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Thread: IBM 5154 Problems & Electrolytic Caps Question

  1. #11

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    One interesting thing about the 5154 monitor, the generation of the EHT (for the final anode voltage for the CRT) is independent from the horizontal scan system (it has to be because its multi-standard for CGA & EGA).

    This means that the scan circuits can be working just fine and just the EHT for the CRT final anode can fail independently. In the process of doing that, the picture can change size (because in magnetic deflection, the amount of deflection the beam experiences is inversely proportional to the square root of the EHT voltage). So for example, if the EHT was dropping due to a fault, the picture would expand in size, or say if the EHT increased , the picture (in both width & height) decreases in size. The changing size picture before it failed and the popping sounds , suggests an EHT failure. The all green screen suggests a problem in the video amplifiers or the circuits that drive the cathodes and grids of the CRT.

  2. #12
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    I have two spare 5154 power supplies, took a picture of one and marked the positive (red + sign) end of the caps.
    The polarity appears to agree with the half moon type markings on the board....the white side being the negative.
    Some of the caps do also have a + sign on the board which is correct.

    5154-PS-Cap-Polarity.jpg

    If the power supply caps polarity are all correct, I would still check those power supply voltages from the page I posted earlier.
    Maybe you have a bad (new) cap.
    Last edited by mikey99; July 14th, 2019 at 02:27 PM.

  3. #13
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    Just curious, which capacitors need to be low ESR? I didn't make sure to replace any of the caps with low ESR ones, which could explain failures.

  4. #14
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    Quote Originally Posted by willmurray461 View Post
    Just curious, which capacitors need to be low ESR? I didn't make sure to replace any of the caps with low ESR ones, which could explain failures.
    None that I am aware of in the power supply. I used generic Xicon 105C temp rated capacitors that I purchased from partsexpress.com.

  5. #15

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    Quote Originally Posted by mikey99 View Post
    None that I am aware of in the power supply. I used generic Xicon 105C temp rated capacitors that I purchased from partsexpress.com.
    I seem to remember the one on the neck board being low-esr, along with one or two other caps on the HV board.

  6. #16
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    I still have the original caps, so I might put back the neck board one as well as the other low esr ones, if you can remember which ones they are.

  7. #17

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    Looking at the schematic for the 5154, there is something interesting about it, compared to the CRT gun controls on the 5153.

    In the 5154, the input R,G & B signals, after they pass through the input buffers, are direct coupled by the amplifier chain all the way to the cathodes of the CRT. There is no AC coupling. blanking signals etc are common to the 3 channels, as are the CRT grid voltages as they are common too in the CRT.

    In summary, there are no unique electrolytic capacitors in that system for each of the R, G and B amplifier channels. This means that, the drift in the green level you saw cannot possibly have been due to an electrolytic capacitor issue.

    It was instead one of two things:

    Either there was a DC offset or drift in the green channel amplifier that altered the green CRT cathode voltage, or it was a CRT issue with the emission on the R and B guns being slow to warm up. The only way to tell would be to monitor the CRT's cathode voltages and see if the green channel (or R & B together) had a drift after the set was turned on. If not, it must be the CRT.

    When the set is working again, if the green drift issue reappears, this is something that could be easily checked.

  8. #18
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    I just looked at the old caps I took out and remembered that some of the 4.7uf 50v caps have a black casing, whereas others have a blue casing. Is it possible that these black ones are the low ESR caps mentioned? Otherwise, is there any way to look at a capacitor and tell if it is low ESR?
    IMG_0939.jpg

  9. #19
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    I just opened it up again, unplugged the video input board and plugged it back in and now the sets back to working (sort of)! The weird pulsing/vibrating issue is still there. How would I check the EHT? Also, what would cause EHT failure?

  10. #20

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    Quote Originally Posted by willmurray461 View Post
    I just opened it up again, unplugged the video input board and plugged it back in and now the sets back to working (sort of)! The weird pulsing/vibrating issue is still there. How would I check the EHT? Also, what would cause EHT failure?
    An EHT probe will measure , they need to be at least 30kV capable, and the probe tip is slipped under the EHT cap on the CRT,can be tricky if you haven't done it before, however, there is mostly no need to measure it, IF there is a visible scanning raster of the correct size AND a focused CRT beam, both these things imply that the EHT is normal and a final check is if the raster doesn't change size much with alterations in brightness and contrast (CRT beam current) that implies that the internal resistance of the EHT generator is low enough to be normal.

    If there is nothing visible on the CRT face, the CRT's heater is running, and the CRT's grid and cathode voltages are in the low range of 0 to 10V (grid more negative than cathode) there should be a beam visible, but if say the grid is 60V or more negative the CRT beam current will be cut off (even with normal EHT). So if that is checked, still no picture, then it probably is the EHT generator.

    Often, when a set turns off & on you can hear an electrostatic "crackles" when the EHT comes and goes, that is another clue.

    The EHT generator in the set is practically identical to the H scan output stage (but it does not power the H coils) . So it has an output transistor, damper diode, transformer and EHT rectifiers and is driven from a similar drive source as the H scan stages. If your main power supply voltage peaked up, that could take out the output transistor, so that is the very first thing to check (after checking these circuits have their correct power supply voltage).

    A pulsing effect of some kind (not exactly sure what you mean by that), suggests that some protection circuitry is shutting down your main power supply at intervals, this happens if one of the stages it powers draws too much current, which of course happens if the HOT is shorted out, or the similar transistor in the EHT generator is shorted out.
    Last edited by Hugo Holden; July 24th, 2019 at 02:32 PM. Reason: typo

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