View Full Version : IBM 5151 monochrome monitor - how to convert to worldwide voltage

August 2nd, 2013, 10:23 PM
Gidday people,

My winter project has been to investigate if it's possible to convert several 110VAC-only items in my much-loved IBM collection to run on our country's unforgiving 230 VAC mains. I dislike modifying things from stock but will do so when the chance of an accident is significant.

I've tackled the 5151 mono monitors first as it's by far the easiest, and at a guess probably one of the very few CRT monitors ever made that uses a line-frequency power transformer. In fact, once past the grossly-inefficient linear voltage regulator circuit, the monitor runs entirely on 15 VDC at 870 mA, a mere 13 watts!

The first step was to purchase a suitable switch-mode power supply to replace the transformer and regulator. The best I could find is a Lambda 26W 15V @ 1.7A, Digikey part number 285-1885ND, at only US$17.50. Even in NZ it was a bargain at $23.50 and I got (2) shipped free by piggy-backing on our weekly order at work.

I removed the power transformer and the incoming AC terminal strip and fuse, just clipping off the line cord connections. The transformer output terminates in a plug on the main PCB, easily removed, as is the connection to the remote TO-3 style regulating transistor - which I left physically in place. As it presents a short to external DC, you'll also need to extract the 7815 three-terminal regulator from the main PCB, desoldering and removing the screw from the heat sink. Easy, when there's no RoHS involved.

By complete luck the threaded mounting holes on the switcher exactly match two of the open holes where the transformer was located. I used one M3 screw from the AC terminal strip and a shorter M3 screw at the rear-most hole to secure it. Naturally you have to ensure the screw ends don't touch anything internal to the switcher. The AC line cord had to be moved in slightly within the strain relief clamp and wired with crimp-on terminals as shown in the photo. The ground on the switcher is provided via the mounting to the plate, no wire required. The connection to the PCB is equally simple, see the photo, brown is +15V. All that's left is to check the output is 14.9 to 15.0 V and screw it back together.


August 2nd, 2013, 10:43 PM
I don't know much about this topic, but i'd say from those pics, it looks like a professional job. does it all fit inside the case of the monitor itself? It seems like all you'd have to do at this point is plug it in and see what happens.

August 3rd, 2013, 03:14 AM
If you need another:

http://www.vintage-computer.com/vcforum/showthread.php?38382-IBM-5151-mono-monitor-in-excellent-condition (http://www.vintage-computer.com/vcforum/showthread.php?38382-IBM-5151-mono-monitor-in-excellent-condition)

August 3rd, 2013, 04:00 AM
Nice to see another kiwi PC collector! I have a room dedicated to IBM here in Wellington, just added three more desks. Only one UNIX machine though, a 6150 RT.

Excellent work on the PSU conversion. Finding MDA, CGA or EGA displays here can be a challenge so it's good to have alternatives. For some reason I never actually thought about replacing the PSU board with something more modern.

August 3rd, 2013, 04:02 AM
Huh I just learned something interesting.. The CRT used in the 5151 is the same as my Sampoo black and white TV :P Just with different phosphors.

That's kinda cool!

August 3rd, 2013, 08:16 AM
Perhaps this is too late to matter, but I'll offer something that I discovered in some old Panasonic laser printers. They built them for 240VAC operation, then, for US/Canada/Japan operation, included an extra transformer to raise the 120V line voltage to 240. It was really odd to see a product with a transformer tying into a transformer.

In the UK, there's a type of transformer that's made for operating portable power tools, such as drills and saws. They're usually pretty beefy (http://www.amazon.co.uk/3-3KVA-Transformer-Power-Tools-Heavy/dp/B007WRTKLY). Does such a thing exist in Kiwiland?

August 3rd, 2013, 02:59 PM
Yeah, I have a couple of step-down transformers including a 1kVA construction site type for my laser printer. But with the IEC connectors used on all this IBM kit there is no way to distinguish 110 from 230 other than being very careful when I set them up for occasional use. I would like everything to be "natively" 230 V so I don't have to think about it when I plug things in.

The PC 5150s are next. Have considered swapping the internals for a Baby AT PS but that really damages the originality. Have looked at the 110V 63 W PS schematic and believe it can be modified to take 230.

The 5153 CGA monitor is another story as it's a typical color CRT design with a line-powered degaussing coil. That will take some thought.

August 3rd, 2013, 03:14 PM
Be careful--the original 5150 black PSU used a 120V line-powered AC fan. Later PSUs used a DC fan.

August 3rd, 2013, 03:40 PM
Digikey part number 285-1885ND
Input voltage range: 88 to 264 VAC

So, when the local mains stepdown transformer partially fails, dropping your house's 230 VAC to say, 100 VAC, your 5151 will continue to work.
If you can get your 5150 to be brownout tolerant as well, you'll have something to do while you are waiting for the brownout to be fixed. :)

Although, as a friend of mine discovered, modern TVs deal with significant brownouts quite well.