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IBM 5155 Portable Project: Looking for ideas and direction

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ButINeededThatName

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So a little over a year ago now I picked up an IBM Portable 5155 that I found on the side of the road of all places. Anyways, to keep a long story short, virtually everything aside from the case, power supply (though that went out in a blazing glory when I tried to test it) and CRT was consumed by the onslaught of battery acid brought on from two expansion cards and their barrel batteries. Seriously, the board and expansion cards were toast as it just creeped along most of the traces it could get to and, making matters worse, the floppy drives had been pulled and the springs and switches in the keyboard were utterly ruined by rust.

So with that said, after a lot of hard cleaning and partial re-assembling, sourcing a replacement power supply (still need a keyboard and CGA card) I'm now at the point where I'm considering different paths to take on this restoration, seeing as how I currently have a blank canvas, so to speak. The first is pretty straight forward; restore it back to it's original state. I'm sure this is the preferred method of a lot of you here, but seeing as I already have a PC, XT and AT, I kind of what to do something different and more creative here, which brings me to my other two concepts...

1) Track down the original system board, a MACH 20 card, and two 1.2Mb floppy disk drives (or one disk drive and a hard drive) and make this machine an example of what it could have been later in it's life after being upgraded by a tech-savvy owner. This is my preferred method, though MACH 20s are hard to come by, especially with their additional memory and disk controller add-ons.

2) Track down the system board from an XT-286 (or alternatively the later revision system board from an AT, both should still fit), the drive controller from an AT, a 1.2Mb disk drive and a 5.25" hard drive and skip the whole "tracking down a", and "paying out the nose for a MACH 20" thing.

Anyways, what do you all think would be the best route or alternatively, what other route would you take with this project were it yours?
 
I like option 1, but that's my personal taste.
Also with motherboards, going with an AT style could leave you in a pickle when it comes to the keyboard.

One mod I'd love to see on one of these would be a colour CRT. Chosen to fit the chassis and hooked to the composite colour output on the CGA card.
 
I love my 5155, but I really wish they had made the internal monitor a true-CGA. Composite creates weird artifacts on certain colors.

But, once you get past that, it’s a fabulous machine. Also, setting DOS to black & white with the “mode bw80” command helps with the screen. The amber display is very nice.

I just recently put a HardCard 20 in mine to make it more useful.
 
compaqportableplus said:
I love my 5155, but I really wish they had made the internal monitor a true-CGA. Composite creates weird artifacts on certain colors.

But, once you get past that, it’s a fabulous machine. Also, setting DOS to black & white with the “mode bw80” command helps with the screen. The amber display is very nice.
Dead easy to fix, in the CGA card just snip the resistor that introduces the color subcarrier to the composite video out (it feeds with others the base circuit of the video output transistor) , and the artifacts seen on the amber monitor vanish.
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Click to expand...
 
ButINeededThatName said:
2) Track down the system board from an XT-286 (or alternatively the later revision system board from an AT, both should still fit), ...
Click to expand...
The following measurements may prompt you to do further research.

12" x 8.5" = IBM PC (5150) and IBM Portable PC (5155) and IBM XT (5160) motherboard [As measured by me.]

13.8" x 12" = IBM AT (5170) motherboard - type 1 [As measured by me.]
13.8" x 9.4" = IBM AT (5170) motherboard - types 2 and 3 [As measured by me.]

13.04" x 8.57" = IBM XT-286 (5162) motherboard ???? [Unconfirmed. I do not have one to verify.]
 
compaqportableplus said:
Really? Cool! Which resistor is it? A picture would really help.
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I'm travelling, but when home I will look it up. If you look at the schematic for the IBM CGA card, on minuszerodegrees, you will see that the sync pulses, the r,g b and I signals and the color subcarrier signals are mixed into the base circuit of the video output transistor (the only transistor on the board as I recall), simply disconnect the resistor that couples in the subcarrier, it's easy to see on the schematic.
 
Chuck(G) said:
Why not retrofit it with a modern nATX board, power supply and LCD monitor? You should have more than enough room to do it.
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I'd really like to swap out the monitor in my 5155 for an LCD, I'm not keen on the composite due to my eye sight, Other than the monitor i'm happy with my 5155, While out shopping a few weeks ago i was looking at one of those POS type LCD's above the counter, It was a "Wincor Nixdorf" and showed lovely clear text / pictures and wondered if one of those could be used, I've seen them on Ebah at a reasonable price.
 
Hugo Holden said:
I'm travelling, but when home I will look it up. If you look at the schematic for the IBM CGA card, on minuszerodegrees, you will see that the sync pulses, the r,g b and I signals and the color subcarrier signals are mixed into the base circuit of the video output transistor (the only transistor on the board as I recall), simply disconnect the resistor that couples in the subcarrier, it's easy to see on the schematic.
Click to expand...

Okay, I’ll see if I can make sense any of that.
 
compaqportableplus said:
Okay, I’ll see if I can make sense any of that.
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The resistor to disconnect is R8 a 750R (750 ohm) resistor feeding the base of Q1 the video output transistor. You will see it on the schematic. One easy way is with a very sharp pair of flush cutters just to cut one of the resistor's wires near its solder pad so you can solder it back in later. You could de-solder one resistor wire or remove the resistor entirely, or cut the IC pin on the buffer IC feeding the resistor (but that is a rough thing to do). Be aware that this will kill the color carrier on the composite output connector on the card too, mostly that is not used.

In my 5155 I use an EGA card, with a home made feature adapter, and simply omit the signal that feeds the 750R resistor. Because the Amber CRT in the 5155 has such a good video high frequency response the color subcarrier plays havoc interfering with the luminance signal, so it's better to eliminate the colour subcarrier in the signal feeding the Amber VDU. IBM only did it the way they did because the connector on the CGA card, that feeds the monitor, was intended for an RF modulator and without adding an extra transistor, they could not get a monochrome signal for the 5155, that is if they still had a color signal on the RCA video output socket on the card.

Perhaps somebody with a CGA card (that is out of a computer) could post a photo with R8 labelled for you.

If you look at the images on page 18 of this article you can see the interference produced by the subcarrier:

http://worldphaco.com/uploads/FITTING_AN_EGA_CARD_TO_AN_IBM_5155.pdf

On my feature adapter I simply grounded the input to the buffer IC feeding the 750R resistor, I think just disconnecting the resistor will work just as well.
 
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SpidersWeb said:
I like option 1, but that's my personal taste.
Also with motherboards, going with an AT style could leave you in a pickle when it comes to the keyboard.

One mod I'd love to see on one of these would be a colour CRT. Chosen to fit the chassis and hooked to the composite colour output on the CGA card.
Click to expand...

Yeah, option one is my preference as well, though finding a complete MACH 20 may prove to be a royal PITA.

That's right, I completely forgot that the AT doesn't work with an XT keyboard.
 
ButINeededThatName said:
Yeah, option one is my preference as well, though finding a complete MACH 20 may prove to be a royal PITA.

That's right, I completely forgot that the AT doesn't work with an XT keyboard.
Click to expand...

For me, the amber CRT is part of the charm of having the 5155. However, if you were so inclined, I would bet you could swap the entire CRT assembly with an assembly from one of those common 9" CRT VCR combo portable TV things. They often have a chip on them that displays menus, volume indicators, and on-screen numbers. With some modification, you can actually tap into this input on the CRT controller and supply it with a digital RGB signal from the internal CGA card! I believe it was the 8 bit guy who did a video showing the modification of a cheap CRT tv to become a digital RGB monitor. I am thinking of doing this mod with a compaq Portable, since those are pretty readily available.

Also, XT to AT keyboard converters exist, and wouldn't be that hard to make yourself if you have any experience with programming microcontrollers.
 
compaqportableplus said:
Thanks for the info! Now that I’ve got the number of it, I should be able to find it.
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Probably when you have done this small modification you will be much more impressed when you see this monitor display graphics and not want to replace it with a modern screen.

That way you will retain the charm of the original monitor. There is the risk that putting a modern screen in, that it will degrade the originality and value of your 5155.You might end up creating an anachronistic Frankensteinian monster. But I know that is only one view of the notion of things like fitting modern screens in old housings. Each to their own I guess, but in my case I'm fairly strict about how far I go modifying vintage electronic equipment, especially an iconic item. Small reversible mods are just fine I think or when there really is no choice due to total lack of parts.

The Amber monitor is a very well made item, especially the CRT its self to which (assuming good condition) has a very sharply focussed beam , good contrast ratio, excellent video high frequency response and a good raster geometry. I think Zenith did a really good job on it.
 
dkarguth said:
For me, the amber CRT is part of the charm of having the 5155. However, if you were so inclined, I would bet you could swap the entire CRT assembly with an assembly from one of those common 9" CRT VCR combo portable TV things. They often have a chip on them that displays menus, volume indicators, and on-screen numbers. With some modification, you can actually tap into this input on the CRT controller and supply it with a digital RGB signal from the internal CGA card! I believe it was the 8 bit guy who did a video showing the modification of a cheap CRT tv to become a digital RGB monitor. I am thinking of doing this mod with a compaq Portable, since those are pretty readily available.

Also, XT to AT keyboard converters exist, and wouldn't be that hard to make yourself if you have any experience with programming microcontrollers.
Click to expand...

The two problems I see using RGB monitors or TV/video to subs in for GGA monitors:

Most of the monitors made for general video work have an upper video frequency response in their gun driver circuits that is not as high as a computer grade CGA monitor and the phosphor pitch is often coarser in the actual crt, so the graphics resolution won't be quite as good, unless it's an excellent grade monitor type, some of the Medical grade Sonys were pretty good though with very fine pitch phosphor CRT's. Some digital flat screen LCD/TFT monitors might be better in this respect.

Also, a sub circuit needs to be added, it can be done just the way they did it in the IBM5153 with some open collector gates and a resistor matrix so that the I signal is accommodated, as well as the RGB signals from the CGA card. Otherwise the intensified colours won't be correct and there also won't be a 16 level greyscale. (It's an RGBI system after all not just RGB)

Pages 21 and 22 of the article I mentioned in post #12 cover how IBM used the I signal to modify the RG&B levels.
 
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Hugo Holden said:
Probably when you have done this small modification you will be much more impressed when you see this monitor display graphics and not want to replace it with a modern screen.

That way you will retain the charm of the original monitor. There is the risk that putting a modern screen in, that it will degrade the originality and value of your 5155.You might end up creating an anachronistic Frankensteinian monster. But I know that is only one view of the notion of things like fitting modern screens in old housings. Each to their own I guess, but in my case I'm fairly strict about how far I go modifying vintage electronic equipment, especially an iconic item. Small reversible mods are just fine I think or when there really is no choice due to total lack of parts.

The Amber monitor is a very well made item, especially the CRT its self to which (assuming good condition) has a very sharply focussed beam , good contrast ratio, excellent video high frequency response and a good raster geometry. I think Zenith did a really good job on it.
Click to expand...


Oh, I had no plans of replacing the CRT anyways. I had had learned to live with the CRT’s quirks, but this little workaround is something I definitely didn’t know about. Still need to lug my 5155 out and try it,


Yes, I do like to try and keep mods reversible as well. I’m not big on taking Dremels to my old machines in order to make stuff fit for example.


I also agree that the Zenith CRT is very well-built.
 
I've always sort of wondered whether or not it'd be possible to make a 5175 using a XT-286 motherboard and reusing the stock 5155 CGA card...
 
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