Hey Marc,

I feel for you.  I am working on so many Amiga projects right now and it can be very frustrating when things that worked a few minutes ago, suddenly don't work any more.  When that happens to me I just go work on a different project, or quit for the night.  I guess that explains the dozen Amiga projects that I have unfinished around here.  Oh wait... I did get the CD32 w/SX32 Pro installed and working after having to replace the 2.5" HD because the old one would not format successfully after ten tries.

I might suggest that if you have an emergency floppy to start your A4kT w/CSPPC from maybe you should remove everything except the CSPPC and see if it will boot from the floppy disk.

Then start replacing everything else one piece at a time starting with the hard drive connected to the CSPPC and ending with the last Zorro card.  Start up the Amiga after each item is installed to make sure that item is not causing the problem.  It is a hassle, but there is a method to my madness.

Send me a picture of your ribbon cable installation on the A4kT module and I will check it against mine that is open now.

 Sounds like a bad contact between the CS and one of the SIMM banks. Take them out and reinsert.

@marcfrick:

one of the worst weekend in my entire life: for some reasons I  had opened my A4000T (maybe I was adding a SCSI terminator) and noticed that the coin battery was not soldered but in a battery-holder (sorry I don't know the right term)...since it was discharged I tried to remove it...but  doing so the battery-holder detached form the board. Even though I did not  need it (I was using external batteries) I decided to repair it. So I dismounted everything took the motherboard to a nearby electronics shop and in few minutes they resoldered in place the holder. I returned home, re-assembled the Amiga and... blank screen. No sign of life. I felt completely lost.
My dream machine dead. For 2 days I tried and checked the cpu board connectors, video module and disk module connections, SCSI connectors, changed cpu boards, simms...
then I saw it... it was one of the 50 pin ribbon cables connecting the motherboard with the small board which terminates the SCSI bus. I reconncted it leaving the leftmost pins unconnected (so any pin was in a wrong hole).
It was hard to see, inside the case. Corrected and the Amiga  came back, alive n' kickin'.
So my advice is to recheck connections of the disk module, if they're wrong prevent the entire system from booting. In my case the wrong connection didn't do damages, hopefully this your case too.
I told you my story to restore your hope!

good luck!

make sure you didn't leave anything rattling around...no orphan screws that might be lodged on or under the motherboard.

I'd suspect the daughter cards...

make sure they're back on the motherboard properly if you removed them.  

make sure they aren't touching the edge of your processor card at some point.

make sure the insulating paper backing on the audio card is in tact or add an index card or something between it and the scsi card.

check the orientation of your 4000T scsi and floppy cables and that the floppy cable is seated both on the card and on the floppy.

make sure you haven't managed to remove/lose the little red grounding wire from the audio card...and that you reattached it when you put the machine back together.

Thanks Sprocket, I checked tonight, I'll double-check tommorow in better light....
BUT, have some more questions for you guys. I'm pretty sure my power supply is suspect, ... in fact, it's pretty much dead, ATM. OK, is the PSU in an A4000T standard? using a multimeter on the PSU just before it went, I wasn't getting +12V on the yellow wire, more like 8V (on the 4-pin molex connectors) . Is it common, or even possible, that a PSU can 'start to go bad' before it finally dies? If, by some merciful chance, this is my problem, how hard is it to replace the 4000's PSU? Many Many Thanks everyone.
Marc

@rkauer: Thanks for the tips... I will pull the PSU and test tommorow, busy getting drunk tonight  :pint:
edit; Actually, I decided to un-plug everything and try the PSU by itself right after I read your post, checked the voltages ... now I am even more confused. OK, all I had handy for an electric drain was a 15V DC fan, I got +5V on the +5V line, exactly ... but the +12V line read +11V ... As always, more questions, I got +8V from the 12V line when everything was connected, how power-hungry is the CyberStorm? Mind you, I tried both with and without RAM installed. The 4000T motherboard has the max. 16MB installed. Also, how picky are the terminators in a SCSI chain, in other words, do they need full power to work at all? I had read that if the SCSI chain(s) on a 4000T aren't terminated properly, the machine won't boot. (Doh! sorry just read ALL of your last post, sorry too tired

I think it still could be a PSU problem, but I can't seem to make 100% certain...
I know my dad's accelerated A1200 had lots of probs. until I got him a dedicated PSU (actually MADE for A500/A1200, would you believe, Applied Engineering..)

My A4000T with CSPPC stopped booting due to the +5V user going too low with everything plugged in.  Sometimes it would not boot at all, other times it booted with the mouse not working (as you have said you have seen).

It always booted as long as I removed the keyboard, this sometimes dropped the +5V user voltage below usable when connected.  Maybe you could try to boot with no mouse or keyboard also connected.

This seems to occur after I use the DVD drive continuously for a period of time or did many reboots one after the other.