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TI-99/4a - Worth fixing?


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#1 jmetal88 OFFLINE  

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Posted Tue Jul 6, 2010 6:22 PM

I got a free TI-99/4a from a guy a few days ago. It has four missing key caps and the video display gets progressively more scrambled after the system is powered up. I can't see much happening on screen, but it makes two tones on start up and beeps when I press keys.

I've discovered if I re-seat the TMS9918ANL, the display temporarily clears up, then quickly corrupts again. I found some of these chips for sale on eBay for about $11 apiece (albeit out of Hong Kong).

So, what do you guys think? Do I have the problem pinned down? Is it worth attempting to fix?

And on a related note, is there anything I can do about the missing key caps that doesn't involve looking for another broken TI-99/4a? The caps that are missing are, specifically, 1, I, A, and B.

EDIT: Forgot to mention, the power switch is also missing. I don't mean the physical switch, but the bit that slips over the physical switch so you can flip it on from outside the case.

Edited by jmetal88, Tue Jul 6, 2010 6:55 PM.


#2 BDW OFFLINE  

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Posted Tue Jul 6, 2010 7:15 PM

I got a free TI-99/4a from a guy a few days ago. It has four missing key caps and the video display gets progressively more scrambled after the system is powered up. I can't see much happening on screen, but it makes two tones on start up and beeps when I press keys.

I've discovered if I re-seat the TMS9918ANL, the display temporarily clears up, then quickly corrupts again. I found some of these chips for sale on eBay for about $11 apiece (albeit out of Hong Kong).

So, what do you guys think? Do I have the problem pinned down? Is it worth attempting to fix?

And on a related note, is there anything I can do about the missing key caps that doesn't involve looking for another broken TI-99/4a? The caps that are missing are, specifically, 1, I, A, and B.

EDIT: Forgot to mention, the power switch is also missing. I don't mean the physical switch, but the bit that slips over the physical switch so you can flip it on from outside the case.

I say that it is worth fixing. If you do fix it, I have a bunch of spare keys if you want them.

#3 jmetal88 OFFLINE  

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Posted Tue Jul 6, 2010 7:49 PM

I say that it is worth fixing. If you do fix it, I have a bunch of spare keys if you want them.


What color are the keys? For some reason I have the odd combination of a silver computer with beige keys - most of the silver ones I've seen online have black keys, I notice.

Not to say I wouldn't take mismatched keys, but I'd be better off being prepared for it. :lol:
If I manage to fix it, I'll definitely want the keys, if you have what I'm missing (or some of what I'm missing) in there. I'm mainly trying to make sure I have the right idea of how to go about fixing the problem with this thread.

Do you have a spare power switch too? I figure I should ask rather than just being resigned to either sticking a screwdriver in to turn it on or leaving it on and just unplugging it.

#4 BDW OFFLINE  

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Posted Tue Jul 6, 2010 8:45 PM

I have an entire set of black keys. But, I dont have an extra power switch, sorry.

#5 jmetal88 OFFLINE  

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Posted Tue Jul 6, 2010 9:03 PM

I have an entire set of black keys. But, I dont have an extra power switch, sorry.


Ah, well if it's the full set of keys then I guess there won't be any mismatch problems. :lol:

I might as well take 'em, I think I will at least try to fix it. I guess just PM me your terms and we'll take it from there.

#6 jmetal88 OFFLINE  

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Posted Tue Jul 6, 2010 10:08 PM

Alright, well, does anyone have a spare TI-99/4A power switch slider then? :lol:

#7 jmetal88 OFFLINE  

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Posted Tue Jul 6, 2010 10:56 PM

Oh, wow, I just found a complete TI-99/4a keyboard out in the garage. I didn't even know I had one. :lol:

It's still the beige one though, so with Brandon's key caps I'll be able to change keyboard colors whenever I want!

#8 JamesD OFFLINE  

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Posted Wed Jul 7, 2010 8:27 AM

To be honest it might be cheaper to just buy another one if there is much to fix.

#9 jmetal88 OFFLINE  

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Posted Wed Jul 7, 2010 9:07 AM

To be honest it might be cheaper to just buy another one if there is much to fix.


Yeah, that's what I'm trying to figure out. Hopefully it's just the VDP. I went ahead and ordered one of the $11 chips off ebay and I guess I'll see what happens.

#10 jmetal88 OFFLINE  

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Posted Wed Jul 7, 2010 11:46 AM

Oh, another question.

The guy that gave me this also gave me a bag full of cables, among which was something labeled a "TI Flexcable Interface".

From what I gather, this is used to connect the TI to some kind of expansion box. Given that I probably won't be seeking out such an expansion box, what should I do with the flex cable interface? Is it worth the trouble of trying to sell it?

#11 jmetal88 OFFLINE  

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Posted Sat Jul 10, 2010 4:03 PM

Well I have Brandon's key caps now. For some reason, TI has the stem of the right shift key off center for the black keys and centered for the gray keys, so I can use all the black keys except for right shift. :lol:

#12 Mad Hatter OFFLINE  

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Posted Sat Jul 10, 2010 5:24 PM

The flex cable interface connects the TI-99/4A to the "Peripheral Expansion Box", which it sounds like you don't have. The expansion box contains a disk drive and slots for different modules, such as an RS-232 interface, memory expansion and many other things. You could probably get $20 bucks for the interface on eBay. If you ever get an expansion box you will absolutely need this flex cable for it to work properly.

Oh, another question.

The guy that gave me this also gave me a bag full of cables, among which was something labeled a "TI Flexcable Interface".

From what I gather, this is used to connect the TI to some kind of expansion box. Given that I probably won't be seeking out such an expansion box, what should I do with the flex cable interface? Is it worth the trouble of trying to sell it?



#13 JamesD OFFLINE  

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Posted Sat Jul 10, 2010 7:34 PM

To be honest it might be cheaper to just buy another one if there is much to fix.


Yeah, that's what I'm trying to figure out. Hopefully it's just the VDP. I went ahead and ordered one of the $11 chips off ebay and I guess I'll see what happens.

I only paid $22 for my machine in box with everything.

#14 jmetal88 OFFLINE  

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Posted Sat Jul 10, 2010 11:16 PM

To be honest it might be cheaper to just buy another one if there is much to fix.


Yeah, that's what I'm trying to figure out. Hopefully it's just the VDP. I went ahead and ordered one of the $11 chips off ebay and I guess I'll see what happens.

I only paid $22 for my machine in box with everything.


I probably would have gone for another machine if there wasn't shipping cost to account for. The $11 chip had free shipping, so I thought it was worth ordering as compared to another TI-99/4a at $x+shipping which would under the best of circumstances probably come out far over $11.

Edited by jmetal88, Sat Jul 10, 2010 11:16 PM.


#15 jchase1970 OFFLINE  

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Posted Sun Aug 1, 2010 5:49 AM

To be honest it might be cheaper to just buy another one if there is much to fix.


Yeah, that's what I'm trying to figure out. Hopefully it's just the VDP. I went ahead and ordered one of the $11 chips off ebay and I guess I'll see what happens.

I only paid $22 for my machine in box with everything.


I probably would have gone for another machine if there wasn't shipping cost to account for. The $11 chip had free shipping, so I thought it was worth ordering as compared to another TI-99/4a at $x+shipping which would under the best of circumstances probably come out far over $11.

i bet the display problem is just a faulty rf adapter. you can get composite cables real cheap or make your own, very simple to do, check the ti programming forum for details.

#16 channelmaniac OFFLINE  

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Posted Mon Aug 2, 2010 10:01 AM

More likely a bad RAM chip than a bad VDP.

#17 jmetal88 OFFLINE  

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Posted Mon Aug 2, 2010 10:50 AM

More likely a bad RAM chip than a bad VDP.


Yeah, you're right - I discovered that a week or so ago, just didn't update the thread.

Someone on the vintage computer forum suggested a bad power supply might've caused the ram chips to go bad, but my multimeter is showing the voltages to be pretty steady. I'm thinking about ordering a full set of eight 4116 RAM chips from Jameco to give this another shot.

Still though, I wonder why re-seating the VDP helped at first? It doesn't do anything now, but at first it caused the graphics to clear up temporarily.

#18 jmetal88 OFFLINE  

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Posted Mon Aug 2, 2010 10:51 AM

To be honest it might be cheaper to just buy another one if there is much to fix.


Yeah, that's what I'm trying to figure out. Hopefully it's just the VDP. I went ahead and ordered one of the $11 chips off ebay and I guess I'll see what happens.

I only paid $22 for my machine in box with everything.


I probably would have gone for another machine if there wasn't shipping cost to account for. The $11 chip had free shipping, so I thought it was worth ordering as compared to another TI-99/4a at $x+shipping which would under the best of circumstances probably come out far over $11.

i bet the display problem is just a faulty rf adapter. you can get composite cables real cheap or make your own, very simple to do, check the ti programming forum for details.


I never had an RF adapter, I just bought a MIDI cable from Radio Shack and spliced a couple of RCA-style cables into it.

#19 jmetal88 OFFLINE  

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Posted Sat Aug 7, 2010 6:39 PM

Alright, I ordered a set of eight 4164 chips from Jameco yesterday, along with a 512k SRAM chip I'll be attempting to adapt for use in my PCjr.

I'll be removing a couple of chokes from the PCB to cut off the -5v and +12v supply to the RAM, and I'll be splicing +5v into the circuit where the +12v used to be. This should make the board 4164 compatible. I know I'll still only be able to access 16k of RAM, but from what I've heard, the single supply 4164 RAM chips are much more reliable than the multiple supply 4116 RAM chips. The fact that 4164 and 4116 chips are the same price at Jameco made the decision for which to buy pretty obvious to me.

#20 JamesD OFFLINE  

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Posted Sun Aug 8, 2010 2:20 PM

So, have you spent more fixing it than it would have cost for another one?

#21 jaybird3rd OFFLINE  

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Posted Sun Aug 8, 2010 2:28 PM

So, have you spent more fixing it than it would have cost for another one?

In the case of the 99/4A, you don't have to spend much before it becomes cheaper to simply replace the whole machine. That's probably what I would have done, although I would have kept the old machine as a parts donor.

#22 jmetal88 OFFLINE  

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Posted Sun Aug 8, 2010 3:15 PM

So, have you spent more fixing it than it would have cost for another one?


Not yet. I've spent about $20 on it so far, including shipping, and I bet a working one would cost me at least that much on ebay with shipping also included.

EDIT: Actually, yeah, I just had a quick look on ebay and most of the TI-99/4a computers on there have shipping alone higher than $20. The exception I saw was a known broken system that had shipping at just under $15.

Edited by jmetal88, Sun Aug 8, 2010 3:27 PM.


#23 zylon OFFLINE  

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Posted Tue Aug 10, 2010 5:01 PM

I got a batch of 2 of them for 35 including shipping. Only problem was the rf cables were bad.

#24 jmetal88 OFFLINE  

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Posted Wed Aug 11, 2010 1:46 PM

Well, now it's complete and it works! And at a minimum cost, thanks to nice guys sending me their spare case parts. :)

4164 RAM modification went ultra-smoothly, and there are no more errors on the display.

#25 JamesD OFFLINE  

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Posted Wed Aug 11, 2010 6:52 PM

Congrats! Not surprised about the RAM. It's one of the least reliable components in my experience.

Just an FYI, my TI was $25 with S&H and it had extras.... boxed and working.
My speech synth was under $10 shipped.
A box full of carts (including a couple rare titles) was $15 shipped.
And I picked up a box of TI stuff for under $20 shipped... and ended up getting my money refunded totally due to a shipping error.

You just have to be patient to get a deal on ebay.




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