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Troubleshooting 520ST


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Hi - This is my first attempt to troubleshoot any ST.  Also I am a novice when it comes to electrical matters and I don’t know how to read a schematic so please go easy on me 😁


This 520ST came to me with a frozen power rocker switch (in the off position) and an original PSU that I measured showing the 5V pins were putting out 14V instead of 5. So bought and wired-up a new PSU and I replaced the rocker switch with a new one. When I fired it up for the first time I got a glorious desktop screen!!!!  I was so excited!!  I switched it off, plugged-in the mouse and switched it back on. Nothing. Removed mouse.  Nothing and nothing ever since. 

I took power readings at the pad circled in red on the back of the PCB with and without the 520 power rocker switch on (see images below). Do my power readings seem normal?  Is 30V on the two ground pins normal when on?  For all my power measurements I had the black probe on the ‘COM’ terminal of my new PSU and the red probe on the individual pins on the back of the 520 PCB. The pins that I believe are the 7 pins of the power connector. Is my method of testing the power on these pins valid (black probe on the PSU ‘COM’ terminal)?

Picture of the PCB for identification. Shown with diagnostic cart inserted. When I got the one and only boot to a desktop I did not have the diag cart installed. I was just seeing if the diag cart made any difference.  It does not. 


I believe the 7pins circled in red are for the power connection. These are the pins I read the voltages for. My multimeter black probe was on the ‘COM’ terminal of the PSU. 


My voltage readings on the pins from Left to Right with 520 switch both off and on. Do these readings look OK/normal?


Assuming these voltage readings are OK and given the fact that my first power-on got me to a desktop and no screen image since… What are the recommended next troubleshooting steps?  TIA


Note - I detect no hot or even warm board components. No bulging or leaking caps. 

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Quick UPDATE - The plot thickens… I have had it boot to desktop and to the diagnostic cart several times since my first post.  However it is inconsistent. I get no screen image many times when I switch it on. I can’t make any sense of it. I did connect the keyboard and got one boot of the diagnostic cart but none of the keys seem to work. 




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Second Update:  One of the times it booted, with the keyboard reconnected and with the diagnostic cartridge I noticed a message quickly scroll off the top of the screen before the Diag screen that I attached above.  Here's what the message says:


"Testing MFP, Glue timing, Video

K1 Keyboard not responding

Keyboard failed, connect RS232 terminal"


The little LED in the lower left corner of the keyboard always lights-up when I turn the 520ST on, regardless of whether or not I get a screen image or not.  So at least that LED connection of the keyboard is working.


So... in addition to troubleshooting why it is only booting-up intermittently...   How do you troubleshoot a 'K1 Keyboard not responding" ?  Are bad ST keyboards a common failure?

Edited by 800_Rocks
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2 hours ago, 800_Rocks said:

Are bad ST keyboards a common failure?

I don't think it's a common problem, my 2 ST's have original keyboards with no problems, I have had

issues with an 800 and 130XE, but they are very different.


I would start by carefully lifting out any chips that are socketed and re-seat them it's not uncommon for some

corrosion to occur between the sockets and the chip legs, I would also reseat the 2 square chips, although

you may need to use a "puller" to get them out safely.

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I'm not sure but it seems to be more than one issue going on here, it is probably worth looking into this issue first: --- > MANDATORY FIXES < --- ST/F/M/E /MEGA ST - Reset fix (Reset circuit is malfunctioning due to bad design and 30 year old bad caps. Machines fail to reset on power up and often need the reset button to be pressed to start the machine.) https://www.exxosforum.co.uk/forum/viewtopic.php?f=17&t=2175

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  • 4 weeks later...

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