General Stuff(tm)

[rubber_jonnie]

I'm coming more and more to the conclusion that this is an issue with one of the three custom chips.

When I press the reset button the amplitude of the processor clock decreases markedly and the frequency doubles. In fact it looks a lot like the raw output of the crystal. The 82C482 chip deals with the system clocks, reset and some of the memory management.

If the CPU isn't being given a clock signal which is within specification during the time the reset signal is high then it's likely that it's not resetting properly as it needs 15 clock cycles to do so, which is probably why it's not properly trying to read the ROM.

The trouble is that without either a working reference system or a schematic it's a case of shooting in the dark.

I may pack the machine away and get back to the Apricot stuff.

Sounds like a fairly heavy duty issue you have there :(

[stephen_usher]

A bit of progress, which may be related to the clock issue...

If I manually hold the CPU in reset (RESET line held high) for a short period after releasing the reset switch the machine boots/resets reliably. This suggests that the CPU isn't coming out of reset correctly and the reset line is going low before the CPU is ready. Seeing as this depends upon the CPU having seen 15 clock cycles suggests that it's not seeing those clock cycles whilst the reset line is being held high by the chipset.

[stephen_usher]

OK, a 1uF capacitor between GND and the CPU RESET line works.

I just need to install it properly now.

[stephen_usher]

Installed and it's fixed!



The scream from the speaker is due to the BIOS complaining that there's no video card.

[stephen_usher]

Yesterday I got gifted a non-working Tektronix MSO4054 Oscilloscope.

The fault was that if you turned it on the fuse would blow, so it was scrapped as Tektronix wanted an insane amount of money to fix it, more than an equivalent 'scope from another manufacturer.

Anyway, I took a look...

Well, it seems that a spring grounding clip from the top of the power supply housing had come out and dropped into the power supply. Can you see what happened?

IMG_2552.jpg

IMG_2553.jpg

There is actually no reason for this clip being there as there's nothing in that region underneath the plastic case. The three other spring clips do actually press against the lower shielding. It's almost as it was only added so that it could jump out and drop into the PSU.

Anyway, as the PSU only outputs 12V I've ordered a generic one with 16.7A output. This is slightly lower than the 20A of the original but should do amply.

[rubber_jonnie]

Installed and it's fixed!



The scream from the speaker is due to the BIOS complaining that there's no video card.

Splendid!!

[rubber_jonnie]

Yesterday I got gifted a non-working Tektronix MSO4054 Oscilloscope.

The fault was that if you turned it on the fuse would blow, so it was scrapped as Tektronix wanted an insane amount of money to fix it, more than an equivalent 'scope from another manufacturer.

Anyway, I took a look...

Well, it seems that a spring grounding clip from the top of the power supply housing had come out and dropped into the power supply. Can you see what happened?
IMG_2552.jpg
IMG_2553.jpg

There is actually no reason for this clip being there as there's nothing in that region underneath the plastic case. The three other spring clips do actually press against the lower shielding. It's almost as it was only added so that it could jump out and drop into the PSU.

Anyway, as the PSU only outputs 12V I've ordered a generic one with 16.7A output. This is slightly lower than the 20A of the original but should do amply.

Nice gift and good find, although it makes you wonder how many of the scopes failed for the same reason!

[derkom]

This is slightly lower than the 20A of the original but should do amply.

rimshot.jpg

[stephen_usher]

:)

[stephen_usher]

The replacement PSU has arrived. It'll need some thought about how to secure it as, of course, the mounting holes won't line up with the originals. Also, I hope that the 'scope doesn't actually try to draw the full 20A@12V as I could only find a 16.7A supply in the right sort of form factor.

Looking at the motherboard of this machine last night it seems that it's almost an early noughties Mac... PowerPC processor, AMD Radeon Mobile graphics. There's an FPGA in there too, which is probably doing most of the logic and glue.