Electronics > Repair
Replacing fried component on Encoder PCB...don't know what it is
Michael86:
Hello,
Attachment descriptions:
3x Datasheets
1st image shows the PCB before components were removed (other than the missing component in question). As well as closeups of the chip identification markings
2nd image shoiws the bare PCB front/back with the exception of the two resistors still attached. The bottom I've traced the circuits. I apologize for using a crazy color scheme for PWR / GRD.
3rd image shows and image from a google search (aliexpress russia). Main point here is that it shows the original ceramic part. It appears to be the same thickness as the original part.
I'm trying to repair an Encoder for a wheel balancer and am hoping for some assistance. I like to play around with electronics but this is not my specialty. Please keep that in mind.
Basically I got a really good deal on the unit because the previous owner couldn't spare the time to try to repair the encoder board. The motors bearings were going and the heat from the bad bearings destroyed one of the SMD ceramic components near the Optical sensor (current theory anyways). The issue is that it had no markings at all. The previous owner had someone try to replace it. They believed it was a capacitor and apparently tried a number of different values but were unsuccessful with the repair – though they didn't use a scope at all and didn't have access to the shaft with the slotted ring for testing.
The PCB part is not available so replacing the entire unit is not an option.
I've removed most of the components to come up with an understanding of the circuit – yes...2 pads were torn and I'll have those repaired by a professional. I'm hoping to test the chips to understand if they are working or if they are also in need of replacing due to heat, but I don't know how to test them. The chips are listed below:
74HC14D – Philips NXP – Nex inverting Schmitt trigger
24C32 – believe it is Atmel
HEDR-8000 HP8K – Agilent Technologies (going off image of device and HP8K marking)
The two resistors test great and are both 100kOhm +/- 1%
The ceramic SMD capacitor measured (removed from circuit) at 106nF so I'm assuming it's a 100nF Capacitor.
The part of concern is the unknown part. It's a ceramic part, assuming its a capacitor but unknown value. It goes across the GND and VCC pins (Pin 4/5) of the HEDR-8000 which I believe is the 2K4 model.
Notes:
1) The 4 pin connector is not in use when installed in the machine – My assumption is that it's simply a port to diagnose encoder issues. Vcc and Ground are directly connected to it (assume to test for power to the unit). The two other pins are connected to the 10 pin ribbon cable but not connected to the circuit at all. So I believe one would be the A output and the other the B output. But those are just thoughts since they don't directly connect to Pins A/B of the optical sensor. I plan on hooking my scope up to the 4 pin connector after trying to replace the unknown component to observe the output. If it doesn't show, then I'll directly connect to Pin A / Pin B of the optical sensor with clips.
2) I have access to an identical working machine with identical parts with the exception of a different encoder board (newer) and different ring on the shaft (shinny ring – plastic I believe). The new encoder has a lens on it as well.
My board: eap0201d50a
New board: EAA0344G00A
I've also found reference to 6726387 being a replacement for mine. That appears to be the same as the new board but without the lens. My going theory is that the input (5v) as well as the outputs from the PCB are the same since the main PCB's that the 10 pin cable goes to are identical. The unknown is whether or not the plastic band / metal band have the same number of Bar/Window pairs and whether it would work. Ie: the new board might work if I remove the LENs. That being said, I had the chance to measure the capacitors on a "newer board" (in circuit only though) and all three of it's "capacitors" measured roughly:
Resistance:
350kOhm in one direction and 625kOhm in the other.
Capacitance:
350mF in one direction and 625mF in the other.
I know this is not a good test since it's in circuit. I didn't have the option of removing them.
Questions:
1) Anyone have enough knowledge of these types of encoders or from the datasheet of the chips/optical sensor to suggest the proper replacement / type for the missing component? Or suggestions on what to do? I assume it's a capacitor but not sure.
2) What kind of tests can I do on these specific chips to determine if they are good or not? Or should I just do a full test at the end and if it works...it works.
3) Expecting 90 deg phase shift between A/B on scope (with A leading). The input on this PCB is 5v but what are the expected outputs on this circuit? Also 0-5v on the scope from Channel A / B?
DavidAlfa:
Being connected between vcc and gnd, can only be a decoupling cap.
Try a 100nF - 1uF cap.
HwAoRrDk:
The 74HC14 is simply being used to invert and buffer the A and B signals coming from the HEDR-8000. Each signal gets inverted once with a single inverter, then the output from that goes in parallel to another pair of inverters (which will inverter the signal back again to its original polarity), whose output are again paralleled and go out to the pin header. I suspect they're paralleling a pair of inverters for output so that the signal has a strong drive strength. Possibly it passes over a long cable harness, or whatever is receiving needs a low-impedance signal.
The 24C32 is an I2C 4KB EEPROM, and most probably just used for identification and/or serialisation of the module. It should have no impact on the sensing function of the module other than perhaps the main unit may refuse to operate if it can't interrogate the EEPROM to find out whether the encoder is present and/or what it is.
You can test the EEPROM by hooking up the VCC, GND, SDA, SCL pins on the header to something like a Raspberry Pi or Arduino and trying to read out its contents over I2C. From the way the A* pins are wired, the slave address for it should be 0x54.
--- Quote from: Michael86 on November 24, 2024, 07:58:09 pm ---The two resistors test great and are both 100kOhm +/- 1%
--- End quote ---
These are pull-up resistors for the I2C SDA and SCL lines. Bit odd to have such a high value of 100k, as normally they're <10k, but maybe the main unit has its own pull-ups, and these are just fail-safes.
--- Quote from: Michael86 on November 24, 2024, 07:58:09 pm ---The part of concern is the unknown part. It's a ceramic part, assuming its a capacitor but unknown value. It goes across the GND and VCC pins (Pin 4/5) of the HEDR-8000 which I believe is the 2K4 model.
--- End quote ---
I would say it's almost certainly just a decoupling capacitor, probably of same value as the other one - 100nF.
The datasheet for the HEDR-8000 doesn't mention the need for any other external components apart from a 220-ohm current-limiting resistor for the LED (which curiously is not featured on the PCB at all!), so it's unlikely to be anything else.
--- Quote from: Michael86 on November 24, 2024, 07:58:09 pm ---2) What kind of tests can I do on these specific chips to determine if they are good or not? Or should I just do a full test at the end and if it works...it works.
3) Expecting 90 deg phase shift between A/B on scope (with A leading). The input on this PCB is 5v but what are the expected outputs on this circuit? Also 0-5v on the scope from Channel A / B?
--- End quote ---
I would try to do an end-to-end test. It should be fairly simple with a 'scope and multimeter to determine whether the module is functioning. You should be able to do it on the bench by just providing a 5V power supply. I would go about it as follows:
1. Check whether the LED in the encoder is illuminating. Jump between VLED and 5V with a 220-ohm resistor, and see if it lights. Datasheet doesn't say whether it might be a non-visible infrared LED, so if no light appears, check with camera (e.g. smartphone) to see if it lights - IR LEDs usually can be seen to glow dim purple-ish colour on camera screen. If LED is bad, you need a new HEDR-8000.
2. With the 5V powered and the LED illuminated, try to replicate the encoder wheel pattern in some kind of to-hand medium (e.g. cut some holes in paper - dimensions given in the datasheet) and see what signals you get out of the A and B pins of the HEDR-8000. The datasheet says they should have a high level at least 2.4V. No signal? Again, possibly bad HEDR-8000.
3. If you get what's expected there, then move to probe the 74HC14 input pins and see if the same signal appears. Then move to the 74HC14 outputs and check again for a signal. No output signal? Replace the 74HC14. Outputs from the inverter should have a high level around 4-5V.
4. Check the EEPROM functions and seems to contain valid data - as described above.
Other than those things, there's not really much else to go wrong. This module is pretty simple.
Michael86:
For the output pins of the 74HC14, I take it that one output would be Pins 6, 8 (Bridged by PCB trace) and the other pins would be Pins 10, 12 (Bridged by PCB trace). I assume that's the part that you mentioned the parallel aspect and inverting of the signal / strengthing. The cable harness is quite long - at least a meter.
I ended up pulling a bunch of SMD caps off scrap boards and found mostly 4-10 microFarads BUT one of the last measured at 110 nF! I put the board back together after the first response (DavidAlfa's) to try caps between 100-1000nF and then saw the second answer (HwA0RrDk's). I decided to try as is and it worked! The first time... Then it didn't work. I manually rotated the motor and kept trying and kept getting "Loose hub" on the display, but then I got it to run again for a bit. I have a feeling that there is some bearing adhesive underneath the ring reflecting / interrupting the pickup.
Next I plan on hooking up the scope next as well as doing some of the other tests that you recommended on the bench. Even if it had worked I want to under stand what's going on.
I really appreciate the quick answers! And thank you for all that detail HwAoRrDk I'll update again once I have some time to play around again on this project.
themadhippy:
--- Quote ---The 74HC14 is simply being used to invert and buffer the A and B signals
--- End quote ---
it also a pretty common method of reducing contact bounce
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