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Diagnose this sick TI


dphirschler

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I have three beige TI consoles. Two have bad keyboards, one has a good keyboard but looks like this when I boot it.

 

post-39934-0-64907900-1415493251_thumb.jpg

 

I replaced the one chip that comes out of the socket (this is the one I removed for the eventual F18A mod) from another console. Didn't help. Can anybody else diagnose this from just a photo?

 

 

Darryl

Edited by dphirschler
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I don't think it's the VDP because I already swapped it out with a known good one (removed from another TI in preparation for F18A). I was very hopeful this would solve it, but it didn't. So that leaves video RAM then, right? Can I assume that the video RAM is #14 in the below picture?

 

post-39934-0-83999900-1407978003_thumb.jpg

(picture from old-computers.com)

 

I should point out that TI Invaders crashed shortly after entering the game. That seems to point to bad RAM too, doesn't it?

 

 

Darryl

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I didn't realize the F18A had its own RAM. So it doesn't even use those bad chips after the upgrade. This post made my day! And now (after re-reading the thread), I see that Stuart said the same thing but I just missed it.

 

 

Darryl

Edited by dphirschler
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On the screen you see "PPGQQ" instead of "PRESS". P is 0x50 (or $50 or >50, depending upon your assembly culture), R is 0x52, S is 0x53, Q is 0x51, E is 0x45, and G is 0x47. In bit-speak that is:

P  %01010000
Q  %01010001
R  %01010010
S  %01010011

E  %00100101
G  %00100111

Just looking at "PRESS" we see this pattern:

BAD            GOOD
%01010000      %01010000
%01010000      %01010010
%00100111      %00100101
%01010001      %01010011
%01010001      %01010011

This does not give a consistent dead or stuck state, but it does show that bit 1 is usually not what we expect. Looking around the screen, it is pretty consistent that bit 1 is often wrong, and wrong on the same characters: E->G, S->Q, N->L, M->O, etc.

 

I do not know which chip is bit 1 of the VDP RAM byte, but I am sure someone else here does. If you are going to replace your VDP with an F18A then this is academic unless you want to save good RAM chips for other computers.

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If you're handy with a de-soldering tool (I'm luck, I actually have a Weller de-soldering station :) ) you could de-solder all of the VRAM chips, throw away the bad one, and sell the rest on eBay. They actually go for good money - they're getting increasingly rare. It could offset some of the cost of the F18A :-)

 

Another benefit: Your TI console will pull considerably less power from the built in power supply board, meaning the whole console runs cooler - that's also good from a "stressing the hell out of vintage ~30 year old components" perspective!

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Talking of old components, are there many electrolytic capacitors in the TI console? We're gonna start hitting a wall soon whereby those old caps are gonna dry out and need changing. It's already being seen quite a lot in the ZX Spectrum world (degrading PAL video output which is corrected by changing a capacitor). The components in our consoles are now 30 years old (32 years, and 31 years in my two consoles) - some of those caps are gonna we waaaaay off spec!

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I was gonna suggest swapping a working motherboard and keyboard into one case but the F18A answer is no doubt better yet! If the RAM chips don't feel hot like one or another might be shorted, you may be better off leaving them in. It's a lot of desoldering that has its own risks of board/trace damage. Or just swap in one of the working boards, but you'd have to reinstall the F18A into it.

-Ed

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Removing the chips will be a snap. My coworker has a professional desoldering station and wouldn't mind taking a few mins to remove those chips. My main concern is identifying the bad chips so I can throw them out. The good chips I want to keep on hand as spares, or possibly sell them on EBay.

 

Darryl

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Some Timex Sinclair models use the same 4116 or compatible RAM, plus other vintage iron as well. Sometimes you can piggy back a good chip by hand, but not so sure how easy or reliable that test is. About the only way I know of testing in circuit other than a short check on the power pins or feel for heat is substitution with a known good chip. Which usually means having a working system with one chip socketed for testing the unknown chips.

 

Maybe the hardware gang has something better. I have a Timex 1500 with bad RAM, so I'm watching this thread as well.

 

-Ed

Edited by Ed in SoDak
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Removing the chips will be a snap. My coworker has a professional desoldering station and wouldn't mind taking a few mins to remove those chips.

 

Be aware that you'll probably find that two diagonally-opposite pins on each chip were folded flat against the board prior to soldering, so as well as melting/removing the solder you've also got to bend up a couple of pins that are soldered to the board. It's not just a case of melting the solder and the chips fall out ...

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Be aware that you'll probably find that two diagonally-opposite pins on each chip were folded flat against the board prior to soldering, so as well as melting/removing the solder you've also got to bend up a couple of pins that are soldered to the board. It's not just a case of melting the solder and the chips fall out ...

 

Is that part of the wave-soldering process, to hold the chips in-place until they hit the tub?

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I am also planning on removing the a/v connector in the back left corner. I will put either an RCA there or a 1/8" audi connector. I might install a small amp and a volume wheel too.

 

Darryl

 

I just built a 5-pin DIN connector with nothing but audio output.

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