This procedure was written for the IBM 5150 motherboard and the IBM 5160 motherboard.
The IBM 5155 contains an early 5160 motherboard, so treat the 5155 motherboard as a 5160 motherboard.
You are here because all of the following are true:
- You have an IBM 5150 or IBM 5155 or IBM 5160; and
- The fan in the power supply IS turning; and
- When you ran the 'minimum diagnostic test' test at here, you did not hear the expected beeps; and
- When you ran the 'minimum diagnostic test' test at here, the +5V, -5V, +12V, -12V, POWER GOOD, from the power supply all measured good.
Because all of the above are true, we know that the IBM 5150/5155/5160's motherboard is faulty, and faulty in a way that does not overload the power supply.
Preface
Ideally, I would provide a procedure that, using suitable test equipment, would lead you to the fault on your motherboard.
If that sounds simple to you, then it would inform me that you are completely unaware of the complexities of a motherboard. Pretty much, any component (or PCB trace) could be faulty. A procedure to logically diagnose a fault down to component level on a 5150/5160 motherboard could be say, several hundred pages long. I am not going to create such a procedure. Instead, below, I will take you through some common problems, and some things that are relatively easy to check.
Step 1 of 8 - Visual inspection
Perform a thorough visual inspection of the motherboard. You may spot a problem.
Step 2 of 8 - Socketed chips
Various chips on the motherboard are in IC sockets.
• Are the socketed chips plugged in the correct orientation?
• Try re-seating the socketed chips, in case there is a poor connection between a chip and its socket. If in the re-seating process, you decide to fully remove the chip from its socket, then when putting the chip back in, ensure that you do not accidentally bend a pin up underneath the chip.
• If you have access to known-good replacement chips, try swapping those into the sockets.
• If you acquired the motherboard in a faulty state, then consider the possibility of incorrect chips (i.e. bad attempted repair). An incompatible RAM chip is an example.
Step 3 of 8 - Faulty BIOS ROM ?
Sometimes, the BIOS ROM chip is faulty. Consider acquiring a known-good replacement.
Per here, the 8284A chip generates a 4.77 MHz clock. It is critical that the 8088 CPU and 8288A chip receive that.
Verify the presence of a clock on pin 8 of the 8284A chip.
- Ideally, you would use an oscilloscope to do that.
- The next best tool is a logic probe, and with that, expect it to indicate pulses.
An unreliable method is to use a multimeter in 'DC voltage' mode. In that mode, the multimeter will read the average voltage of the clock signal. You can see at here, that I measured about 1.8V on one of my motherboards. Perhaps expect between 1.5V and 2V. I cannot stress enough how unreliable this method is. Consider acquiring a logic probe (and the knowledge of how to use it).
Obviously, if the 8284A chip is not generating a 4.77 MHz clock, then you need to establish why that is. A related diagram is at here.
If the 8284A chip is generating a 4.77 MHz clock, then check that it is reaching the following pins of the following two chips:
• Pin 19 of the 8088 CPU chip - see here.
• Pin 2 of the 8288A chip - see here.
Step 5 of 8 - 8253 chip - Pulses out of channel #1
If you have a logic probe (or oscilloscope), see if it reveals pulses on pin 13 of the 8253 chip - see here.
Pulses present
For the IBM 5150 motherboard, the presence of pulses indicates that:
- The POST in the IBM BIOS ROM is being executed; and
- The POST in the IBM BIOS ROM has progressed to at least step 9 at here.
For the IBM 5160 motherboard, the presence of pulses indicates that:
- The POST in the IBM BIOS ROM is being executed; and
- The POST in the IBM BIOS ROM has progressed to at least step 13 at here.
No pulses
If there are no pulses on pin 13 of the 8253 chip, then the possibilties are:
- The POST in the IBM BIOS ROM is not being executed; or
- The POST in the IBM BIOS ROM is being executed, but a very early test is failing (resulting in the CPU being halted).
You could try the procedure at here to see if the IBM BIOS ROM is being executed.
Step 6 of 8 - 8253 chip - Pulses out of channel #0
If step 5 revealed pulses, and you have a logic probe (or oscilloscope), then see if the logic probe reveals pulses on pin 10 of the 8253 chip - see here.
Pulses present
For the IBM 5150 motherboard, the presence of pulses indicates that:
- The POST in the IBM BIOS ROM is being executed; and
- The POST in the IBM BIOS ROM sucessfully tested the first 16 KB of RAM; and (Step 13 at here.)
- The POST in the IBM BIOS ROM has progressed to at least step 19 at here.
For the IBM 5160 motherboard that has a 1982 dated BIOS ROM fitted, the presence of pulses indicates that:
- The POST in the IBM BIOS ROM is being executed; and
- The POST in the IBM BIOS ROM sucessfully tested the first 16 KB of RAM; and (Step 15 at here.)
- The POST in the IBM BIOS ROM has initialised the video card; and (Step 21 at here.)
- The POST in the IBM BIOS ROM has progressed to at least step 23 at here.
For the IBM 5160 motherboard that has a 1986 dated BIOS ROM fitted, the presence of pulses indicates that:
- The POST in the IBM BIOS ROM is being executed; and
- The POST in the IBM BIOS ROM sucessfully tested the first 64 KB of RAM; and (Step 15 at here.)
- The POST in the IBM BIOS ROM has initialised the video card; and (Step 21 at here.)
- The POST in the IBM BIOS ROM has progressed to at least step 23 at here.
No pulses
If there are no pulses on pin 10 of the 8253 chip, then see the next step.
