CPC6128 Voltage issues..

[kbjunkie]

Hey All,


I've just received a 6128 to repair off of fleabay and whilst it had most of the DRAM's dead (which have now been replaced), I have got it working again - albeit with a small issue..


When powering it up with 5v from my bench supply, it will draw around 900mA but not come on - just a black screen.  When I check the VCC pins on the logic chips etc, they're all down at about +3.1v...


If I up the supply voltage to +5.9v, it will come on and appear to work fine - all chips has around +4.1v - nothing getting warm except the AY and the CPU (RAM just barely warm)..


Is this strange?  I've repaired a couple of 464's in the past and don't remember them needing almost +6v to get 'em working..


I've replaced all the electrolytic caps on the board with no change..


Reading about 500R between the power socket VCC and GND pins.. which also makes me wonder if I have a short somewhere?


Any insight would be appreciated!


Mark.

[Bryce]

Clean the power socket contacts and the contacts inside the power switch. But DON'T power it with 5.9V after you've done this!

Bryce.

[kbjunkie]

Hey Bryce :-)


I've replaced the power socket too .. no change.  The switch is also out of circuit as it's also faulty - using a jumper cap over the 2 pin header.. should have mentioned :-)


Any other ideas? 


Cheers,

Mark.

[Bryce]

What bench supply are you using and what settings (especially current limits) have you set?


Bryce.

[kbjunkie]

I'm using an Owon ODP3063.


Setting it at +5v with a current limit of 2A - although the 6128 never draws anything more that about 900mA..


If I up the +V to about +5.8v, I can get it to power up, current goes up about 20-30mA


Should also say that if I supply +12v (Limited to 1A) on channel 2 of the Bench PSU to the floppy drive, it all works - can read disks etc.


It's just the weird no start at +5v - all the logic reads +3.2 (ish).  When upped to about +5.8v, they all read about +4.1v


Ta,


Mark.

[Bryce]

Can you trace the voltage with seperate meter to find out where the drop is occuring. It must be a single point (ie: a bad joint) somewhere. 900mA definitely isn't pulling the rail down.

Bryce.

[Liartes]

I experienced recently a voltage drop issue that was not related to the motherboard or the PSU : The wires I used to build my power cord were too thin to handle the requested power, with a voltage drop when powering up the CPC as the result. I could tune the voltage up by increasing the voltage on the PSU side. Did you recycled an old PSU cable ?

[kbjunkie]

I've measured the low voltages right on the power socket - +3.2v.  Measuring the input on the power plug, unplugged and its +5.001v - plug it into the 6128 and measured on the socket (which I've replaced), its +3.2v.


I haven't recycled any old cord - I'm using a new power cord connected straight to the 6128 with a DC jack.  I've tested this on 2 464's and another 6128 and they power up fine and have correct voltages on the logic - around +4.8v


Really pulling my hair out :-)


I've socketed all of the DRAM and replaced them all now (with TMS4164-15NL DRAMS - this is ok?).  Different Z80 too - still the same issue.. I think I have a haunted 6128!


Cheers,


Mark.

[Bryce]

Due to the low current, it can't be any sort of short-circuit bringing down the voltage, so it has to be a resistance in series causing the issue. I still suspect that there is a bad connection somewhere along the power rail.

Bryce.

[kbjunkie]

I'll check the power rail more closely.. see if I can spot any bad joints etc.


Ta!


Mark.