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vans1932

TI 99/4A black screen and beep. (Power Supply?)

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Hello I am new to AtariAge.

I recently picked up a TI 99/4a computer off of ebay and expected to turn it on and relive my childhood.  However it shows a black screen or a black screen with lines and a loud continuous beep from the TV.  I've done my normal troubleshooting steps,

1. Reseated and cleaned all the chips. (their aren't that many to reseat)

2. Checked the TIM9904 Clock Generator (for voltages)

3. Taken the grom connector out (cart connector)

4. As far as I am aware the video chip gets very warm but don't know what the temp is actually supposed to be. (TMS9918A)

5. Checked for cracked traces, etc etc.

 

I've read other forms online including these from AtariAge:

https://atariage.com/forums/topic/270436-ti994a-black-screen-and-continuous-beep-sound/

https://atariage.com/forums/topic/172939-ti-994a-console-not-working-anymore/

And many others including this one:

http://www.vcfed.org/forum/showthread.php?65903-TI-99-4a-black-screen

 

Now here's the interesting thing, when I check the voltage coming directly from the Power supply (AC 9500) It only shows between 2-8 volts on all wires.  Not the -5 volts on pin 1, 12V that should be coming out of pin 2, (ground on pin 3 is even 3-4V) and pin 4 which is the only one that seems to be closer to the 5V but still at 2.5V. http://www.mainbyte.com/ti99/hardware/power_supply.html

 

Now from the info I've seen it could be a ram chip but I don't want to desolder anything until I have some evidence otherwise.  Could it be a power supply issue or is it a ram chip (or something else)

 

Note: The power is changing a lot sometimes it will jump from 4-7V and sometimes it will go down as well.

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Another thing of note is that some of the RAM chips (TMS4116) report 4V and some report 1.5V

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15 minutes ago, vans1932 said:
Hello I am new to AtariAge.
I recently picked up a TI 99/4a computer off of ebay and expected to turn it on and relive my childhood.  However it shows a black screen or a black screen with lines and a loud continuous beep from the TV.  I've done my normal troubleshooting steps,
1. Reseated and cleaned all the chips. (their aren't that many to reseat)
2. Checked the TIM9904 Clock Generator (for voltages)
3. Taken the grom connector out (cart connector)
4. As far as I am aware the video chip gets very warm but don't know what the temp is actually supposed to be. (TMS9918A)
5. Checked for cracked traces, etc etc.
 
I've read other forms online including these from AtariAge:
https://atariage.com/forums/topic/270436-ti994a-black-screen-and-continuous-beep-sound/
https://atariage.com/forums/topic/172939-ti-994a-console-not-working-anymore/
And many others including this one:
http://www.vcfed.org/forum/showthread.php?65903-TI-99-4a-black-screen
 
Now here's the interesting thing, when I check the voltage coming directly from the Power supply (AC 9500) It only shows between 2-8 volts on all wires.  Not the -5 volts on pin 1, 12V that should be coming out of pin 2, (ground on pin 3 is even 3-4V) and pin 4 which is the only one that seems to be closer to the 5V but still at 2.5V. http://www.mainbyte.com/ti99/hardware/power_supply.html
 
Now from the info I've seen it could be a ram chip but I don't want to desolder anything until I have some evidence otherwise.  Could it be a power supply issue or is it a ram chip (or something else)
 
Note: The power is changing a lot sometimes it will jump from 4-7V and sometimes it will go down as well.
IMG_0210.thumb.jpg.9b8e1cbb394649bd0e07696b434c7b35.jpg
IMG_0209.thumb.jpg.46d2b9dada35aec1afdd3e667889d633.jpg
IMG_0208.thumb.jpg.f37a10c4d9c25b77b70352cf91a367d8.jpg
IMG_0207.thumb.jpg.4ddb9d9618a7878a48f8295b5d6374b5.jpg
IMG_0206.thumb.jpg.8d93421e09120adebf0c6bb52a9f1fe5.jpg
IMG_0205.thumb.jpg.5807473f9153d7b00956715de8a08bd9.jpg
IMG_0204.thumb.jpg.8b9d0e9308fc5cc76dc091ba48a3d024.jpg

That's the AC power supply.. The DC supply is in the console.

For more information on the supply's http://www.mainbyte.com/ti99/hardware/power_supply.html also the FAQ:

 

Greg

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Posted (edited)

Oh, so sorry. I was in a hurry yesterday to test and it takes a little while to get an account activated here.  I retested the voltages in DC mode and you are correct.  Ok, so do you think with the circumstances (black screen and continuous noise) that it could be a vram chip? dram chip? ROM?  If so where would you recommend buying one.  Amazon or eBay would be best.  Also what does the chip look like?  I see these chips (picture below). Once again I had this as a kid but never opened it, and this is my first motherboard repair. Most of the time I try to get working items, but I got a good deal on this. 

unnamed.jpg

unnamed (1).jpg

Edited by vans1932

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1 hour ago, vans1932 said:

I had this as a kid but never opened it

I had this as a kid too! ...But I did open mine.:evil:

 

I hate to do this but... I wonder how familiar you are, taking measurements with a multimeter?

 

I imagine you know that measurements are taken between two points, whether AC or DC. So I am quite curious as to where you have placed the common probe.:ponder: I'm guessing it's either in the NEUTRAL or GND of the electrical utilities' supply receptacle(ouch)... This would result in measuring leakage voltages and stray static fields. This is because the electrical transformer OUTPUT is isolated from it's INPUT. This would work to measure the output from an AUTO-TRANSFORMER, where there is no isolation!

 

Anyway from what you describe, it sounds like the transformer is O.K..

 

"Most of the time I try to get working items, but I got a good deal on this." Perhaps a good deal of work ahead as well...:D

 

Those are the VRAM IC's alright. I'm not sure that's the best place to start though. More likely the ROMs, but they're hard to source.:(

 

A full view of the board might help a little.:cool:

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2 hours ago, vans1932 said:

Oh, so sorry. I was in a hurry yesterday to test and it takes a little while to get an account activated here.  I retested the voltages in DC mode and you are correct.  Ok, so do you think with the circumstances (black screen and continuous noise) that it could be a vram chip? dram chip? ROM?  If so where would you recommend buying one.  Amazon or eBay would be best.  Also what does the chip look like?  I see these chips (picture below). Once again I had this as a kid but never opened it, and this is my first motherboard repair. Most of the time I try to get working items, but I got a good deal on this. 

unnamed.jpg

unnamed (1).jpg

 

No worries.. So you tested the AC EXTERNAL power supply? or the INTERNAL DC power supply? 

 

Based on your motherboard pictures it likely looks like this: powers_square_994a.jpg


power_supply_out.jpg
 

 

 

The DC voltages out are:

Pin 1 = -5 volt

Pin 2 = +12 volt

Pin 3 = GND

Pin 4 = +5 volt.

the Vram requires the +12 to be there..

 

Greg 

 

 

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If your power supply is OK (the voltage on each output pin should be marked on the PCB right next to the connector) then you might be better off by first replacing the two static RAM chips - those are the two larger chips marked "MCM6810P". You can get those off eBay. The VRAM RAM chips (the block of eight marked MCM4116... and TMS4116...) can and frequently do go bad, but they usually result in a corrupted display rather stopping the machine booting altogether. Get some decent quality cutters so you can cut the IC pins individually with the IC still on the board then you can remove the IC body, then you can unsolder and remove each pin individually. This will reduce the risk of damaging the PCB. Fit a turned-pin socket to the PCB to plug the new IC into.

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