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Thread: IBM 5155 Portable Project: Looking for ideas and direction

  1. #11

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    Quote Originally Posted by Hugo Holden View Post
    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.
    Okay, I’ll see if I can make sense any of that.
    Compaq - “It simply works better”

  2. #12

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    Quote Originally Posted by compaqportableplus View Post
    Okay, I’ll see if I can make sense any of that.
    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/FITTIN...N_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.
    Last edited by Hugo Holden; July 17th, 2019 at 07:20 AM.

  3. #13

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    Thanks for the info! Now that I’ve got the number of it, I should be able to find it.
    Compaq - “It simply works better”

  4. #14

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    Quote Originally Posted by SpidersWeb View Post
    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.
    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.

  5. #15

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    Quote Originally Posted by ButINeededThatName View Post
    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.
    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.

  6. #16

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    Quote Originally Posted by compaqportableplus View Post
    Thanks for the info! Now that I’ve got the number of it, I should be able to find it.
    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.

  7. #17

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    Quote Originally Posted by dkarguth View Post
    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.
    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.
    Last edited by Hugo Holden; July 18th, 2019 at 02:16 PM.

  8. #18

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    Quote Originally Posted by Hugo Holden View Post
    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.

    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.
    Compaq - “It simply works better”

  9. #19
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    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...

  10. #20

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    Quote Originally Posted by MrArgent View Post
    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...
    The IBM Portable Personal Computer Model 286
    Compaq - “It simply works better”

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