exxos blog - random goings on

[rubber_jonnie]

@exxos

I had 2 takes on this kind of report, but the 000008 I believe is the start address for the RAM tests. Based on this I think you're looking at bank 0.

Interpretation 1 was that the first block is the test value, and the second block is what's returned.

Interpretation 2 was that the first block is bank 0 and the second block is bank 1.

On thing I am confident of is that both blocks represent bits 0-15, with bit 0 at the RHS and bit 15 at the LHS of each block.

Problem is my service manual is missing the section on interpreting!

The RAM/Data line error definitely means a data line problem, in my Mega it was D2 & D4, there wasn't a problem with the RAM.

In the end I used a logic probe to test against what I'd expect from a working machine then buzzed the lines out when I found the bad data lines. Looks more like an address line problem though.

[exxos]

Problem is my service manual is missing the section on interpreting!

Yeah know what you mean... biggg post to follow any second...

[exxos]

So A0 tested short to GND. The waveform on my scope was basically just noise which is why it caught my attention. On first inspection I couldn't see anything wrong... So back to the low resistance meter...

Testing GND to A0...


TEST POINT 1 - 0.441R

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TEST POINT 2 - 0.396R - Getting closer...

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TEST POINT 3 - 0.357R - Getting closer...

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TEST POINT 4 - 0.316R - Getting closer...

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TEST POINT 5 - 0.275R - Getting closer...

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TEST POINT 6 - 0.229R - Getting closer...

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TEST POINT 7 - 0.185R - Getting closer...

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TEST POINT 8 - 0.141 - Getting closer...

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TEST POINT 9 - 0.182 - GONE UP!

So ah-ha! TEST POINT 8 is where the short is.. On the second to last socket ont he bottom right of the board... sooo... Magnifier time...

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Marked A0 with marker pen to see where it was going.. Something looks suspect...

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Zooming in.. so tiny....

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Re-solder....

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and...........

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:cheer: :girldance: :cheer: :girldance: :cheer: :girldance: :cheer:


So what have I learned in all this ? I'm glad I don't do this for a living anymore :lol:

[PhilC]

LOL, well done and a very informative way to test for shorts.

Re ram test, I always presume that the bit indicated with a 1 has a problem and work from there as the manual is less than hopeful with its descriptions. Presume that left 16 bits is 1 bank and right is another. This is how I found the short on my mega the other day.

[czietz]

Re ram test, I always presume that the bit indicated with a 1 has a problem and work from there as the manual is less than hopeful with its descriptions. Presume that left 16 bits is 1 bank and right is another. This is how I found the short on my mega the other day.

Guys, there's no need for guesswork. It's as simple as this with the initial diagnostic cartridge RAM test:
Expected value (bits D15-D0), value actually read (D15-D0), address (in hex).
The affected bank you have to decode yourselves from the faulty address.

[PhilC]

@czietz thanks.

[rubber_jonnie]

@czietz Cool, so the first interpretation I had was the correct one, that's good to know. The service manual(s) I have don't seem to describe this at all well, other than working out which bank it is.

[rubber_jonnie]

@exxos Good job, and proves that a multimeter is sometimes all you need :D

:girldance: :girldance: :girldance:

[exxos]

@exxos Good job, and proves that a multimeter is sometimes all you need :D

Yeah, I think the diagnostic tests "lead me up the garden path" into thinking there was a faulty data bit, when actually there wasn't, it was a address fault.

I3 RAM address check 0000000000001000 0000010000001010 000008

8 = 1000 so that must be the address. it gets back 1010 = 10. Even so it doesn't make sense. As if A0 is shorted to GND, then "1000" is still A0=0.. So it shouldn't have mattered anyway at that point.

I have been talking to @Icky off and on about doing a better low level diagnostic board for the past year. Mostly intended to debug a dead H4 but could be used on ST's as well. As when the machine is "dead" nothing runs from ROM anyway.. It would be like a "CPU EMULATOR" which would have a bank of LED bits to relay data on the bus properly.

[rubber_jonnie]

I have been talking to @Icky off and on about doing a better low level diagnostic board for the past year. Mostly intended to debug a dead H4 but could be used on ST's as well. As when the machine is "dead" nothing runs from ROM anyway.. It would be like a "CPU EMULATOR" which would have a bank of LED bits to relay data on the bus properly.

@exxos A proper 'Dead Test' cartridge would be a great idea.

I know when my Mega 4 was first on the workbench with the dry joint on the ROM socket GND, even with the diag cart, I got nothing out of the serial port, presumably because the diag cart couldn't be mapped into the ROM space.

A cart that will work and report back, even on a dead board would be a great tool to add to the collection.