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TI-99/4A from 1980, had a TI-99/4 psu and board help

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Hi everyone and thanks for having a look at this post. I've had my ti994a since it was initially released back in 1980. Never had a problem with it recently eye spotted too good of a deal to pass up on eBay and bought a second ti994a both models are the black and silver kind for 20 bucks with box complete too good to pass up. From looking at it, I saw no differences with this new TI from ebay and my 1980 original. I decided to fire up my original ti994a but the power supply I had was put away somewhere so since the ti I just got off eBay seemed exactly similar including the power supply which was the old wart version I connected it powered it on and my computer worked for about a second then went to a black screen rebooted it a few times sometimes got to the initial screen other times started to get garbage on certain games like Donkey Kong where the text would be messed up I dismantle my machine to take a look at it to see that the inside power motherboard was actually from a ti994! computer my guess is this was an early one and then we were using anything they could to keep costs down but basically the power supply I had was a 16 volt where are the newer ti994a for 18 volt so obviously I screwed something up has anyone had any experience in these matters. I purchased f18a thinking maybe I tried video Ram or the video processor but it made no difference. I did notice a lot of bad solder joints so I repaired every single bad solder joint I saw power the machine on with the original proper power supply and the two I worked about a minute perfectly but now all I get is a black screen with the initial sound beep.

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Hi Moonlight .... sounds to me like you need to get a 4A motherboard ... reason I say that and not just another console is because from looking at your 1980 4A it is actually a Tan colour rather than silver, like the 994, but with the 4A keys right? .... that IS a rare beast. Keep the console! :)

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