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Davvel

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In my Teens I was the lucky owner of a Beige TI 99/4A which my parents bought me for Christmas (1983). I was thrilled with it and in a few weeks I was writing small games and sharing these with my friends. My Software Developer Career started on that day.

32 years later exactly 6 months ago I joined the local Vintage Computer club and 2 months ago I told them that I would start displaying my TI 99/4A in the shows we do in schools etc... I tried switching it on again but to my dismay a long Tone and a black screen came up on the CRT TV.

I started looking for solutions on the Internet and happened to hit ATARIAGE, it was like a miracle for me, I could not believe that so many interesting people would congregate onto one forum.

From the tips I was getting I started the journey to fix my TI.
I started by checking the voltages of my power supply and found that the 8.5v AC was not coming out correctly, only 2.5V.

I took the transformer for new copper winding in a factory we have locally and the TI 99/4a booted up properly. The problem was that it only lasted 1 hour and I had the Long Tone and Black screen again. Now all the voltages were correct but TI was still not working.

I wanted to be bolder and fire up my trusty soldering iron and hand de-soldering tool (2 euro pump). I read that in many cases it is either the Video Chip (TMS9929A) or the VDP RAM (4116). So I purchased a new video chip. Replaced my chip with the new one but to no avail.

Then I ordered 8 VDP RAM chips, which are still on the way. I will socket all the chips so that I can replace the RAM chips easily if need be.

Today the 6th of December 2015, I started the de-soldering process and will keep this blog updated with the progress. The VDP RAM is difficult to remove as the solder is double sided and the two white strips interfere with the process of pulling the chips. I really would have finished this in an hour if these were not so in the way, but patiently today I removed 5 chips. More on this soon. :)

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Today I removed the last three 4116 VDP RAM chips. Now I need to wait for the new chips to arrive so that I can fit them in the TI and see if it works. Fingers crossed.

 

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Today I soldered the 8 IC bases and placed the new TMS 4116 chips. Turned on the TI but to no avail, the dreaded long tone and black screen persists. I am frustrated that after all the TIME and money spent on trying to repair my childhood TI, I am still where I started.

 

Up till now I changed/repaired the AC transformer. Repaired the power supply circuit. Replaced the TMS9929 chip, Replaced 8 RAM chips.

 

I really need some help now to try to diagnose from where the fault is originating.

 

Starting the task to replace the 8 RAM chips

Replaced RAM

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After many hours of randomly trying to find the issue in my TI 99/4a, I thought that someone must have written some kind of step by step manual to fix a TI 99/4a. I found the following invaluable material from John Guion that clearly explains how one should test your motherboard and identify the source of your problem. If I knew this before I would not have done so much surgery to my TI.

 

I hope that this weekend I can apply the following and identify the source of my problem.

 

 

 

@&CONSOLE^DEBUGGING^HELP

By John Guion

Dallas TI Home Computer Group

 

@PROBLEM^AREAS

1)^^Console will not power up

 

&TI-99/4A &Console &and &Peripheral &Expansion &System &Technical &Data manual available from Texas Instruments' Dealer Parts Department [(806) 741-2265] will serve as an excellent source for schematics and part location guide.

 

The information contained herein is only intended for use as a reference for possible debugging procedures. It is not intended as a repair guide for the common user with little or no knowledge of digital electronics or the basic structure of the TI-99/4A system. The author assumes no responsibility for damages resulting from improper use of this information.

 

This information should only be reproduced in its entirety with credit to the original author.

 

@1CONSOLE WILL NOT POWER UP

 

@1.1 General information.

 

Failure of the TI-99/4A console to power up and produce the TI title screen is a common problem that is also the hardest to track down and fix since failure of nearly any component in the console or power supply can cause this.

 

The following are not intended as solutions to the problem, but merely as points to check that may aid in finding the actual problem and fixing it.

 

Unless a particular part is suspected, replace any socketed chips possible with known working equivalents before de-soldering any components. Since the socketed chips are common causes of lock up, eliminating them as possible problems first may save excess soldering on the board. The console will power up if the sound chip is removed entirely, but not if that chip is shorted internally.

 

A simple TTL logic probe can be used for tracing signals in the circuit. An oscilloscope may also be used and has the advantage of being able to check clock signals for proper frequency. When a signal should exist as an output from a particular device, be sure to check that device's input for proper signals before attempting to replace the component. When checking for locked up signals, try to trace all signals back through the circuit to the point of origin. A set of schematics (available from several sources, including TI) will help greatly in this part of debugging.

 

Tracing locked signals can determine whether or not the signal is missing due to a faulty component that it must pass through or what power up operation was occurring during lock up.

 

 

@1.2^Console^power^up^procedure.

 

A. TMS9900 CPU resets and addresses low ROM locations.

 

B. TMS9900 initializes.

 

C. TMS9900 sets up workspace registers in MCM6810 RAM.

 

D. TMS9900 begins GROM read.

 

E. TMS9900 enters delay loop for about 1/4 second.

 

F. TMS9919 sound chip is disabled.

 

G. TMS9918A VDP chip is initialized.

 

H. 4116 VDP RAM is initialized (requires about 1 second).

 

I. Title screen is loaded into VDP.

 

J. TMS9919 sound chip emits beep.

 

K. TMS9900 CPU enters keyboard scan.

 

L. System is ready for use.

 

 

 

 

@1.3 Voltage/signal checklist.

 

 

A. Check power supply for +5V, +12V, and -5V. Lack of -5V often results in a grey flickering screen on power up.

 

Check for +5V on chips throughout board.

 

Check TMS9900 for -5V at pin^1; +5V at pins^2, 33, 59, and 64; and +12V at pin^27. If any voltages are missing, check for shorts on main board. Replace power supply if necessary.

 

B. Check TMS9900 pins^8, 9, 25, and 28 for clock signal. If not found, check TIM9904 clock generator pins^1, 2, 3, and 4 for clock signal. If not found, check TIM9904 supply voltages (+5V at pin^20, +12V at pin^13), crystal, and tank circuit. If no external problem can be found, possible TIM9904 failure.

 

C. Check TMS9918A pin^39 and pin^40 for the 10.73863 MHz clock. If missing, check crystal and oscillator circuit. Otherwise, check TMS9918A pin^36 and pin^37 for clock outputs. If not found, remove GROMs and sound processor (located next to GROMs) and test again for clock. If missing, possible TMS9918A failure. Reinsert GROMs and sound processor after tests.

 

D. Check TMS9918A pins^14 (-CSW) and^15(-CSR) for lock up. If locked up, check memory enable from pin^6 of 74LS32 and pin^13 of 74LS138 located next to MCM6810. Trace signal to find possible failure.

 

E. Check TMS9918A pin^13 (MODE) for lock up. If locked up, trace signal back to TMS9900. Also check for other components that may be locking up this line (it is used as A14). If no other fault can be found on that line, possible TMS9918A failure.

 

F. Check TMS9918A pin^1 (-RAS), pin^2 (-CAS), and pin^11 (-R/W) for lock up. If locked up, possible TMS9918A failure.

 

G. Check TMS9918A pins^17 through 24 (data lines) for signals. If missing, trace to fault. Possible TMS9918A or TMS9900 failure.

 

H. Check TMS9918A pins^3 through 10 (RAM address/data lines) for signals. If missing, possible TMS9918A failure.

 

I. Check 4116 RAM pin^14 (DATA OUT) on each chip for signal. Each chip missing signal may be at fault as well as TMS9918A.

 

J. Check TMS9900 pin^62 (READY) for lock up. If locked up, check TMS9900 pin^6 (-RESET) for signal. If pin^6 is locked up low, possible TIM9904 failure. If high, possible TMS9900 failure. If TMS9900 pin^6 is not locked up, trace circuit back from pin^62 to find fault.

 

K. Check all three GROMs (CD2155, CD2156, and CD2157) at pin^10 (-CS) and pin^15 (GREADY) for signals. If either is missing, remove all three GROMs and test pin^10 again for signal. If the signal at pin^10 does not exist, trace back through circuit to find failure. If signal exists, replace GROMs one at a time until GROM that causes lock up on pin^15 is found.

 

L. Check all three GROMs for signal on pin^11 (M0/A14) and pin^12 (M1/DBIN). If missing, trace circuit to find break in signal path.

 

M. Check each GROM for -5V at pin^14, +5V at pin^9, and -.8V to -.6V at pin^16. If missing, check for broken trace. If -.8V/-.6V is missing or at

-5V, check diode connected to that line.

 

N. Remove sound generator. If console powers up, check pin^16 for +5V, pin^4 for clock from TMS9918A, pin^5 (-WE) for signal, and pin^6 (-CS) for signal from 74LS138 closest to MCM6810. If these signals exist, possible sound chip failure.

 

O. Check TMS9918A pin^36 for composite video output. If missing, check TMS9918A crystal and clock circuit and pin^16 (-INT) for interrupt signal. If signals exist, possible TMS9918A failure.

 

P. Check GROMs for clock on pin^13. If missing, check clock output on TMS9918A pin^37. If signal on TMS9918A exists, check for break in signal path. If not, check TMS9918A oscillator circuit. If oscillator operates, possible TMS9918A failure.

 

Q. Check pin^20 (-CS) of console ROMs for lockup. If locked up, trace circuit back to find fault.

 

R. Check pins^7 and 9 through 15 of 74LS138 nearest I/O port to determine memory area accessed during lock up. Check pin^4 (-MEMEN) for lock up. If no signal can be found on pin^7 or pins^9 through 15, possible 74LS138 failure.

 

S. Check pin^11 (-CS) of MCM6810 RAMs for lock up. If locked up, trace circuit back to find fault.

 

T. Check TMS9901 pin^5 (-CE) for lock up. If locked up, check 74LS138 nearest I/O port for failure. Check TMS9901 pin^11, 17, and 18 for lock up. If locked up, trace circuit back to find fault.

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I found the following interesting information on the net.

 

The GRMCLK signal comes from the VDP, which has its own crystal or oscillator. (Note that the TMS9118 and TMS9128 later VDP chips do not have GRMCLK, but were not used on the 99/4A. The 9995 in the 99/8 and Tomy Tutor run off the VDP clock, while the Geneve 9995 and V9938 have necessarily separate oscillators.)

A defective GROM 0 can cause the black screen and continuous beep. Likewise, a defective 9901 can cause the black screen. On many consoles, GROM's and the sound chip (TMS9919 or equivalent) are socketed.

Pull GROM's 1 and 2 (TI BASIC) and the sound chip. These are unnecessary for the console to get to the title screen. If you still have a black screen, check the GROM 0 by putting it into a working console. GROM needs the -5V signal and is PMOS, and maybe movement of the firehose shorted the power temporarily.


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And this....

 

The sound you hear is probably generated by the power-up routine. This routine performs some clean up, turns the screen off (i.e. black), triggers a "beep", and waits for the beep to finish before turning the screen back on.

Since the power-up routine is written in GPL and stored in GROM 0, if it goes as far as generating a beep you can probably conclude that:
- the CPU is ok,
- the console ROMs are ok (they contain the GPL interpreter),
- GROM 0 is likely ok,
- the sound generator is ok.

So why won't the beep stop? Most likely because it is timed by the interrupt routine, itself triggered by VDP interrupts. So I'd say that the most likely failures for the "howling black screen of death" are:
- VDP failure (to generate interrupts),
- TMS9901 failure (to process interrupt and signal the CPU).

The VDP is socketed, so it may be possible to swap it, if you have another console. Or maybe just taking it out and cleaning the contacts will do...

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I feel that the above were important diagnostic steps that could lead people to solve the TI 99 Black Screen of death problem that I am having. Today I opened a good TI 99/4A and started to diagnose it with the TMS9929 chip removed.

 

I found that in the good TI pin 39 and 40 of the tms9929 had a high pitched audio sound when using a logic probe. These pins are connected to a 10Mhz crystal and oscillator circuit.

 

On the bad circuit board there is no signal at all on pins 39 and 40. This leads me to think that the oscillator needs to be replaced.

 

I have an idea, I will try to take the output clock of the good board and feed it to the bad board on pins 39 and 40. I will see if this would boot the TI. I still don't know How to isolate the IC but the only thing I can try is to bend the two PINS 39 and 40 of the tms9929 and solder a wire leading to pin 39 and 40 of the good board.

 

If this works I still don't know if I will find the exact crystal + 56pf capacitor that goes with it.

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Hello, I realize this is a bit of a necrothread, I'm attempting to follow the troubleshooting steps by John Guion above.  There's a lot of parts that mention "checking for locked up signals" - does that simply mean that the pin is stuck high?  Or is there more to it that I need to check with a scope?  Thanks.

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