Commodore C128 PS repair HELP PLEASE!

[LateLesley]

Yeah, That sounds like a sensible reading for the diode. It should only read one way, and since it does, this is a good indication.

The oscilloscope, we can use to see if the IC is outputting a PWM signal. That will come out of pin 15/16, goes through R9 (which is right in between R1 and C5) So I would maybe try probing carefully on that resistor.

As for the settings, I'm not sure. I can't see the model number of the oscilloscope, so we can get a user manual. We need a setting on the vertical that can take at least 30V, so first we need to confirm that A) it can take that voltage, and B) what settings to use, before we go probing the circuit.

I can see it's a heathkit, see if you can find the Model Number anywhere on it, and we'll see what info we can dig up on the old beast. It's impressive!! :-)

[Serge125]

Yeah, That sounds like a sensible reading for the diode. It should only read one way, and since it does, this is a good indication.

GOOD! 

As for the settings, I'm not sure. I can't see the model number of the oscilloscope, so we can get a user manual. We need a setting on the vertical that can take at least 30V, so first we need to confirm that A) it can take that voltage, and B) what settings to use, before we go probing the circuit.

I can see it's a heathkit, see if you can find the Model Number anywhere on it, and we'll see what info we can dig up on the old beast. It's impressive!! :-)

Here it is; https://www.mods.dk/downloadmanual.php?RefFil=IO-12_Oscilloscope_Manual.zip

Hope it helps

THANKS!!

Serge

[Serge125]

Ok just had a thought about this problem. Would it be possible that this ps must be connected to the computer to “enable” the 5vdc part of the ps? There isn’t any on/off button on the ps it’s on the computer itself. So what I’m thinking is the 9vac goes into the computer, it powers a switching circuit that turns on the 5vdc in the ps. Could it be possible? I’ve checked everything up to now and no problems found, I even changed the caps and still no problems. Like I said it’s just a thought.

Serge

[LateLesley]

NO, it's just a switch on and there should be power thing, there is no enable line from the computer to the PSU.

thanks for the link to the manual, i'll download it, and have a read, see if i can point you to the settings you need. it might be a day or two though. i'm tired today, and got a foggy head. Some days I find it hard to concentrate (got a Mental health condition), and the current CV 19 situ isn't helping it.  :-) once i've had a decent sleep, and a read at the manual, ill get back to you. :-)

L.

[Serge125]

Ok I decided to give it a try and see if it would work and yep it did turn on and I get a display so I think that it needed to be connected to the computer. So for now it's working as it should and glad that it does. So could it be what I said that it might have (inside the computer) a little circuit that kinda sends a signal to the 5Vdc to turn on OR something like this? Well it's working for about 2 hours now without any problems.

So I want to say a BIG THANKS for all of your help, your time, patience and teaching!!! It's something that I always enjoyed doing electronics but doing what I did with the ps was just but a dream and you helped me understand allot and I THANK you for this!!! Oh I do fix a few "basic" stuff for a long time but doing troubleshooting like I did with the ps it was a dream come true. Now I'm looking forward to my next fix and if I need help I'll come back to this group to ask and maybe having you to help me again. It was nice to meet you Latelesley and again THANKS for all the help!!!

Take good care!!

Serge

[Bud]

Some switching power supplies require minimum load current in order to work. This looks to be the case with yours. When you connected it to the computer, the computer started drawing current and the power supply  turned on. You can validate it by simply connecting a 10-20 Ohm resistor between the 5V and Ground (with the computer disconnected) and check the voltage across the resistor.

[LateLesley]

@Serge125 You are very welcome, I'm glad it's working.

Bud is probably right, there are cases where a supply needs a load on it before it'll work, and it's possible with the IC checking the current load, that it only switches on when the computer is connected, and it starts drawing current.

So Bud is spot on.

Take care. :-)