Is my A500 rev5 frying RAM chips or what?!

[Turrican_3]

This is a follow-up to two former threads about the same A500 rev5 motherboard that can be found here and here.

The A500 has been running nicely since Agnus socket and RAM IC replacement.

During this week though a worrying thing happened twice in a row.

The system all of a sudden started presenting a green screen at boot time first a few days ago. I was able to (painfully, since as you know this is a staggering 16 RAM chips system) isolate and replace the bad RAM IC, then it resumed working as usual. Or so it seemed.

Today, while I was leaving a game running to check the motherboard was working as expected, I was greeted by a dark grey screen after roughly 1 hour. Ever since then the system booted with the same dark grey picture.

Again, I was lucky enough to find one faulty RAM chip, and now the system is booting fine again.

So my question is: is this motherboard "frying" RAM chips like crazy, or was I just extremely unlucky (the two faulty chips come from the same stock I purchased recently on AliExpress)

And if it's the former, is there anything I should check to ensure I stop destroying RAM ICs?

Power supply is a brand new Meanwell unit, I see 4.8V / -12.9 / +12.2 on the Amiga DC plug for the record.

[Screechstar]

Hi,

So I am definitely no Amiga expert at all... the only comment I have is from what I've found regarding voltage from the PSU, as I've a few and have been re-capping them recently and have a Meanwell RT-50B too.

I tested the Meanwell at the PSU plug and get 5.3V & 11.7V (a bit off spec).
But once connected to my Amiga 600, I get a good 5.01V & 12.14V at the motherboard floppy drive pins (due to losses in the cable etc. as far as I have understood).

I also tested a re-capped A600 PSU (#3910929) and measured similar... 5.3V & 14.5V at the PSU plug and then a perfectly good 4.98V & 12.26V from the floppy drive pins.

Is there any chance the voltage is too low? You may be dropping to 4.5v or lower in the amiga.

Testing at the floppy pins seems to be a recommended way, although don't short-circuit anything! The floppy pins are [5v,GND,GND,12v].
There's an adjustment potentiometer on the 5V rail on the meanwell PSU.

[turrican9]

I have heard of at least one case where C812 (filtering cap for voltage to the ram chips I believe) was faulty and killed RAM chips. Allthough, looking at pictures of Rev. 5 boards C815 is also close to the RAM chips. So could also have something to do with filtering voltage to RAM chips. For Rev. 6A at least C812 is the one.

Caps on A500 boards rarely go bad as far as I understand, but it happens. I don't know if you have recapped your board, if not, might be something to look into.

[duga]

-12.9 V sounds too low. Make sure it's 5, +12 and -12 Volt +/- 5%, which means 4.75 - 5.25 and 11.4 - 12.6 Volt.

[solarmon]

You need to check the power rails when it is under load - i.e. when connected to th Amiga and turned on.

[Turrican_3]

Originally Posted by Screechstar

There's an adjustment potentiometer on the 5V rail on the meanwell PSU.

Originally Posted by duga

-12.9 V sounds too low. Make sure it's 5, +12 and -12 Volt +/- 5%, which means 4.75 - 5.25 and 11.4 - 12.6 Volt.

Sorry for the slow acknowledgment, I'm on vacation.

And thanks everyone for the replies!

Sooo... as far as I understand this power supply is regulated on the 5V rail. So I wonder what would happen if I rise it a bit... wouldn't that make things worse?

Originally Posted by turrican9

Caps on A500 boards rarely go bad as far as I understand, but it happens. I don't know if you have recapped your board, if not, might be something to look into.

It hasn't been recapped yet (and to tell you the truth it has been running fine since then apparently) but it's definitely worth investigating sooner or later... I think I already got most of the required parts.

Originally Posted by solarmon

You need to check the power rails when it is under load - i.e. when connected to th Amiga and turned on.

Yes I apologize, my first post was meant to be that I checked voltages with the A500 turned on, probing the inner side (i.e. the back of the motherboard plug) of the DC connector, which I assume is correct.

[r.cade]

You will get a lot of junk chips from some scrappers on AliExpress. It's just the way it is. It comes from recycled computers.. i.e. junk piles that were outside for months or years, and then chips torched off the boards, sorted and resold. They often don't work or fail pretty quickly.

[Screechstar]

Hi,
That’s great you were measuring voltage while under load.
The voltage is a little on the low side but should normally be ok (from what I’ve read, not from personal experience!). I don’t know if getting closer to 5v would make things worse as it’s what the chips were designed for.

And from what I’ve found, getting closer to the 5v actually helps bring the other voltages further in-spec. I had to increase my meanwell a little when I first got it, to get exactly 5v and get the 12v pretty close (although the 12v is less important).

On the negative side, that’s possibly not the issue to your ram chip problem then!

[Turrican_3]

Originally Posted by r.cade

You will get a lot of junk chips from some scrappers on AliExpress. It's just the way it is.

Yeah I guess it's... the price you have to pay for getting this kind of stuff cheaply.

Originally Posted by Screechstar

I don’t know if getting closer to 5v would make things worse as it’s what the chips were designed for.

The thing that worries me is: would increasing the +5V line also lead to a voltage increase on the other lines? I suppose I might look for a dummy load and see what happens, I definitely wouldn't want to risk frying something else on my precious A500 motherboard.