Ruggers Customs Posted February 9, 2021 Share Posted February 9, 2021 Hello all. Recently, a customer sent me his TI-99/4a for an f18a install. After I was finished, I played around with the console for a while and was hooked. Such a nice little old computer. So, I decided to buy a broke in pieces one to restore for myself. Looks to be intact and in decent shape. Immediately, I noticed a power issue. When the power supply board is disconnected from the main board, it outputs the proper -5, +5 and 12 volts. However, when I plug it into the main board then the output voltages drop to barely under a volt. Nothing is getting hot so this will be a little fun project to troubleshoot and repair. Wish me luck! 4 Quote Link to comment Share on other sites More sharing options...
Ed in SoDak Posted February 10, 2021 Share Posted February 10, 2021 Ok. Luck! Or better yet, Good Luck! (It comes in two flavors) Can the PS handle a load? Try reading current from the supply or check voltage with a load of a 12v light bulb or a resistor of a few hundred Ohms. The -5 line is low current, but the +5 and 12v can handle a few hundred mA. Enough to verify they work till connected to the motherboard at least. Nice pic! I spotted something that looks like a couple stray solder bits near the top right of the VPD chip, it's the one covered in dried thermal compound. This chip runs hot and needs fresh compound and a heatsink to last in service. It's ok as-is for brief tests. It also fails frequently. Next is the RAM, which can short if one of the supply voltages quit or just because they're feelin' ornery. Shorts on chips can be resistance metered on the voltage to ground pins. -Ed 2 Quote Link to comment Share on other sites More sharing options...
Ruggers Customs Posted February 10, 2021 Author Share Posted February 10, 2021 16 hours ago, Ed in SoDak said: Ok. Luck! Or better yet, Good Luck! (It comes in two flavors) Can the PS handle a load? Try reading current from the supply or check voltage with a load of a 12v light bulb or a resistor of a few hundred Ohms. The -5 line is low current, but the +5 and 12v can handle a few hundred mA. Enough to verify they work till connected to the motherboard at least. Nice pic! I spotted something that looks like a couple stray solder bits near the top right of the VPD chip, it's the one covered in dried thermal compound. This chip runs hot and needs fresh compound and a heatsink to last in service. It's ok as-is for brief tests. It also fails frequently. Next is the RAM, which can short if one of the supply voltages quit or just because they're feelin' ornery. Shorts on chips can be resistance metered on the voltage to ground pins. -Ed Thank you sir! That's just the dried heatsink compound. It's stringy and I just haven't had a chance to clean it yet. I fixed it earlier, or rather I found out the issue. The power board was outputting the proper voltages when disconnected from the mobo but would drop out to practically nothing when connected. At first I thought short to ground somewhere on the mainboard but decided to hook a coleco 4 pin jack to the mainboard first broke I out the oscilloscope. Hooked a meanwell switching power supply up and she's alive! Hard part is over, just got to rework the power board. The keyboard is completely dead though. No signs of life so I'll have to read up on that. This one has the brown back so I'm assuming it's the membrane type keyboard. Hopefully it's just the chip responsible for the keyboard function and not the keyboard itself. If it does turn out to be the keyboard, is there a source for old used parts like this? I checked eBay but it's just consoles. Quote Link to comment Share on other sites More sharing options...
Ed in SoDak Posted February 11, 2021 Share Posted February 11, 2021 (edited) 5 hours ago, Ruggers Customs said: Thank you sir! That's just the dried heatsink compound. It's stringy and I just haven't had a chance to clean it yet. After posting, I thought it might be only heatsink compound drips. Good the MB is working at least. Easier to sub a different PS or repair yours, than to troubleshoot/replace all the RAM, for example. You could test the MB end for key response by shorting pins in the motherboard's kbd connector. Jumping pins 1 and 15 would get you a U, and so on. See attachment below. TI's membrane keyboards were a great idea at the time, but their best-if-used-by date came all too soon. There's a couple recent threads on fixing them, or rather trying to, or rebuilding. Good replacement TI keyboards used to be common, sold by many of the surplus outlets BITD, even Radio Shack sold them. Same for the power supply boards. Now I think you either have to get lucky and find one in somebody's stash they're willing to part with or get a whole 'nother console to rob one from. Arcadeshopper may have replacement membrane kbds, which are a dice roll themselves, I'd guess, unless tested first. There are a few kbd alternatives using USB or other methods. Check the FAQ. All of those are outboard to the console which can be good or bad, depending on your desk space versus preference for originality. The FAQ also0 gives details on the various good/bad original keyboards TI used. Attached a few schems to help confuse inform ya! -Ed TI Circuit Diagrams and Schematics.pdf Edited February 11, 2021 by Ed in SoDak 2 1 Quote Link to comment Share on other sites More sharing options...
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