LM338k to replace 5v with Commodore 128 Power Supply

[Kevinwil]

Hi Everyone,

  I've recently had the bug to get out some of my old commies, and alas, most of them were not working!  My 64 just blips out after about 30 secs after running for about 8 hours with no issue.  It looks like it may be fried now.  (Side question, do you guys recap your machines?  Maybe this is why it failed?)  Anyway, I have my old 128 which I thought would work, but it too did not come on.  However, this time, it was the power supply not putting off +5v and the fuse was not blown!  I saw that it was 4.4 amps I believe, and I just happened to have an LM338k 5a variable regulator.  I opened my 128 supply to find some nice crispy transistors, and a few other resistors, jumper wires, etc burned up. Not really sure what happened, but that supply is frankly over my head (IE, I'm too lazy to try and figure it out) and I suspect some of the parts may be hard to find anyway.  I desoldiered everything but the filter caps, the output caps (and coil) and the rectifier.  I then plugged it in to find about (18vac) 23v dc from the caps.  Wow, the output is pretty hot for +5v, but I knew the LM338k could handle the voltage swing, but now, I will need a fan. :-( 

I built a small board with a few protection diodes/caps and the 338, and just wired it back into the existing holes on the power supply circuit board. Only three wires!  One ground, one for input and one for output.  I then just used the existing caps(and coil) on the board as they all seem to be fine. It is small enough to still fit inside the power supply, now that I desoldiered everything else. I wired it all up, and success!  However, I haven't added a fan yet and within about 5 mins, the screen went nuts and the supply was hotter than the sun!  The 338 has overload protection, so it's fine but I don't even know if a fan will work. I will have to drill major holes in the supply case to allow air-flow, so I wanted to ask around before I started butchering it too much.  I may just need an enormous heatsink, but I'm starting to think I just can't use the existing transformer efficiently.


Ok, sorry for the long description, here are my questions.

1. Has anyone else tried this with a 5a 5v regulator and the native supply @23volts?
2. Is there a better circuit I could use perhaps?
3. I know this is more of an electronics question, but I suspect since the transformer is putting out 18vac, it's probably only 2 amps at most.  Will the 338 convert the volts to amps as well as drop the voltage, or have I made a 5 volt 2 amp regulator?  Does this make sense? :-)
3. Should I just get like a 6amp 9vac transformer and run the ac and dc output from a single winding?  Will this cause any problems? (Shouldn't but I know old equipment may be touchy to any ripples in the ac.) 
4. Are there any other preemptive repairs I should make to the 128 before I leave it on for any length of time?  It does seem to work just fine, until the supply went silly.
5. Any other suggestions?  I just want a working machine again!  Preferably, one of my old ones. 

Thanks!
-Kevin

[RobertB]

     Whoa!  That's a lot of trouble to build a p.s. that may or may not work.

5. Any other suggestions?  I just want a working machine again!

Why don't you just buy another working CBM power supply?

                Truly,
                Robert Bernardo
                Fresno Commodore User Group
                http://videocam.net.au/fcug
                July 25-26 Commodore Vegas Expo - http://www.portcommodore.com/commvex

[arnsfelt]

A linear voltage regulator such as the LM338k works by dissipating all power that is not delivered to the output as heat. Thus, the more the voltage is dropped, the more heat is dissipated. The current is not increased; you will dissipate (23-5)V*2A = 36W of heat at 2A, and get 5V at 2A at the output.