Topic: How to check a C128D (Plastic Case) PSU

[Mgarnett]

Hi All,

I'm still trying to resurrect a faulty C128D machine that I have.  The machine powers on, the monitor detects a video signal and springs to life, but it only displays a black screen.  This happens regardless of mode (C64/C128), 40/80 columns or the presence of a cartridge.

I have replaced the entire motherboard assembly in the machine but I still get the same fault, so I am assuming not that the fault lies with the power supply (unless I have two boards with the same fault).  I am new to electronics but I want to be able to check that I am getting the correct voltages from the power supply.  So my newbie question is, how do I go about doing this?

I have a multimeter but I don't actually know what to test in order to check this.  My questions are:

1.  When testing, do you need to make sue the power supply is plugged in and connected to the 128D motherboard, or is it tested in isolation (i.e. plugged into the wall but not connected to the machine).

2.  Which pins/components do I need to touch with the multimeter in order to perform the test?

Apologies for the dumb questions, but I didn't want to start poking around inside a powered on machine with metal probes without knowing exactly how to go about it.

Cheers

Mark

[Hydrophilic]

You can test the power supply without connecting it to the C128.  The plug on the end of the cord should look like

Code: [Select]

+----U----+
| o1   o2 |
|    o3   |
| o4   o5 |
+---------+
You should measure about 5.5v DC between pins 2(ground) and 5(+5V), and you should measure about 10v AC between pins 1 and 3.  This is a bit higher than the normal values as the supply is not "loaded down" by the C128.
 
To test the 'loaded' values, you can connect the power supply and turn on the computer and measure the voltages on the user port.

Code: [Select]

   1  2  3  4  5  6  7  8  9 10 11 12
+-==-==-==-==-==-==-==-==-==-==-==-==-+
+-==-==-==-==-==-==-==-==-==-==-==-==-+
   A  B  C  D  E  F  G  H  I  J  K  L
Here you should measure about 5V DC between pins 1 (ground) and 2 (+5V), and about 9v AC between pins 10 and 11.
 
If you measure the power supply and the voltages are bad, well, then power supply is bad.
 
If you measure the power supply and the voltages seem good but then when you measure the user port and the voltages are bad, then either something is wrong with the board or else the power supply is bad -- it is too weak drive the C128.
 
I had similar symptoms once and it turns out is was the 9V AC from the power supply was bad.  Anyway, good luck!

[Mgarnett]

Thanks for the info!

I will check the loaded values as you have said using the user port. 

In relation to the the power supply, the plastic case 128D has an internal power supply with two long ATX style connectors one going to the motherboard and one going to the 1571 daughter board.  Do you know which colored wires I need to check for the voltages on that type of power supply?

Cheers

Mark

[Mgarnett]

Sorry, I also have one additional question, if the 9v output of my power supply is bad, is there any way of repairing it and if not, is there any way of adapting a more modern power supply to power up the machine?

Cheers

Mark

[RobertB]

...is there any way of adapting a more modern power supply to power up the machine?

     Duncan M. of TOGA adapted a more modern, North American, H.P. power supply to run on his plastic C128D.

          Happy New Year!
          Robert Bernardo
          Fresno Commodore User Group
          http://videocam.net.au/fcug
          The Other Group of Amigoids
          http://www.calweb.com/~rabel1/
          Southern California Commodore & Amiga Network
          http://www.sccaners.org

[Hydrophilic]

I don't have a D model, so I can't tell you about the internal wiring. 
 
If the power supply is bad at all, you'll probably need to take it to an electronic technician.  Ray Carlson (sp?) is often recommended, though I haven't used him.
 
I was lucky with my power supply.  You see the transformer inside the power supply is quite massive and thus has a lot of inertia.  So if the power supply ever gets dropped or kicked, there is a good chance the circuit board will crack around the transformer.  Well, that was the trouble with mine anyway.  I justed needed to solder jumper wires to bridge the cracks in the circuit board and it worked.  But don't count on the problem being that easy to fix!

[dabone]

No 9volt will show that fault, the z80 cpu is powered from the 9v supply. Check the z80 for +5v on pin 11. If this isn't there then the machine only shows a black screen as the z80 is the initial boot processor.
 
 
Later,
dabone

[Mgarnett]

...is there any way of adapting a more modern power supply to power up the machine?

     Duncan M. of TOGA adapted a more modern, North American, H.P. power supply to run on his plastic C128D.

          Happy New Year!
          Robert Bernardo
          Fresno Commodore User Group
          http://videocam.net.au/fcug
          The Other Group of Amigoids
          http://www.calweb.com/~rabel1/
          Southern California Commodore & Amiga Network
          http://www.sccaners.org

Hi Robert,

Do you know how this was done?

Cheers

Mark

[RobertB]

     No, I don't know the details.  However, I will send his contact information to you in private.

          Truly,
          Robert Bernardo
          Fresno Commodore User Group
          http://videocam.net.au/fcug