'scoping the mains.

[DavidA]

I want to do some waveform checking on a couple of old PC power supplies. Actually I don't want to,  but the supplies are part of two very old computers and are no longer available; so I have to try and fix them.
  I do know the operating principle of these switching mode devices,  and am aware of the risks ( Once worked in the lab of an elecrtical manufacturing company) So I know what I am getting into.
So I will need to apply my 'scope to the mains switching transistors to see if they are working.

Question.

What value should the isolating capacitors be ?  I reckon that 1000 Volt should be enough for a safe working voltage,  but what capacity ?

Dave.

P.S.  I'm aware that I need a capacitor on each of the connections (earthy side and probe) before the scope.

[John Rudd]

IMHO, dont scope with capacitors........

Use a proper isolation transformer if you must scope the waveforms, make sure the earth lead on the scope is disconnected too....

Dont ask how I know.........

However, if the primary side is working, then you could safely scope the secondary side of the transformer.

Is there a pwm chip providing the switching signals?

[John Swift]

Hi Dave

what computer power supply  are you repairing and  would  adapting a ATX power supply be an option ?

unless your scope has a differential input to display the difference between two inputs so you don't connect the screen to the circuit under test


I'd use an isolating transformer to power the power supply
since the NPN switching  transistor emitters will be connected to the negative terminal of the mains bridge rectifier

ATX psu circuit -  http://www.pavouk.org/hw/en_atxps.html

as a starting point try a 0.1 uF 2KV capacitor

  John

[DavidA]

John R

I am thinking about getting an 1:1 isolation transformer just to add that extra safety factor.  Maybe two 250 to 110 transformers back to baqck will do it;  I'll have to do some continuity chacks on the windings to ensure the primarys are isolated from the secondarys and not just auto transformers with the windings connected at one end.

John S,

It's a while since I looked at these two machines,  so I can't remember what the psu output voltages are.  I have thought about using an ATX psu if the voltages are ok,  but would like to use the original if possible just to keep the thing 'proper'.

The machine I am particularly keen on fixing is a TRS-80 Model 16 (twin 8" drives, Z80 + 68000 processors..  It was working fine until I had to store it in the outhouse where it must have got a bit damp. On turning it on I was not unsurprised to see a small puff of smoke issue from the vents. I do have a model 2 which is very similar and probably has the same psu. I'll go for a swap first before I get involved in fixing the duff unit.

It will be  awhile before I get around to doing this,  so lots of time to examine the circuit diagrams etc.

Dave.