First impressions and review of my TI-99\4a version 40
Stuck home from work due to health issues so I got bored and picked this project out of the pile on the work bench. I have been experimenting over the years with speeding up the stock 4a console. The result of those experiments created the console that has been reliably running the Hidden Reef BBS for the past few decades. That particular console has the 32k on the 16 bit buss and runs overclocked to 3.58Mhz. Back when I was playing around I think I tested out at least three versions of available designs that put the 32k on the 16 bit buss until finally settling on the design from System Ninety Nine User Group of Germany. The SNUG design required a daughter board which was produced by the group but was the most reliable of the designs I tried and added a couple of features handy for programmers and could be setup to toggle on and off so the console could still be used at stock speed for gamers. Back at that time I had also experimented with running at 4.0Mhz but found that 4.0 broke too much software and wouldn't boot with certain 3rd party hardware in the P-Box.
All that said, I had chose a 4a motherboard with the NEC -3 VDP memory which is 150 nano second instead of some of the other 4a MB's with 200 nano second or slower VDP memory as the base for this project. If you are lucky you might find a 4a MB with NEC -5 VDP chips which are 100 nano seconds. First thing I did was remover the stock clock crystal and install a new 4.0Mhz clock crystal. Sure enough, it did boot but was very flaky and showed dropped characters and wrong colors as has happened in the past when I tried a 4.0Mhz crystal in a stock MB. I next removed the stock TMS9901 and installed a TMS9901-40 and also began replacing the 74LSXXX logic in the memory management circuit with 74HCTXXX logic and replaced the stock electrolytic capacitors with new capacitors. At this point, I rigged up the MB and booted it to make sure of my work to this point. It booted nicely and was helped by the TMS9901-40 being installed. Did some things like run the size command while in X-Basic, access DSK1 just to see what would work and what wouldn't. I then removed the TMS9900 from the MB and installed a TMS9900-40. Booted up the MB to check my work and it did boot and ran as expected. In the photos you can see I installed a RAM disk in the P-Box and formatted it and loaded software to it. I also made a log-in on the Hidden Reef using TELCO at 4800 baud and to my surprise the screen didn't show any dropped characters and I didn't experience any other problems I was expecting from past experience. It was late and I had lost energy so I left the system running the EKMEL Fish Tank software and went to bed. Now 14 hours later the system is still running nicely without having crashed. Tonight I hope to finish changing the remaining 74LSXXX logic with 74HCTXXX logic and also perform the SNUG 32k on the 16 bit buss mod to bring this MB up to running at 16 bit speed at 4.0Mhz. If I'm successful I will install this MB into the Hidden Reef system and make it the first S&T BBS running at 16 bit, 4.0Mhz speed.