TI99/4A Black screen and continuous beep sound

[repetto74]

Hi Guys,

 

I have received from a friend a TI99/4A he got for a few bucks at a garage sale. The guy was not able to say if the machine was working but due to the really cheap price (10 Euros) he decided to pick it up anyway.

 

Unfortunately the machine will not boot up. The power light is on but I do see a black screen with a continuous beep sound. Sound pitch may vary on power recycle.

 

Is there first any tutorial on how to disassemble the TI99? I did repaired some C64 and VIC20 but this one is new for me. I tried to open the case but then leave it as a small PCB inside was attached apparently to both top and bottom cases so did not want to brake it of course. Before doing any stupid thing better to have a tutorial for this one :-)

 

Then concerning the fault where I should start to look at? I have some good experience in soldering/unsoldering + all tools needed.

 

The PSU has been tested working on another TI99 (my machine) so not at fault.

Thanks

Rick

Edited September 28, 2017 by repetto74

[ClassicGMR]

Found this:

http://www.99er.net/phbbarchive/99er.net/99erbb/viewtopic2cbf.html?f=7&t=1712

 

and maybe this:

http://www.mainbyte.com/ti99/console/console_dis.html

Edited September 28, 2017 by ClassicGMR

[repetto74]

Hi Classic GMR

 

Thanks for the disassembling guide this is just what I was looking for . Also found a thread with same symptoms as mine : http://atariage.com/forums/topic/172939-ti-994a-console-not-working-anymore/

 

I will start some troubleshooting then. Hope maybe this will be only a RAM fault which can be easily repaired

[repetto74]

I have disassembled the TI99/4A and have some findings. What I did first is to clean of course the machine from dust. Then what I did was to remove all IC's socketed and to clean legs with light sand paper + alcohol. There are not so many chips with socket (the two with thermal paste I cannot identify and 4 IC's located on top of the CPU (video ram??)). Also cleaned the module connector pins with sandpaper and alcohol.

 

Then here comes the most interesting finding : onto the power input connector there are 3 diodes I believe which are soldered (diodes are marked 104Z on one of them I can also read 50V). I do not think this is a mod but mostly part of the original design of the power circuitry. Now as you can see from pictures one of those diode is cracked. The glass body is broken on one side leaving the internal circuitry exposed. I do not know how much this can have an impact on a black screen on startup but I assume it should maybe not work properly if cracked that way.

How can I measure the integrity of a diode with a multimeter? ref 104 does mean what exactly in terms of voltage?

 

So probably changing this diode is the first thing to do and then retest the machine again. As most of the IC's are soldered I will have to start unsoldering if T/S will have to resume so I would like to try anyway to point out the IC's which are most commonly involved in this kind of failure. (all 8 rams on the left side I suppose).

 

Another finding is on the power supply PCB (the small PCB inside the TI99 where to power switch is located). I can see some components I would identify as diodes also but some and in particular one look quiet odd. Inside the transparent glass cover the circuitry is not straight from lead to lead but looks like bending on one side or looks detached from the diode body (is that possible?? ). I should compare this with the PCB of my own machine to be sure.

Thanks

Edited September 29, 2017 by repetto74

[repetto74]

I would like to test the PSU board of the working TI99/4A to check if the issue is related to that broken diode/resistor (cannot identify it). The very strange thing I do see is that the PSU board of the bad machine is supposed to be for model TI99/4 where here this board is mounted on a TI99/4A machine according to this link

http://www.mainbyte.com/ti99/hardware/power_supply.html

 

I suppose that both PSU are compatible ? The only main difference between the 4 and the 4A was the sprytes addition on the video chip right?

 

 

[repetto74]

So here is what has been done :

 

Cleaning and reseating all socketed chips + cleaning the module connector pins : still the black screen.

It is not possible to swap the PSU boards as the pin connector are not the same on both boards. On the broken ti99 the boards has a molex connector while on my TI99 the connector is on the board so I get a male/male connection. My PSU board do not have the diodes soldered on the power plug chassis so I think the one broken is not causing the black screen (I unsoldered it as it is completely broken off).

NOw next move will be to swap any socketed chip onto my working board and check if any is faulty.

 

Glad to have any advice that may help at this stage

[+Ksarul]

The glass components with a 104 on them are actually capacitors (.1uF), not diodes. Here is a troubleshooting guide (for the 99/4) but it is very similar to the /4A). It is in two parts. You will find a LOT of useful technical information for the 99/4A on this website too (it is also where the manuals are--I put a lot of those documents up there when I scanned what I have in my collection)..

[repetto74]

Hi Ksarul,

 

Thanks a lot that will help for sure . I really want to fix this machine ! I do have some 1uf capacitors available here but they are ceramic so does this make a big difference if I use one of these?

[repetto74]

So today I have tested all socketed chips by swapping them to my working TI99/4A. There are not many of them, the VDP, GROM and the sound chip. All are working good as my TI99/4A will boot up normally.

I could also test the clock generator (socketed on my board) but on the dead machine this chip is soldered to the board but I do not think this chip is faulty considering that I do have a black screen boot with the continuous beep sound.

 

What could be the next move? I am considering to invest in a desoldering station to easily take out all soldered chips. Maybe now removing the full bank of rams?

I still have a doubt on the PSU board, do not know if a missing voltage can cause this problem and do not want to test it on my machine to avoid frying something if the PSU is at fault. Any idea where to measure for correct voltage outputs?

The idea would be to hook the PSU of the working TI99/4A to the faulty board but the molex connector is different and need to find out an adapter for it.

Edited October 1, 2017 by repetto74

[repetto74]

So I have checked voltage outputs at the mainboard and looks ok. I have 11.78, +5.2 and -5.1 so this is within range. Next move will be to desolder all VRAM chips and test them one by one.

[repetto74]

I am continuing my T/S report...alone

I have taken voltages on the 8 VRAM chips and there is something suspicious. RAM 0 and RAM 7 have both less than 1V input while all the other are stable to 4V so I think there is something wrong here.

I have checked also the Clock Generator TIM9904 and voltages of 5V and 12V are ok. Could this low voltages be a consequence of a bad ram?

[repetto74]

Hi Guys,

 

So I had some extra time to continue the troubleshooting of this TI99/4A. What I did is to swap the suspect TIM9904 from the bad board to the board of the working TI99/4A. The good board will boot up to the prompt but now with the continuous beep sound. On the bad board the power up is giving the continuous sound with a black screen.

So I need to consider the TIM9904 as bad but I do think it is not the only faulty chip.

[mervynmcmurray]

Hi repetto74,

 

Did you ever solve the black screen issue? Facing the same issue myself.

Edited February 15, 2018 by mervynmcmurray

[repetto74]

yes I did sole it. it turned out to be one of the two console ROMs (U611) which was faulty.