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COMMODORE CBM PET 2001-N DESPERATE

You missed I4 from the set of readings.

However, the results are consistent, but not correct.

I think I know what is going on, but I could do with some single channel oscilloscope traces of a number of pins to potentially rule out other causes.

G7 pins 8, 9, 10 and 11.
B2 pins 9, 10, 11, 12 and 13.
A5 pin 3, 4, 5 and 6.

If these pins are correct I would suggest replacing I10 and I11. My thoughts are that some of the I10 and I11 buffers are writing to the DRAM when they should be reading. They may be reading as well of course, but (in this case) they will be reading what the write buffer is writing!

When you have replaced I10 and I11, do NOT install the DRAMs - but leave our resistors in place.

I will be very annoyed if replacing I10 and I11 does not fix the problem...

But PLEASE wait until after you have taken the measurements I have asked for and I have analysed your results.

Dave
 
daver2 said:
You missed I4 from the set of readings.

However, the results are consistent, but not correct.

I think I know what is going on, but I could do with some single channel oscilloscope traces of a number of pins to potentially rule out other causes.

G7 pins 8, 9, 10 and 11.
B2 pins 9, 10, 11, 12 and 13.
A5 pin 3, 4, 5 and 6.

If these pins are correct I would suggest replacing I10 and I11. My thoughts are that some of the I10 and I11 buffers are writing to the DRAM when they should be reading. They may be reading as well of course, but (in this case) they will be reading what the write buffer is writing!

When you have replaced I10 and I11, do NOT install the DRAMs - but leave our resistors in place.

I will be very annoyed if replacing I10 and I11 does not fix the problem...

But PLEASE wait until after you have taken the measurements I have asked for and I have analysed your results.

Dave
Click to expand...
I will never be able to reach your level of expertise unfortunately, you could fix any computer!
 
The resistors we placed on the DRAM IC sockets should pull the data pins down to 0V. If these have not been installed correctly, you are leading us both up the garden path.

The ONLY source of an active HIGH voltage level can only come from the 'write buffer' in I10 or I11.

A write can occur OUTSIDE of when I10 and I11 pin 1 is LOW, but not inside. As a result, when I10 or I11 pin 1 is LOW, the data lines must also be LOW - but this is not what we are observing. We are seeing that the data lines are sometimes HIGH when I10 and I11 pin 1 is LOW.

Besides which, the only active components that it can be (that PETTESTER hasn't given a clean bill of health) is I10 and I11!

Dave
 
Ignore it then - but you do know how to deal with this scenario - don't you? We came across this in a previous repair, so I shouldn't have to tell you a second time...

Dave
 
daver2 said:
Ignore it then - but you do know how to deal with this scenario - don't you? We came across this in a previous repair, so I shouldn't have to tell you a second time...

Dave
Click to expand...
I should look for connections with these pins, shouldn't I? But now I have to take care of some errands and I'll have to postpone the rest of the work until tomorrow..ok?
 
Hi, I10 and I11 changed!
I tested the two 244 with my logic tester and they are bad!
Next step?
 
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