IBM 5155  -  Known Problems/Issues

Flawed methodology for determining total RAM size

The IBM 5155 contains an early IBM 5160 motherboard, and therefore suffers from the same RAM size flaw that is in the 5160.
Click here for details of that flaw.

Wrong switch setting information in 5155 Guide to Operations

Page 4-81 of the first edition (January 1984) of the 5155 Guide to Operations is incorrect.
It wrongly indicates that for 256 KB of motherboard RAM, switch 3 is set off and switch 4 is set on.
The correct information is that both switches 3 and 4 must be set off for 256 KB (i.e. enabling all four 64K banks of motherboard RAM).

Short circuited tantalum capacitors

It is common for tantalum capacitors on the motherboard and on expansion cards to go short circuit, stopping the power supply from working (although the power supply fan might still turn).
It will appear that the motherboard is 'dead', although to be noted is that that symptom has many causes.
If you suspect that your 5155 may have a shorted tantalum capacitor, then try the diagnostic procedure here.

Deteriorated lubrication in floppy disk drive

It is often found that in 5.25" floppy drives that have not been used in many many years, that the lubrication in the drive has deteriorated to the point where it 'gums up' and stops the drive from functioning properly.

This is commonly seen on the rails that the head carriage slides up and down on.  Deteriorated lubrication there results in the inability of the head carriage to move properly, or in extreme cases, not move at all.  By hand (and with power off), you should be able to freely move the head carriage along its rails.  You will experience some slight resistance presented by the stepper motor.

What is required is to clean the rails of the old lubricant, then followed by application of new lubricant.  Myself and others have found a silicone based lubricant to be satisfactory.  I sometimes use silicone spray, but there is a tendency for the spray to get to areas where it should not go.  Otherwise, I use 'silicone dielectric compound', a particular type of silicone grease that I also use to get rid of any squeak sound from the drive's front panel latch.

If in any doubt, clean/relubricate the rails.  The clean/relube is a worthwhile maintenance activity even if it doesn't fix a faulty drive.

Some programs display poorly on the IBM 5155's display

Information on this subject is here.

CGA snow

Because the 5155 is fitted with an IBM CGA card, the 5155 suffers from something that is commonly referred to as 'CGA snow'.
You can read more about that at here.

'Jumping' display

Two users at the VCF each had a 5155 that exhibited this symptom.
The fix was to disconnect connector P12 and then reconnect it.  The fix suggests that a poor connection had developed in P12.
P12 is pictured here.  Note that P12 is normally found in a covered channel that runs underneath the (opened) 5155.

Unique IBM CGA card

IBM produced variations of their CGA card, and the variation that is fitted to the 5155 specially suits the CRT unit in the 5155.
You can read about that in the 'Forever Amber' section on page 117 of the document here.
Obviously, if the IBM CGA card needs to be replaced with another, it is best to use the same variation of IBM CGA card.

POST cards do not work

Information on this subject is here.

Cards put in expansion slot 8 must be slot 8 compatible

Information on this subject is here.

16-bit ISA cards

Some 16-bit ISA cards advertised as being 8-bit slot compatible, may not work in the IBM 5155.
Here are some possible causes:

• When the maker wrote "8-bit slot", they meant "8-bit slot in an AT-class computer".  This kind of requirement is usually found in the card's user manual.  
   Click here to see an example.

• Some 16-bit cards need to be reconfigured for 8-bit slot operation.  That configuration is sometimes done by switches or jumpers.  On at least one VGA card, it is done via the card's configuration software.

BIOS ROM replacement

The IBM 5155 contains an early IBM 5160 motherboard.  Early 5160 motherboards are wired for a Mostek MK38000 series ROM in the BIOS ROM sockets (U18 and U19).  On such motherboards, a replacement ROM of type 27256 (or 27C256) usually works, but not always.  See here for more information on this subject.


Swapping out the 8088 CPU for a V20 CPU can sometimes cause problems.  See here for more information on this subject.

Upgrading motherboard BIOS to 05/09/86 revision

The IBM 5155 contains an early IBM 5160 motherboard.  Upgrading the BIOS to the 05/09/86 revision found in the later IBM 5160's is valid, but be aware that the use of the 05/09/86 revision may result in third-party floppy controllers being unable to boot from 1.44M diskettes.  See here for more information on this subject.

RAM failure in motherboard bank 0

Vintage RAM chips have a relatively high failure rate.
The failure of any chip in the 5155's first bank of RAM (bank 0) results in what appears to be a 'dead' motherboard.

Click here to see a diagram that shows the four RAM banks.   (Diagram applicable because the 5155 contains an early 5160 motherboard.)

Note that rather than a RAM chip actually failing, what might have instead happened is that a RAM chip has developed poor electrical contact with its socket.
And so the first thing to try is to simply to reseat all of the chips in bank 0 (wiggle each chip in its socket).

For the 5155, one way of diagosing a faulty RAM chip in bank 0 is to fit the bank 1 chips into bank 0 (assumption: the chips in bank 1 are all good).
Because all 4 banks of RAM use the same type of RAM chip (generic type 4164), you have the option of using RAM chips from banks 2 and 3 as well.