Amiga 600 ... maybe Gayle problem ...

[saronno]

Hi, I am trying to repair and amiga 600 and I noticed (with thermal cam) that gayle is cold. Then I checked the gayle's vcc line (is present) and check also kb_reset.
Now ...

The pullup resistor is good. The impedance to ground from _kb_reset is 1kohm ( Too low?). R514 is ok. The voltage at the base of the transistor is slighty short of 0.5v (when a600 is powered up).
Even when amiga is not powered, impedance to ground is just 1kohm .... to me it makes no sense you have just 1kohm to ground because if so, the pullup resistor can only rise the voltage to 2.5v if the transisto r is not in a conductive state. I put 3v on the kb_reset line to see what is getting hot .... it is the gayle chip. Is it dead? Is it kb_reset active low? If so, why is gayle cold?

TIA

[m k]

Underline first indicates it's active low.
First R514 activates Q511 and later U14 pulls R514 down.
0.5V is 10% of Vcc, so R514-R515 divider is as it should.

How much is from _KB_RESET to GND?

[saronno]

I actually solve the problem changing the 555, but now, I have another problem:

Amiga start, it shows light green, followed by two lighter tone of green .... then white screen.

I check cpu, ram and rom .... there is dialogue going on  ... maybe a crc error in the rom?

[Andy Watson]

Do you have facility to check the power supplies? Not just voltage, but ripple too?

[saronno]

Do you have facility to check the power supplies? Not just voltage, but ripple too?

Yes, I have a scope. There is a about 300 mV peak-to-peak on  5v. Anyway, I try to add a big cap on 5v rail but nothing change.

[screwbreaker]

...
Amiga start, it shows light green, followed by two lighter tone of green .... then white screen.
...

Seems good. Have you tried to replace the kickstart rom?

[m k]

I actually solve the problem changing the 555, but now, I have another problem:

Amiga start, it shows light green, followed by two lighter tone of green .... then white screen.

I check cpu, ram and rom .... there is dialogue going on  ... maybe a crc error in the rom?

Check both ends.

http://wiki.classicamiga.com/Amiga_boot_error_code_colours

[saronno]

A friend lend me a different a600:

1 - I tested the ROM from the faulty one on the working one: the rom is good.
2 - The resistance on the 5v rail of the working one is much higher than in the faulty one (some kohm vs 85 ohms).
3 - I tried to inject some current on the 5v rail with a bench psu ..... even with the thermal camera I can't find a specific component heating up much more than the others. I tried also with low current (50 mA) to see if something heating up before the rest of the board ... nothing clear. So, my guess now is .... something big is sucking this extra current and, having a good thermal mass, you can't see clearly it's him.

What do you think? Am I right?

[saronno]

I actually solve the problem changing the 555, but now, I have another problem:

Amiga start, it shows light green, followed by two lighter tone of green .... then white screen.

I check cpu, ram and rom .... there is dialogue going on  ... maybe a crc error in the rom?

Check both ends.

http://wiki.classicamiga.com/Amiga_boot_error_code_colours

Yeah, I will test again to see if both end are active during communications.
Anyway, right now, I wonder what lowers the 5v rail resistance to 85 ohm.

I also got an original PSU with the second amiga 600 (working) and use it to power up the faulty one ... nothing change. I will anyway recap the faulty one, but, I don't think it's a psu OR caps issue (I tested the caps with an esr meter, nothing seems wrong).

[m k]

85 ohms is quite low.

I remember that some PCB versions had ground plane problems, it was not positioned correctly, means that some holes may be contacting it, it's also pretty nasty problem if a trace of questionable hole has a wrong destination.
Then only solution is to cut the hole away from the trace and reroute, possibly through the same hole, maybe after drilling some extra space.

Can the screen be light gray instead of white?

If checked signal is bidirectional you need a control signal to see what is really happening.

Since screen color is changing you're away from the beginning and can think that basics of basic things are ok.
Maybe CIA stuff next, like even/odd CS from Gayle and INT to Paula, there should be life.

[saronno]

85 ohms is quite low.

I remember that some PCB versions had ground plane problems, it was not positioned correctly, means that some holes may be contacting it, it's also pretty nasty problem if a trace of questionable hole has a wrong destination.
Then only solution is to cut the hole away from the trace and reroute, possibly through the same hole, maybe after drilling some extra space.

That's something I was thinking too .... because, with the thermal camera, I often see ground connection a bit hotter than the rest of the board.

Can the screen be light gray instead of white?

maybe