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Reparing an old DEC Rainbow PC100

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Hello, I've been a long time viewer of Dave's videos on Youtube but on the forums this is my first post.  Hello all!

When I was at my parents I dug out my dad's old DEC Rainbow 100B (with a massive 20 MB hard drive!) and tried to power this computer up.  Made sure the voltage was set to 115V and hit that switch...nada.  I yanked out the PSU to see if there are any blown caps, fuses, etc anything that an amature like me could easily diagnose and replace.  They used Nippon Chemi-con caps as I gather from Dave's videos as being 'the ducks guts' and they looked a ok.   But working with power supplies is a bit of a step up from what I'm used to and I don't know fully everything there is to know.   I've check continuity across the fuse and its ok.  There is a 3.3? 400V power resister that looks alright.  No magic smoke holes or obviously blown parts.

Anyway,  any pointers when dealing with an older PSU?  Where to go next to finding the fault if there is one?  The documents on that I found about it didn't list a troubleshooting flow chart if it doesn't power on, just to remove and get another "Field Replaceable Unit" part from DEC.

By the way, a picture:  And yes, the transformers are a bit rusty.

Are there any markings for DC voltage rails on the PSU anywhere? You need to check if the whole thing is dead or if there are any signs of life (voltages) at the output in an attempt to isolate the fault.

Edit: Found this;

Not sure if this is what you already have. But it appears that there should be a +5Vdc, +12Vdc and -12Vdc rails on the PSU. Check to see if any of these are there.

Also, this block diagram should be of some assistance. Check the outputs of the rectifiers.

Almost certainly the problem is in Control Module, or the V-bias power supply that feeds it. If the supply is OK, then replace any small electrolytic caps in and around that module.

The other thing that can happen, is the capacitors in the output go bad/leaky and shut down the PSU.

Did you check the voltage selection switch for bad contact?

I have a working Rainbow 100B, i had to replace some capacitors in the monitor. I still have to find a way to reformat the disk in my system.

According to that block diagram, the fan is AC powered.  If 'nada' happened when you hit the power switch, the only things that were involved were those few items on the AC side of the circuit.



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