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7854 POST fail, stuck at Step 2


Martin Whybrow
 

I've just powered up my recently acquired 7854 and have hit a problem; it runs POST but stops at what the manual lists as Staep 2, with all V mode and H mode indicators lit and red indicators on the front panel lit; it doesn't beep (the switch is set to on) and doesn't do anything else. I've already checked that all boards and connectors are correctly seated and am testing it without plug ins. Is this a sign of ROM rot?


Martin Whybrow
 

Just seen that Paul's chasing the same issue, so following that thread for now.


Martin Whybrow
 

Pulled JP1306 on the MCU board and it's progressed slightly, showing only the CHOP indicator on the horizontal mode buttons, so appears to be ROM failure. I have a good selection of old EPROMs around so need to see if I have anything suitable for the ROM replacement job.


 

MCM68766 is the goto EPROM for these, but sadly they've got very expensive lately.

David

-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Martin Whybrow
Sent: 21 May 2020 19:44
To: TekScopes@groups.io
Subject: Re: [TekScopes] 7854 POST fail, stuck at Step 2

Pulled JP1306 on the MCU board and it's progressed slightly, showing only the CHOP indicator on the horizontal mode buttons, so appears to be ROM failure. I have a good selection of old EPROMs around so need to see if I have anything suitable for the ROM replacement job.


Martin Whybrow
 

I'd planned to fit EPROMs in place of the ROMs, but decided to use one of the spare 24 to 28 pin adapters to enable me to read the ROMs on my TL866ii (which doesn't support the MK36000 or pin compatible EPROMs); all 4 ROMs match the content of the files retrieved from TekWiki. I also checked the 2 EPROMs and they're also correct. Refitted the ROM board and the scope passed POST, great! I refitted the plugins and powered up, a longer delay this time then it showed all the vertical indicators and the CHOP indicator on the horizontal, indicating ROM failure. I power cycled once more and it stuck at the previous stage with all the lights on. Clearly something is causing this inconsistent behaviour and I no longer suspect the ROMs, I guess it's a job for the logic analyser unless there are known faults that can cause this.


Martin Whybrow
 

The behaviour is decidedly odd, when it does POST correctly, the digital section works for a time then crashes so none of the mode controls work and the OSD disappears. Whilst researching, I found mention of the TI P25 sockets, so I checked and found mine uses them; should I bite the bullet and swap them all for a better type? What's the consensus as to which is the best alternative?