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Thread: ibm 5151 blinking cursor then shuts off

  1. #1
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    Default ibm 5151 blinking cursor then shuts off

    Hi!
    A while back I aquired an ibm 5151 moniter. Unfortunatly ive never been able to get it to work because at most 10 seconds after powering it on, it shuts off and then will not turn back on for a few minutes. The phospher glow from the back of the crt will also disapear for this few minutes. The moniter also only gives me a blinking curser, even when attatched to a mda card. I am hooking it up to an xt clone that displays the ram count on startup, so I know I should be getting a ram count and not just a blinking cursor.
    I went ahead and recapped most of the capacitors with a kit i got off of the consol5 website. However, not all were changed because I was confused over where some should go because they were shapped differently than anything i saw on the board and i didnt want to take a chance and short the board. Even after recapping, the problem was unchanged. Do you guys think the problem even would be the capacitors, or do you think its a problem with the power supply or the connectors?

    Any help would be appreciated.

    Thanks!

  2. #2
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    I see two symptoms, with different causes:
    #1. 5151 shutting down.
    #2. Unexpectedly seeing only a blinking cursor when the 5151 is moved to a computer that is known to display a RAM count-up.

    Regarding issue #2. I can produce that symptom on my IBM XT. It is normally fitted with a CGA card and attached to a CGA monitor. Now, if I bring across the MDA card and 5151 from my IBM PC, but forget to change the XT's motherboard switches from CGA to MDA, I see only a flashing cursor. An XT clone could behave differently, but I think that you should first rule out this (incorrect motherboard switch settings) possibility for us first.

    Regarding issue #1. Confirm for us that your, "The phospher glow from the back of the crt ...", is the glow shown at [here]. That glow is not phosphor. It is the glow produced when a large current is put through a thin wire.

  3. #3
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    Ok, cool. Thanks for the reply. Ill go ahead and rule out issue #2 for now. As for issue #1, I misspoke. It is indeed the glow that you cited, produced when the large current is being put through a thin wire.

  4. #4
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    Regarding symptom #1.

    It sounds like the symptom is triggered by heat: When the symptom appears, the symptom 'cause' cools down, and everything starts working again. A constant cycle of on and off.

    The glow is from the cathode's heater. Per [here], as soon as you power on the 5151, the power supply produces +15Vdc (and +5Vdc), and per [here], the +15V indirectly powers the cathode's heater. So, the fact that the glow from the cathode's heater in your 5151 is cycling on and off, suggests that the +15V may also be cycling on and off.

    If you have a multimeter, you could measure the +15V to see whether or not is cycling on and off. Note that the +15V may be dropping down to 0V, or may be dropping down to a low value.
    If the +15V is cycling, the task then becomes to work out why: power supply section, or something else affecting the power supply.
    If the +15V is not cycling, the cause is in the heater circuitry ([here]).

  5. #5
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    So i went ahead and got a multimeter and took some measurements.

    I took a reading at pins s1 and s2 on the main board leading from the power supply. While i still had a display on the screen, it measured about 13.37 before the display failed. It then dropped down to about 6.68 and the lomger it stayed on, it dropped to around 1.3.

    I also took measurements from the 7 pins on the back of the crt socket board while it was plugged into the back of the crt. I believe these are the 7 pins leading from the heater tube that you wanted me to take measurements from? Correct me if i amvwromg sine I am not great at reading schematics. If going upward clockwise from left to right, then pin 2 reads 3.7 before the display fails, and then drops down to 0, and pin 3 reads 12.35 before the crt fails then drops down to 6.2 and then the longer it stays on, it drops down to 0.

    So in this situation since the reading from the power supply drops down as well, am i then to assume that it is a problem with the power supply or something else affecting the power supply? And if so, where should i begin to look from here?

  6. #6
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    Quote Originally Posted by Seanbar View Post
    I took a reading at pins s1 and s2 on the main board leading from the power supply. While i still had a display on the screen, it measured about 13.37 before the display failed. It then dropped down to about 6.68 and the lomger it stayed on, it dropped to around 1.3.
    S1 and S2 is the secondary of the power transformer. The SAMS circuit diagram of the power supply, at [here], shows that when your 5151 is working, you should be measuring about 22 Vac. Did you have your mulimeter in AC volts mode, with one probe on S1 and the other on S2 ?

    Quote Originally Posted by Seanbar View Post
    I also took measurements from the 7 pins on the back of the crt socket board while it was plugged into the back of the crt. I believe these are the 7 pins leading from the heater tube that you wanted me to take measurements from? Correct me if i amvwromg sine I am not great at reading schematics. If going upward clockwise from left to right, then pin 2 reads 3.7 before the display fails, and then drops down to 0, and pin 3 reads 12.35 before the crt fails then drops down to 6.2 and then the longer it stays on, it drops down to 0.
    Looking at the fitted yoke board from the rear of the my 5151, pin 1 is the bottom pin, with the numbering going clockwise. But the +15 Vdc cannot be measured there.

    The +15 Vdc can be measured at many points on the mainboard. How about you measure it at pin 3 of the 7815 chip, as shown at [here]. Negative lead of multimeter to the chassis (or to pin 2 of the 7815). Do the measurement when the 5151's video/sync cable is disconnected.

    Also, have you done a very good visual inspection? Have you re-seated the connectors on the main board?

    Also, as a possibility for later, do you have the ability the solder, just in case certain parts of the circuit need to be isolated from the power supply as part of diagnosis ?

  7. #7

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    Quote Originally Posted by Seanbar View Post
    While i still had a display on the screen, it measured about 13.37 before the display failed. It then dropped down to about 6.68 and the lomger it stayed on, it dropped to around 1.3.
    For these faults , if you could attach a schematic of the monitor on one if your posts, it makes a huge difference to anyone trying to help you with the fault.

    As I recall from the monitor schematic, the power supply uses a regulator IC, assisted by and additional pass transistor a MJ2955 I think. The regulator IC of course has thermal shutdown. So lets say, for argument's sake, the pass transistor (that handles the bulk of the supply current to the monitor's circuitry) failed, then the regulator IC would have to pass the whole amount. Likely the voltage then could be a tad low, something like 13.37, before the IC regulator overheated and went into shut down mode.

    Its also possible of course that the pass transistor is still ok, and say, for example, the regulator IC has lost thermal contact with its heat sink.

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