*** M E M O R U M ***

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.

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.

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".

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."

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

Ok I will check that out later

Martin A wrote: ↑01 Sep 2024 14:21 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".