Agilent 8753ES A9 CPU Board repair

[inaxeon]

Hey Folks

Recently picked up this Analyser for a steal, but the CPU board is dud (I was aware when purchasing, and drove a hard bargain ;-)

Does anyone know anything about it? I found a few old threads on the internets of others who have tried to diagnose these things without any luck. I've attached a picture of the 8 LEDs which apparently are diagnostic but the service manual has no table of what these mean, other than saying "Replace A9 CPU Board". Arrrgh.

I made up a cable to attach it to a known-good ATX power supply outside of the instrument, same deal, same error code, so not a power supply problem. I'm thinking maybe dud DRAM or something but hard to prove without doing full logic analysis on it, would rather not unless I have to!

Whatever it is, it is a very very early fault, it only cycles through maybe 2 or 3 codes before it bombs out.

Thanks in advance

[inaxeon]

Have spent a bit more time on it. Don't really know much more other than it is executing a bit of the pre-bootloader then crashing out. Doesn't get anywhere near turning the screen on frustratingly.

Cause still unknown.

I whipped a quick PGA-179 pin spy PCB, currently with PCBWay. When I get it I'll hook it up to my 16702B and see what I can learn.

[inaxeon]

Back again, talking to myself ;-)

The pin spy has arrived, made it up, and have been on the LA for an hour now.

The CPU is stuck in an endless loop trying to write 0x55555555 to address 0x30000000 but reading back 0x55551555. Definitely looks like some kind of memory pattern test.

It seems to be the four SRAM chips which respond to this address, specifically it's the SRAM U111 which appears to be the culprit, at least as far as I can see. A little surprising to see a dead SRAM. I'll replace it and see what happens. Next post probably 3 weeks away when that arrives.

[Teichhermelin]

That's what logic analyzers are for. Looks like a plan...;-)
Keep us informed

[inaxeon]

haaaaaaaaaa!

I found some suitable SRAMs in my parts box. Not quite the right package, but just fits.

It's alive!

Full write up: http://www.mattmillman.com/acquisition-and-repair-of-an-agilent-8753es/

[SoundTech-LG]

Congratulations!!!

Great work, troubleshooting, and determination.

Quite impressive.

[inaxeon]

Congratulations!!!

Great work, troubleshooting, and determination.

Quite impressive.

Well thanks. I am hoping my experience will inspire more people to attempt to repair these. It is a shame to see people forking out $900 for a used board when an entire instrument can be had for $2000!