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The parent web page is at here.

You are here because you were following the procedure on the parent page, and due to a particular measurement, directed to this web page.

The particular measurement was that, on your motherboard, the resistance between pins 4 and 6 of the relay is not between zero and say, 20 ohms, when the motherboard is powered on.
That 'breaks' the orange path shown in the diagram at here resulting in the 131 error.
Now we need to determine why, on your powered-on motherboard, the resistance between pins 4 and 6 of the relay is not between zero and say, 20 ohms.

Possible causes include:
•  When the motherboard is powered on, the relay is somehow being commanded into the on state.
•  Faulty relay. This includes electrically 'dirty' contacts in the relay.
•  Bad solder joint on pin 4 of the relay.
•  Bad solder joint on pin 6 of the relay.
•  Faulty PCB.

The power-on self test (POST) configures the 8255A chip to output a TTL high on pin 21 of the 8255A.  That signal is manipulated by some other circuitry, ending up such that the relay is commanded to be off.  We will now follow the signal from the 8255A to the relay.


Step 1 of 5 - Is relay commanded to be off ?

Step 1.1 Power on the motherboard.
Step 1.2 Wait for the 131 error to appear.     <---- IMPORTANT   (see note 1)
Step 1.3 Refer to the blue path in the diagram at here.

Measure the DC voltage on pin 21 of chip U36 (the 8255A).  See here for a pinout of the 8255A.

If a TTL high (+2V to +5V), then proceed on to step 2.

If not, either:
•  chip U36 (the 8255A) is faulty; or    <-------------------------------------------------------- most likely of the three
•  chip U63 is faulty in a particular way, dragging down pin 21 of chip U36; or
•  a PCB problem.



Step 2 of 5 - Relay driver input

Step 2.1 Power on the motherboard.
Step 2.2 Wait for the 131 error to appear.     <---- IMPORTANT   (see note 1)
Step 2.3 Refer to the blue path in the diagram at here.

Measure the DC voltage on pin 2 of chip U95, a 75477.  See here for a pinout of the 75477.

If a TTL low (0V to +0.8V), then proceed on to step 3.

If not, either:
•  Chip U63 is faulty; or    <-------------------------------------------------------- most likely of the three
•  Chip U95 is faulty in a particular way, dragging down pin 6 of chip U63; or
•  A PCB problem.



Step 3 of 5 - Relay driver output

Background: The 75477 chip on the IBM 5150 motherboard contains two drivers. One is used to 'drive' the speaker, and the other is used to 'drive' the relay.

Step 3.1 Power on the motherboard.
Step 3.2 Wait for the 131 error to appear.     <---- IMPORTANT   (see note 1)
Step 3.3 Refer to the blue path in the diagram at here.

Measure the DC voltage on pin 3 of chip U95, a 75477.  See here for a pinout of the 75477.

If about +5V, then proceed on to step 4.

If not, either:
•  Chip U95 is faulty; or    <-------------------------------------------------------- most likely of the two
•  A PCB problem, dragging down pin 3 of chip U95, turning the relay on.

BTW. There are some other possibilities for not measuring about +5V, but they would not cause a 131 error:
•  The probes of your multimeter did not pierce the oxide layer on the pin/s of the chip.
•  Open-circuit between pin 3 of chip U95 and pin 16 of the relay (faulty trace or solder joint).

Note: On pin 3 (an output pin) of the 75477, TTL high's and low's are not applicable.  The inputs to the 75477 are TTL, but the output pins are not.  Expected is that the 75477 is not pulling current though the relay's coil, and because of that, the voltage expected is whatever voltage is on the other side of the coil, namely +5V.



Step 4 of 5 - Input to relay

In step 3, you checked the output of the 75477 driver used for the relay, and found the output to be as expected, about +5V.

Because of the way that the 75477 and the relay work, the +5V that you measured in step 3, must be coming from the relay's coil.

I see no point in measuring pin 16 of the relay, but if you want to satisy yourself, follow steps 4.1 to 4.3 immediately below.

Step 4.1 Power on the motherboard.
Step 4.2 Wait for the 131 error to appear.     <---- IMPORTANT   (see note 1)
Step 4.3 Refer to the pinout diagram of the relay at here.  Measure the DC voltage on pin 16 of the relay.

Approximately +5 volts is expected.



Step 5 of 5 - Conclusion

You have determined that when the motherboard is powered on:
•  Good: As expected, the relay is being commanded into the off state.
•   Bad: Pins 4 and 6 of the relay are open-circuit.

The conclusion of that is that either:

•  The relay is faulty: The relay is off but certain contacts that should be closed-circuit are open-circuit.  The subject contacts within the relay might be 'electrically' dirty.  You should be able to find instructions online on how to clean relay contacts.

•  Bad solder joint on either pin 4 or 6 of the relay.



END OF PROCEDURE



Note 1    There is a delay before the POST programs/configures the 8253 chip to generate the tone for the 131 test, and turns off the relay.  When you see the '131' on-screen, you know that the POST has done both.