HP 82143A / HP 82162A foto coupler (encoder) - substitute

[jebem]

(01-01-2017 01:59 PM)AndiGer Wrote:  I discovered that in a HP 82143A thermo printer the foto coupler is defective. In the printer service manual it is referenced as encoder to be replaced in whole.
I found out it is not working (properly) when I changed it with a working one.

(01-02-2017 04:39 AM)AndiGer Wrote:  If I compare the not working one with the working one with a DMM I can not find many differences. Maybe I did not use the right measurement methods.
My findings:
Working one and not working one both give me only one result when I use the diode tester:
- on black, + on red/white 1524 and 1644 resp.
Using resistance tester:
- on green, + on red/white: if I cover the diodes gives open circuit, if I give direct light from a LED torch gives about 5.4 kOhms one the working, 3.2 kOhms on the not working one.

(01-02-2017 05:18 PM)AndiGer Wrote:  Why do you think the printer works properly when I switch the photo coupler with the other one from the working printer? Vice versa the other printer runs into the left side wall with the supposed to be defective photo coupler.

Hi,

Sorry, this is a long report.

If I understood correctly: the faulty photo-coupler does not work in any of your two printers, while the good photo-coupler does work on both of them.

From all your description, the photo-coupler is defective or the LED/photo-sensor windows are blocked/dirty.
One other possible cause is the encoder wheel reflective surfaces. Is it of the same type on both printers?

I don't have this printer, so I can't give you the correct voltage readings. However, from your description, I could calculate the approximate voltage values based on the HP service guide schematics.
I'm attaching two diagrams that may help.

Assuming the photo-coupler windows are not blocked, a faulty LED is the mostly common cause, although I have seen defective photo-transistors showing anomalies in their dynamic behavior.

In this case, theoretically both photo-sensors should work, as either measured static value (5K4 and 3K2) will set the positive input (pin 11 of U3 comparator) at a minimum of +4.2VDC (5K4) or +4.5VDC (3K2), well above of the estimated +3.5VDC threshold voltage present at the negative input (pin 10 of U3).

Why estimated 3.5VDC at pin 10 of U3?
Because the typical IR LED will have a forward voltage of 1.5V to 1.8V.
So 5Vp - 1.5Vf = 3.5V. This will be the worst case scenario. If the IR LED operates at 1.8V, then the threshold voltage will be just 3.2V.

However, what you really need is a oscilloscope to measure the U3 C comparator input at pin 11 and output at pin 13.
This will show the dynamic behavior of the photo-transistor.

You can test the suspected LED by comparing it with the good working photo-coupler LED in a safe way:
Use a external 5 or 6VDC power supply and apply the voltage to the LED terminals using a 220 ohm current limiting resistor and a current meter in series with the power supply.
Take note of the current reading on the good photo-coupler and compare it with the faulty one. The faulty one will show a reduced current.

Should the IR LED be faulty, there are plenty of 1.5V IR LEDs in the market that can be fitted inside the photo-coupler capsule.
Same as for the photo-transistor: virtually any NPN type will do and there are plenty to choose from.

As mentioned by cruff, Despite all diagnostics done by you are pointing to a faulty photo-coupler, component values in the PCB can also trigger this issue specially when using the 5K4 photo-transistor item (or using the 3K2 one, not sure which one is not working, specially if this is a dynamic behavior issue).
One obvious thing to check here are the two resistors R10 (120 ohm) and R11 (27Kohm).
The U3 section C IC can be faulty by presenting a lower impedance at pin 11. It will work with the 3K2 photo-transistor but not with the 5K4 one.

If all the above does not apply, you can try a simple test: cut one leg of resistor R11 and insert a additional 10K or 22K in series for a total of 37K or 49K at pin 11 of U3 C.
This will allow to use the 5K4 photo-transistor if it is out of tolerance.