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Need help with 5170 Memory Refresh

S

Sabertooth

Member
Joined
Apr 20, 2019
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36
Location
Texas USA
My 5170 Type 1 MB started having issues recently. I threw in the trusty LankMark ROMS and it shows memory refresh failure. Sometimes it will show it right away, other times maybe the 2nd or 3rd round it will fail.

I broke out the oscope and started tracing things out. The 14.318MHz signal is good and is being converted to 2.28 on pin 2 which goes directly to pin 1 of U88. The output on pin 5 of U88 appears to be the issue which of course feeds the timer for the memory refresh.

Sometimes the output is correct at 1.19MHz and then it fails and starts jumping all over the place for 2 to 12MHz and everywhere in between. Of course as soon as that happens is when the memory refresh will fail. After looking at the schematics, I really don't see any reason for this to happen except for a defective U88 which I changed thinking that's what it had to be. That didn't fix the issue.

Can anyone enlighten me as to why the output from U88 to the timer would just go bezerk like this.
 
Diagram at [here].

You have stable 2.38 Mhz but unstable 1.19 MHz, and have tried changing U88.

Are pins 2 and 3 of U88 a stable +5V ?

If you added a socket when changing over U88, then I suggest that you lift the pin 5 leg out of the socket, then see if the 1.19 MHz on the 'floating in the air' pin 5 is stable. If it is stable, then that would suggest that U103 is having some sort of effect on U88.
 
modem7 said:
Diagram at [here].

You have stable 2.38 Mhz but unstable 1.19 MHz, and have tried changing U88.

Are pins 2 and 3 of U88 a stable +5V ?

If you added a socket when changing over U88, then I suggest that you lift the pin 5 leg out of the socket, then see if the 1.19 MHz on the 'floating in the air' pin 5 is stable. If it is stable, then that would suggest that U103 is having some sort of effect on U88.
Click to expand...

Thanks modem7. I did what you said and lifted pin 5 of U88 from the socket and the problem persisted. The 1.19MHz would be good and then it would go crazy. Of course this eliminated u103 issues.

Based on the diagram, pin 2 of u18 goes directly to U88 pin 1 and nothing else. To make a long story short, I thought I had a stable 2.38MHz signal from u18 because everytime I checked it, it was fine. I eventually found that I would occasionally lose the signal depending on how much pressure I would put on pin 2.

I finally decided it had to be the solder joint. Since I don't have a microscope to find a very small crack I just resoldered the pin and low and behold it works perfect every time.
 
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