can get these fixed I reckon you'll become the Nige of circuit boards I'm of no assistance whatsoever but will be watching with interest.
^^^^^^^^^^^^ What he said ! Welcome to HEF @Rhaugen
The units were never designed to be repaired. Schematics would only have been available to top engineering staff at the manufacturer and be a top secret kept from end users. The service manuals for those machines in that era even stated to dealer service personnel to not reveal how to access the diagnostics programs in the electronic packages. What is even worse is that the manufacturer has moved on and likely doesn't have any replacement units available. You likely can't buy one from the manufacturer. What I have done with old Komatsu machines is bypass the electronics altogether. I don't know if that is possible on your make and model of machine. I have also been lucky a couple of times to identify a part and could order it through https://www.digikey.com/ Problem with that is you might have to twenty pieces or more. Good Luck
Interesting. It is too bad that no one has been able to repair or troubleshoot these boards as I anticipate that there are still a number of them out there. I believe I have identified the necessary replacement diodes and am on the track to be able to identify the values of the zener diodes. Surprisingly the capacitors do not seem to have any leakage, which is normally the weak point in circuit boards. That being said it would also be most helpful if someone had the pinouts for the connectors on these boards. John C. thanks for the info and I will keep the forum updated on my results.
Module pinouts would normally be shown on the electrical schematic for the machine. If someone (paging @LACHAU ) can come up with that document it will probably have the information you need.
Sometimes you need to do a little reverse engineering of the circuit and can figure out what they are. They appear to be rated for decent current.
Clearly, the primary function of a diode is to allow current to flow in the forward direction and prevent it from flowing in the reverse direction. However, one important technical characteristic to consider is the voltage drop across the diode when it operates in the forward direction. The diode you are interested in has a low voltage drop, so it must be a Schottky diode. In addition, the diode's response speed needs to be fast; switching the windings of stepping motors and high-speed solenoid valves is another reason why Schottky diodes are necessary. In high-power inductive-load control systems, such as the Stepping Motor (for throttle control) and the regulator solenoid on the Hitachi EX120-2 pump, the instantaneous current (peak current) during reverse discharge is very large. The prediction of 3A and 5A is very close to the actual design specifications of Japanese engineers at that time. Based on my experience with older Japanese construction equipment circuit boards, I'd like to add some common "speciality" codes that match the current range you mentioned: 1. High-power diode grouping on vintage Japanese circuit boards. In Hitachi excavator controller (PVC/MC), the components are often significantly larger than the standard 1N4007 type. These are typically large-bodied Fast Recovery or Schottky diodes (power axial-lead diodes): Group 3A (Commonly used for Stepping Motors) 31DF2 / 31DF4 / 31DF6 (NIEC): This is the most common code. Line: 3A. Voltage: 200V~600V. Characteristics: Black, large, round body, excellent heat resistance. Forward voltage drop (V_f) is typically around 0.55V when measured cold, which matches your measured value. RU 4 / RU 4A (Sanken): A highly durable, fast-recovering diode series for older crane and excavator models. Group 5A (Commonly used for Solenoid Regulators/High-speed Solenoids) The solenoids controlling the tilt angle of the hydraulic pump's rotating disc require high switching frequencies and large discharge currents to protect the MOSFETs/power transistors. 51DF2 / 51DF4 (NIEC): An upgraded version of the 31DF series, capable of handling a continuous current of 5A and very high peak surge current. ERA81-004: A power Schottky series from Fuji Electric, commonly found in circuits requiring extremely high response speeds for precise pump flow control. 2. Why are they burning like in the picture you took? With the Hitachi EX120-2 series, the Stepping Motor and Solenoid pump control operate almost continuously from the moment the engine starts until it stops. Solenoid Regulator: Operates in PWM (pulse modulation) mode. If the solenoid coil overheats, its resistance changes, or the magnetic core jams, a high current will continuously surge back to the protective diode. Component ageing: After more than 30 years, the epoxy resin casing protecting the diode becomes brittle. When exposed to high temperatures from the discharge current, this leads to cracking and scorching, as shown in images D23, D24, and D25 . 3. Modern equivalent alternative (High Power) If you can't find genuine Japanese-made salvaged components, use the following modern power diodes (they are readily available and offer even better performance):
Not that I've understood a thing you have written LACHAU but looks informative.
For more details.
After you have completed the controller repairs check the fuses. If my memory serves me correctly there are 5 or 6 1-amp fuses in the fuse panel. The engine controller and pump and valve controller are fused by 1-amp fuses. Many times, i have seen higher amp fuses installed when there was a short in the wiring harness.
That is great stuff LACHAU. I have a question on the software that runs these boards. When I worked for the Link-Belt dealer the factory rep talked about proms that ran these board’s systems. I’ve been lucky at times finding and replacing burned parts. But were there times when replacing those parts did nothing. I always wondered if whatever happened took out the software in the prom.
ex hitachi fitter back when this was new, they were servicing as a complete part eg no internal on factory training did not go into their. you can convert back to a mechanical/hydraulic
Actually, its full name is EEPROM, or alternatively written as E2 PROM, which stands for Electrically Erasable Programmable Read-Only Memory. The EEPROM is usually surrounded by very good protection circuits, making it very difficult to damage. Usually, such incidents cannot corrupt the program stored in the EEPROM. If we happen to lose the program, we can easily copy it from an EEPROM on the motherboard of a compatible machine. In the past, we used to copy and save the program to a new EEPROM as a backup.
So there is a software component to these controllers. In this country I've never heard that the EEPROM could be reprogramed in the early machines. I always thought that that component was programed at the factory and end users would never have access to that feature. I remember on some machines that the model had to be turned on, I suppose because multiple model characteristics were already loaded into the chip. I've also seen the model numbers that the boxes fit written inside the covers. Would the failure modes above apply more to EEPROMs being used in industrial systems rather than heavy equipment?
1. How to erase the program from an EPROM? It's very simple; just shine ultraviolet (UV) light on the EPROM window to erase it. 2. Copying the EPROM program is done using specialised tools that are sold at a very low cost.
Thanks so much LACHAU . I wish I had seen your post earlier. I certainly did not change the diodes for big enough replacements. The transistors however that I replaced are bigger than the the ones that were bad, so I have no concerns there. I will change out the diodes with your recommendation. And thanks for the info on the eeprom. I may read those eeproms so I have a copy on hand in the event something catastrophic happens in the future. Thanks Ray
@LACHAU , You wouldn't happen to have any knowledge of what the zener voltages might be on these boards ? The other parts that seem to be starting to corrode are the glass zener diodes. I was hoping that someone had the data on what there values might be so I could get a replacement.
I currently have my EC and PVC apart and ZD1 and ZD2 on both boards are labeled T-5Z27...5 watt 27V
Just to update everyone, I went out yesterday with my friend and we reinstalled the EC and PVC modules. The engine fired right up. Test of all the hydraulics and indicator lights on the console and the modules all are working great. Special thanks to @LACHAU for the valuable information about the diode requirements.. Regards Ray