A590 & Weird Rev 6A Issue(s)

[GadgetUK]

I have a rev 6A A500 board that has been acting really strangely with an A590.


The A590 works perfectly with a rev 5 board, whilst FDD is connected up to the A500.


The A590 when connected to the rev 6A board is fine WITHOUT the FDD connected. The rev 6A board works fine with FDD connected up (without the A590). However, when I connect up the A590, and the onboard FDD (DF0) on this rev 6A board, it struggles to boot from the FDD (the FDD led on the keyboard doesn't even light), and if I leave it more than 7 or 8 seconds trying to boot from floppy, the 4 RAM chips on the A590 start to get super hot (it's killed 2 RAM chips that way already).


The rev 6a passes every single diagnostic including serial and parallel ports.

PSU has been ruled out. I've changed Gary to rule it out, and both CIAs too.

As a seperate (maybe related??) issue - I tried measuring the DC voltage on pin 2 and pin 3 of the MC1488, at that point the system black screened and the first CIA (u7) went super hot - I cut the power but it had already died. I thought maybe the CIA caused the issue, so I swapped it out. The same thing then happened again when I measured pin 3 again on the MC1488. Is this normal behaviour?? I mean, is it unsafe to measure DC voltage using a multimeter on that MC1488? Or does that sound like part of the cause of the whole problem maybe?


Bottom line, everything works fine on the rev 6A and the 590, provided the FDD is not connected. Move the FDD and A590 to rev 5 and all is well.


Any ideas?

EDIT: Could the MC1488 perhaps be causing all this wierd behaviour? Despite passing loop back test?

[DigitalKeeper]

The "1488 & 1489" are only for Seriell In- Output; these ICs are sensitive, and go fast "defective".

The 1488 also "drives" the LED from/for Audio Filters - LED is off if the filters are "bypassed".

U8 is for "Floppy Control" - if this CIA is defective - Floppy won't work.


The EIA Line Driver is 1488.

The EIA Line Reciever is 1489.

Both ICs are directly connected to U8 (8520).

[GadgetUK]

Originally Posted by DigitalKeeper

The "1488 & 1489" are only for Seriell In- Output; these ICs are sensitive, and go fast "defective".

The 1488 also "drives" the LED from/for Audio Filters - LED is off if the filters are "bypassed".

U8 is for "Floppy Control" - if this CIA is defective - Floppy won't work.


The EIA Line Driver is 1488.

The EIA Line Reciever is 1489.

Both ICs are directly connected to U8 (8520).

Thanks! That's just the thing that makes no sense! Both CIAs have been swapped for known good chips - the problem remains. The LED toggles when filter is switched on and off. But it sounds like the filter is not working either. the FETs are fine, the transistor, resistors etc in that filter circuit are also fine, yet filtering doesnt seem to work.


And measuring voltage on pin 3 of the 1488 instantly kills the U7 CIA (again). I dare not measure pin 3 again... I am going to swap the 1488 and 1489 and will report back.

[DigitalKeeper]

Have a look at the "EMI's" E502 & E503 -> +12 Volt & - 12 Volt -> look, if there is a "short"

Pin 2 of 1488 -> Input and the Output is Pin 3 - the Pin 3 of the 1488 come from Vee via a diode; if there is a short IN the 1488 -> ya have full VCC on Pin 3