A600 KB_RST high, _HLT low giving black screen

[Deeshay]

Morning all,

I have read and searched as many A600 posts as I can but I can't get to the bottom of this problem on my own. The TL;DR is that I have the black screen issue and am measuring 0V on the CPU and Gayle _HLT pins but 4.8V on the _RST pins.

The details.
I picked this A600 up for a bargain so was prepared to have to do some work like recapping it.
I know very little about Amigas and this is the first one I have had. I have no spares to swap in or out.
On first power up I had a black screen but there was a signal. Reading around the first fix was going to be a recap. All the electrolytic capacitors have been replaced and there was minimal board damage with only one pad lifting and being repaired which was connected to C822. No other obvious corrosion damage. The unit did not boot. It does this with the keyboard connected or disconnected. This is when I started digging around further.
I have checked the output of the 555 timer U14 that controls the _RST pulse and it has a 4.8V input and pulses high then drops low when the power is turned on. This lead me to a post somewhere about the KB_RST being low causing the _HLT line to go low. This is when I found the _HLT low issue. The next check was the pull up resistor R951A but that is within tolerance. I have got to the point in my limited understanding of the system that Gayle is sending out the _HLT signal but I am not sure what could cause this.
Powering the system on and leaving it running gives warms up the chips U1, U2, U3, U4, U7, U8 and U13. The RAM chips U16 and U17 stay at ambient. U4 gets the hottest at 48 degrees C, the rest are around 40.
The voltages out of the PSU when power is on are 12V is 11.65V, 5V is 4.85V and -12V is -11.85V.
The ROM chip installed is an Amiga Forever ROM version 3.1. The until also has Fast RAM and Chip expansion boards as well as a IDE to SD card adaptor with an SD card. With the expansions boards removed the _HLT still stays at 0V.

Any guidance at all as to where to look or what to replace would be appreciated as I would like to get the system powered up and show my children what real games are like! And play it myself...

[lesta_smsc]

Try removing the capacitor that controls 555 timer. It is the one just above it. This prevented my A600 from booting. It was leaky and had a fishy smell when removing.

[solarmon]

What is the revision of this A600 board?

[solarmon]

I have drawn up a reset schematic for the A600 rev 2B at:

https://sites.google.com/one-n.co.uk...-reset-signals

I find this useful to remind me where the reset signals are and what it is supposed to be doing.

Both _KB_RESET and _RST (and _HLT) should pulse low and then stay high. The CPU can also assert _HLT low if it detects some other hardware issue.

Make sure you disconnect everything - keyboard, floppy drive, any expansion boards, etc.

You'll need to get the Kickstart ROM tested, just to rule it out.

I would recommend getting a DiagROM and see what that says. Also, a logic probe is highly recommended to check the signals, rather than use a multimeter.

[Deeshay]

Thanks for your suggestions guys.

I forgot to mention that the board is revision 1.5 and that is pretty important...

For the reset line, I have removed the capacitor above the 555 timer as I read a thread on here about that causing it not to start but it still gives a black screen with the HLT low.

My thoughts were with the Kickstart ROM and swapping it out for a Daig ROM but I wasn't sure if a bad Kickstart could cause a HLT signal or if the board would still fail to boot with the HLT line low and DiagRom installed.

I'll get a DiagROM from ebay, test it out and report back.

What I am really after are reasons the HLT line can go low so I can start probing those areas so the suggestion that a DiagROM can help is really what I am after.

[LIV2]

HLT low but RST High is caused by the CPU hitting some exception (bus error, address error etc) and halting itself.

This can be caused by many things, damaged traces, damaged rom socket, bad ram, etc.
It's also possible that the system isn't being reset - does doing a CTRL-Amiga-Amiga reset make a difference? Can you verify that resetting that way brings RST+HLT low?

[Deeshay]

I've just tried doing an CTL-Amiga-Amiga and the RST stays high and HLT remains low. I haven't verified the keyboard works although the ribbon does look in good condition.

All the visible traces on the board look good and only one pad lifted on C822 which was repalced. I want to try a DiagROM and see if that gives me something but not knowing exactly how it works I thought there was a high chance it still would not boot.

Hopefully it is not a problem with a custom chip.

[Deeshay]

So. The DiagROM has arrived and the unit now boots so I'm assuming the HLT low must be the result of a bad ROM. I'll get a replacement 3.x ROM on order soon.

The trap door RAM expansion works giving me 2048K of Fast RAM but the CPU RAM expansion either causes a black screen on boot or boots with no extra RAM being detected. the IDE to SD card adaptor passes all the tests so that looks good too.

Just a bit more research required on the CPU RAM expansion module now.

[Deeshay]

So the CPU clip on RAM expansion just had a bad connection due to being removed and refitted too many times. A clean up and careful tweaking of the pins on the socket resulted in a full 4Mb of Fast RAM detected by the DiagROM.
Next job is to order a replacement Kickstart to prove that the old one is the issue and then time for games.