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IBM 5150/5155/5160 - Faulty motherboard symptom

NOTE:   NOTE:

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 not faulty in a way that overloads the power supply; and
- An ISA card is not faulty in a way that stops the motherboard from executing the BIOS ROM.


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 9  -  Visual inspection

Perform a thorough visual inspection of the motherboard.  You may spot a problem.  Use a magnifying glass.

Of particular note is the physical integrity of the trimmer capacitor.  There have been a few reported cases of a broken one (visually found to be broken).
IBM 5150 motherboard: Trimmer capacitor T1.
IBM 5160 motherboard: Trimmer capacitor C1.

See here for some examples of problem causes that I have spotted visually.


Step 2.1 of 9  -  Socketed chips - General

Various chips on the motherboard are in IC sockets.

•	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.
•	Are the socketed chips plugged in the correct orientation?
•	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 2.2 of 9  -  Socketed chips - Remove unnecessary ones

Some (as listed immediately below) of the socketed chips are not required in order for the IBM BIOS to start and display something on-screen.  And so, in case one of those chips is the problem cause, remove them, and then see if the computer starts.

•	If present, remove the 8087 math co-processor chip (an optional chip).  After removal, set switch 2 in switch block SW1 to the ON position.  The 40-pin socket for the 8087 is adjacent to the 8088 CPU chip.
•	For something to appear on-screen, the IBM BIOS ROM only requires RAM bank 0 to be populated.  Therefore, remove the RAM chips from RAM banks 1, 2, and 3 - see here.  After doing that, in the case of the IBM 5155/5160, also set the two relevant switches in switch block SW1 for 'enable bank 0 only' operation.
•	Note that unlike the IBM BIOS ROM, Ruud's diagnostic ROM and the Supersoft/Landmark diagnostic ROM do not require RAM bank 0 to be populated.  (To see something on-screen.)

Step 3 of 9  -  Faulty BIOS ROM ?

Sometimes, the BIOS ROM chip is faulty. Consider acquiring a known-good replacement.

See here for IBM 5150.
See here for IBM 5160.

Even if the BIOS ROM chip is not the problem, I think that having a spare is a good idea.

And if acquiring a BIOS ROM chip, the supplier may also be able to supply you with a suitable Ruud's diagnostic ROM and a suitable Supersoft/Landmark diagnostic ROM.


Step 4 of 9  -  Faulty power supply ?

Even though the +5V, -5V, +12V, -12V, POWER GOOD, from the power supply all measured good, that is not a 100% check of the power supply.

There needs to be an adequte delay between the production of the +5V and the production of the POWER GOOD signal, otherwise the motherboard will not start.  An example of the delay in a particular power supply is shown at here.  In the IBM 5150 and 5160, the minumum allowable delay is 840 ns.

If you do not have the means to measure the delay for your power supply, then try a known-good power supply.


Step 5 of 9  -  Verify 4.77 MHz clock

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 6 of 9  -  8253 chip - Pulses out of channel #1

If you have a oscilloscope, then see if the oscilloscope reveals pulses on pin 13 of the 8253 chip, per the diagram at here.

Expected pulses present

For the IBM 5150 motherboard, the presence of pulses (of expected frequency, width, polarity) 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 (of expected frequency, width, polarity) 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, or what is seen is not as expected, 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.

You could also try a suitable Ruud's diagnostic ROM or a suitable Supersoft/Landmark diagnostic ROM.


Step 7 of 9  -  8253 chip - Square wave out of channel #0

If step 5 revealed pulses, and you have a oscilloscope, then see if the oscilloscope reveals a square wave on pin 10 of the 8253 chip, per the diagram at here.

Square wave present

For the IBM 5150 motherboard, the presence of a square wave (of expected frequency) 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 a square wave (of expected frequency) 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 a square wave (of expected frequency) 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.

Square wave missing

If there is no square wave on pin 10 of the 8253 chip, or what is seen is not as expected, then see the next step.

You could also try a suitable Ruud's diagnostic ROM.


Step 8 of 9  -  Faulty RAM in bank 0 ?

If the expected pulses were observed at step 5, but no square wave at step 6, then faulty RAM in bank 0 is the likely (but not the only) cause.

See here for IBM 5150.
See here for IBM 5160.


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