Earlier in 2025, 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 5151 is a 12″ TTL monochrome 720 x 350 50Hz CRT monitor, which shipped with the original IBM PC for use with the IBM Monochrome Display Adapter (MDA) card and other compatible graphics cards. The 5151 uses P39 high-persistence green phosphor, and is unusual in that it does not have an internal horizontal oscillator, instead using the HSYNC input signal directly for horizontal drive.
The display was in reasonable physical condition, with a lot of scratches and marks to the paintwork. It also looked like it had been dropped, as there was a lot of plastic damage – the worst of which was to the front panel, where one of the upper supports had broken off, and a large crack along its lower front.
I wanted to disassemble the display so I could repair the plastic damage and perform some preventative maintenance before attempting to power it on for the first time.
Display Disassembly
The display is easy enough to dismantle – it has six cross-head screws which hold on the rear case, two on its top (hidden below two screw covers which just pop out) and four on its underside, two of which are brass and hold it to the front panel, and two of which are black and hold it to the power assembly. With these removed, the rear cover should come away.
The inside is then exposed, comprising the CRT, mainboard, and power assembly.
The power assembly can be removed by disconnecting the transformer secondary and transistor cabling from the mainboard, removing the mains cable from its strain relief on the case, removing the brass screw which connects its metalwork to the ground strip that runs around the front of the CRT, and removing the two black cross-head screws which hold its metalwork onto the case.
The mainboard can be removed by pulling off the two control knob covers for the brightness and contrast controls on the front panel, removing two black cross-head screws that hold it into the case, disconnecting the ground lead to the CRT dag, disconnecting two connectors which go to the CRT yoke, and disconnecting the two connectors which go to the video input cable.
The CRT can be removed by removing the four black cross-head screws in each corner, including one which has the cabling for the dag ground connection to the mainboard.
Power Assembly Modifications
The original linear (technically hybrid) power supply in the IBM 5151 requires a large, heavy mains transformer, which puts a lot of stress on the brittle plastic case and probably contributed to some of the case damage that had occurred previously.
There have been a lot of technological advancements in electronics since the 1980s, including power supply design – inspired by this thread which points out that the 5151 is supplies by a single 15Vdc rail at approx. 870mA, I decided to replace the original linear supply with a modern SMPS as per their recommendation.
This modification also adds the benefit that the 5151 can now be powered from any typical mains supply worldwide, as the chosen SMPS operates happily from 88Vac to 264Vac line voltage at 47Hz to 63Hz.
This involved removing the original mains transformer, fuse holder and cover, and switching transistor – I fitted a new covered 5x20mm cartridge fuse holder with T1A fuse using one of the original fuse holder mounting holes, and a Lambda LS25-15 SMPS module using two of the original transformer mounting holes.
I connected the mains input to the new SMPS using the original mains cable via the internal fuse, and the DC output of the SMPS to the original TR24 cable with T1 (red) as 15V and T2/T3 (black/blue) as ground – this allows the power assembly to still be easily disconnected from the mainboard without major physical modifications, but it does require some rewiring of the mainboard, which is covered below.
Mainboard Modifications
The earlier modifications to the power assembly required modifications to the mainboard, as per the schematic below – these remove some unnecessary parts but primarily isolate and ground T2/T3 of TR24, allowing us to use this connector for power input.
This included removing 15V regulator IC601 and its heatsink, AC coupling capacitors C601 / C602, bridge rectifier diodes D601 / D602 / D603 / D604, power resistors R601 / R602 / R603, and regulator input decoupling capacitor C604, then bridging R603 and C604. I also redirected the positive side of C603 onto the 15V rail, to add some bulk capacitance onto the output of the SMPS to add extra filtering.
While the mainboard and neckboard were already out, I performed some preventative maintenance, including:
- Cleaning and lubricating the brightness and contrast controls.
- Reflowing solder joints on components that are likely to crack (transformers, connectors, controls, power resistors).
- Cleaning off any browning adhesive, which is water-absorbing, and therefore potentially corrosive.
- Replacing all of the electrolytic capacitors as these can be subject to ageing, including the bi-polar 18uF 25V horizontal output capacitor C505, which I replaced with a high-frequency 18uF 250V axial polypropylene film capacitor.
Case Repair & Respray
I managed to find most of the broken plastic pieces inside the depths of the display, and used these along with superglue to reconstruct the case – I also patched up all of the cracks, and the case was sturdy again.
The paint was still in poor condition, though, coupled with superglue marks – as such, I decided to repaint the bulk of the case. I went with RAL 9002 “grey white” for the colour, which seems to be a very close match to the original IBM paint.
I filled any large cracks and chips using plastic putty; I cleaned down the plastic using pre-paint cleaner to remove any dust and grease, carefully masked the areas that I didn’t want to paint using high-quality painters tape, then painted the exposed surface using a couple of coats of white plastic primer followed by a couple of coats of matte R9002 spray paint.
I didn’t paint the grey inner bezel of the display, and I also didn’t paint most of the underside of the front and rear case pieces, as these are not easily visible and would be tricky to paint correctly – the masking tape gives nice crisp edges around the textured and non-textured plastic areas where I decided to draw the line.
Once the front panel had finished drying and the paint had hardened, I cleaned off the CRT and refitted it to the front panel, then refitted the mainboard and power assembly.
Now that I was happy with the finish on the front panel of the display, I repeated the process for the rear cover – this was a larger surface area and had vents so required extra cleaning and preparation, it also had two badges which needed to be removed.
Reassembling the display was just the opposite of its disassembly – I also added some washers to the upper screws on the rear case to help prevent plastic damage in future.
I then connected the 5151 to my IBM 5160 which I was in the process of refurbishing – I knew this was producing POST codes from its speaker beeping, but I hadn’t tested its MDA video output yet as I didn’t have another display.
Sure enough, the 5151 worked first time after tweaking the brightness and contrast controls – with some tweaks I also managed to get the 5160 booting from its internal 5.25″ FDD, and ran some diagnostic tests to put the MDA card and display through their paces.
This restoration has been a major success – the display weighs a lot less now that the original transformer has been removed which puts far less stress on the brittle plastic, and with the new paintwork it looks practically brand new!
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