Voltage problems 4000-05 board

[Crazyboss]

Hi I did some repair on a 4000-05 board, replaced the power regulators and the tip.
I removed all socked ics. The 5v line seems to be quite low, the PSU i known to be good and work on another MTX, the VRAM gets burning hot after very short time.
Could it be a short in one of the vram ics ?

It seems especially the first row from the left get burning warm first. After maybe 10-20seconds.

The 7805 gets around 7v in, but outputs around 4v

I guess there are no easy way to disconnect the vram.

[Crazyboss]

Update. One of the big 4700uf caps, was bad or maybe bad soldered, so now it get some kind of voltages, but no "Ready Prompt".
+12V is like +11.30
+5v is -5.01V
-5v is +0.5v !!!!!!

I swapped z80 cpu, and ctc with no better result.

[Martin A]

I'm no expert on the power supply, but the -5v supply is "generated" by ZD3. Whether you problem is a busted ram shorting -5v to ground or the zenner itself going up in smoke, I have no idea!

C49 is also across the -5v supply a short there wouldn't be good either.

With no -5v to the vram getting a Ready prompt seems "doubtful".

[Dave]

If the voltages are really as advised, particularly -5VDC, then I think that the VRAMs are highly likely to have been damaged anyway TMS 4116 datasheet states that "-5V must be applied before, or at the same time as, the other voltages and removed last."

[1024MAK]

With 4116 DRAM chips (and equivalents) they must have the -5V supply. This supply is a bias supply to ensure that the silicon junctions in the chip work correctly.

If the -5V is not present when the +12V supply is present (even if this is out of tolerance), they may be damaged. If they get hot really quickly, that's often a sign that they have self destructed and are now dead.

Some may actually work with an out of specification -5V, but may not last long.

Also note that the main supply rail (the most current/power) for these is the +12V. The +5V is only used for the interface to/from the outside world.

Mark

[Crazyboss]

Ok I will check that out later

I'm no expert on the power supply, but the -5v supply is "generated" by ZD3. Whether you problem is a busted ram shorting -5v to ground or the zenner itself going up in smoke, I have no idea!

C49 is also across the -5v supply a short there wouldn't be good either.

With no -5v to the vram getting a Ready prompt seems "doubtful".