A few weeks ago, I bought my first IBM PC, a 1983 5160 (XT) with matching monochrome IBM 5151 CRT display and IBM Model F keyboard. I’ve been wanting to get hold of an IBM PC for a while now, as it’s an interesting piece of history, and I’ve enjoyed several videos from Adrian’s Digital Basement on them.
This was a literal barn find, having came from a barn on a farm in York, about an hours’ drive for me – everything was dirty, a bit rusty, and was apparently untested so was being sold-as-seen on Facebook marketplace.
The IBM Personal Computer XT (PC/XT), model 5160, is the successor to the original IBM PC, model 5150, which it is very similar to except for the addition of three additional ISA 8-bit expansion slots (for a total of eight), a base RAM of 128KB expandable up to 640KB, a 10MB MFM hard drive with disk controller, and an upgraded 130W internal power supply. Like its predecessor, the 5160 has a 360K 5.25″ floppy disk drive, IBM BASIC in ROM, and a socket for an Intel 8087 arithmetic coprocessor. By 1985, the IBM PC AT made the XT obsolete for most customers.
The 5160 has an internal XT-style power supply, which are apparently now becoming unreliable due to their age – they also need to be serviced before powering on, as they contain RIFA-brand metallised-paper mains filter capacitors which like to fail short-circuit and produce volumes of nasty, acrid smoke, so need to be replaced.
To get to the power supply, the 5160 top case needs to be removed – this means removing five slot-head screws at the rear, and pulling the top case forwards and off.
The power supply itself can then be removed – it is held in place by four slot-head screws on the back of the 5160, and two tangs underneath it that mean it needs to be slid towards the front of the machine, then pulled out. To make room for this, the two 5.25″ drives need to also be slid forward – these are each held in place by two slot-head screws on their outer side, and one slot-head screw on their inner side underside, which is accessible from a round hole on the bottom of the machine.
This particular PSU is an Astec AA12151, IBM part number 1501438 – the upper case is secured with two cross-head screws and two rivets, which I drilled out.
Our first look inside the power supply shows its construction, as well as how dirty it is: a relay board with the explosive mains filter capacitors is mounted to the side of the inner case, which switches mains to the power board, fan, and female IEC mains socket for supplying the monitor without overloading the power switch; the cooling fan, which runs directly on mains AC and is very heavy and loud; the power board, which provides the DC supply outputs for the computer.
The power supply internals can be removed by unscrewing the power and relay boards and fan, and disconnecting the relay board from the mains sockets and switch.
The problem with the original parts is their advanced age, complexity and unreliability, large physical size, significant weight, and the loudness of the original fan. I therefore wanted to replace them with more modern equivalents.
The original XT PSU is the predecessor of more modern AT and ATX style PC power supplies – it is capable of 130W maximum output with +5Vdc @ 15A, -5Vdc @ 0.3A, +12Vdc @ 4.2A, and -12Vdc @ 0.25A, as well as a “power good” signal which indicates that the power supply has stabilised and is ready for use.
A common modification for the XT is to replace the original XT PSU with a more modern ATX-style PSU, which can have all of the required voltages as well as some additional supplies and signals which are not required – such as 3.3V, 3.3Vsense, 5V stand-by, and power-on. I say “can“, because the -5Vdc rail was optional in ATX and ATX12V v1.2 and removed since v1.3, as the use of the ISA bus which requires it became less commonplace, so you’ve got to be careful to get a power supply that has it*.
*The -5Vdc rail is is used on IBM 5150 mainboards for the cassette interface, early IBM 5150 mainboards for the 4116 DRAM ICs, early ISA RAM expansion cards which use 4116 DRAM ICs, and a limited number of ISA cards, primarily sound cards – many ISA cards which require -5Vdc, particularly more modern ones, derive their own supply onboard instead of using the bus supply. As such, not all system configurations actually need it, so some people simply leave it out.
I sourced a 200W TFX-style PSU on eBay for < £10.00 shipped, an AOpen FSP200-60SAV (PF) designed for Pentium 4 mainboards – this has a small form factor, features a 110/230Vac switch so can be used with multi-region mains, and supplies all of the voltages at the required currents for full compatibility* with the original 5160 PSU.
*There is one exception: it is only rated for -5Vdc @ 0.2A versus the 0.3A from the original 5160 PSU. This rail is typically used as a reference voltage so does not require much current, hence I doubt that the 100mA difference will ever be a problem.
The donor power supply was easy enough to dismantle – it has a filter board on the back of its IEC mains socket which I removed (I don’t need this 1980s machine to be EM compliant), a mains transformer on a set of flying leads which would need to be relocated, and even a fan speed control board with a remote thermistor, which I removed (I’m happy for the new high-flow low-volume fan to run all the time, it’s already 90% quieter than the original mains fan anyway).
With the new PSU board removed from the donor, I measured up where I wanted to fit the board, mains transformer, and voltage selection switch into the original case – I then drilled and countersunk holes for each of these, and used brass standoffs and countersunk screws to install the board (ensuring that the earth resistance to the chassis was still low), and countersunk screws to install the transformer and switch.
I also fitted a 5x20mm 3A slow-blow (T) glass cartridge fuse and covered fuse holder, wired between the mains live input and the power switch, and wired the output from the power switch to the auxiliary mains socket and the PSU board.
When I was happy that everything was test-fitted correctly, I removed the PSU board to make some modifications. I needed to:
- Connect the PS-ON wire (typically green) to ground, to power the PSU all the time – modern PCs use this for soft-power, but I wanted to keep the original power switch.
- Connect the 3.3Vsense wire (typically brown) to the 3.3V output of the PSU, in case leaving this open-circuit could lead to instability – the PSU uses this to detect any voltage drop across the 3.3V cabling to account for it.
- Remove all of the 3.3V output cabling (typically orange), as these are not required – the IBM PC does not use 3.3V.
- Remove all of the 5Vstandby output cabling (typically purple), as these are not required – a standby supply is not required.
- Remove all but three of the +5Vdc output cables, all but one of the +12Vdc output cables, and all but four of the GND output cables on the main ATX cable, as the others were not required and this would make wiring neater.
I left the two Molex-style power connectors and FDD power connector as they were.
I also fitted a JST-XH flying lead for between +12Vdc and GND, for powering the new cooling fan, and replaced the electrolytic capacitors on the output side of the PSU, as these have to work pretty hard in an SMPS.
I could then wire up the output cabling for the XT mainboard accordingly, as per the pinout table on the side of the PSU – I just soldered these and covered the joints with heatshrink:
- +5Vdc: red (ATX) -> red (XT P9-4/5/6)
- -5Vdc: white (ATX) -> yellow (XT P9-3)
- +12Vdc: yellow (ATX) -> orange (XT P8-3)
- -12Vdc: blue (ATX) -> blue (XT P8-4)
- GND: black (ATX) -> black (XT P8-5/6, P9-1/2)
- Power good: grey (ATX) -> white (XT P8-1).
I then fitted the new 80mm 12Vdc cooling fan to the upper case using standard self-tapping PC fan screws, and connected it to the new fan power connector.
Before reassembling the power supply, I wanted to check that the output voltages seemed to be wired up correctly before attempting to power up the computer – ATX-style power supplies are usually happy to be powered up without load, however the voltage readings will probably not be particularly accurate as the rails usually require some form of load to be able to regulate themselves properly.
I powered up, and the fan spun, which was a good sign; +5Vdc read about 5.3Vdc, a bit high; -5Vdc read about -4.9Vdc; +12Vdc read about 11.7Vdc; -12Vdc read about -10.2Vdc, quite low; power good read the same as the 5Vdc rail.
I also cleaned up the case using a wire brush to clean off most of the rust and corrosion, and flushed the power switch out with contact cleaner.
I was happy that these would work OK under load, so I reassembled the power supply using four standard PC screws, then refitted it to the PC – I’d already done some other preventative maintenance on the XT at this point, including replacing all of the tantalum electrolytic capacitors on the mainboard and ISA cards which apparently have a habit of failing short-circuit, so it should be safe to power up.
With the power supply refitted and reconnected, I powered the machine up and tested its output voltages under load: +5Vdc read 5.03Vdc; -5Vdc read -5.16Vdc; +12Vdc read 12.28Vdc; -12Vdc read -12.09Vdc; power good read the same as the 5Vdc rail.
The XT takes a little while to boot up as it counts/tests the installed memory, but eventually the 5.25″ FDD resets to its home position and it chimes with a POST code – I connected it to my refurbished 5151 CRT and the XT is working well, so the modified power supply seems to be working correctly!
Overall, I’m very pleased with this power supply upgrade – it’s tidy and looks professional, and it’s reliable, cool, quiet, efficient, much lighter, and cheap.
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