Daily driver F18a enabled TI-99/4A console broken

[+retroclouds]

So today I wanted to try out my newly received strangecarts. Before I could do so I noticed that my daily driver TI-99/4a system is acting weird.

 

This is what I noticed.

 

1. The system kinda works, as longs as there is no PEB box connected. If the PEB box is connected it locks up on a blank cyan screen while accessing my stock TI Disk controller

2. If the PEB is disconnected I get the normal TI-99/4a title screen. I can jump into TI-Basic. Fine so far.

3. If I start to type in a basic TI program like “100 PRINT HELLO WORLD” some strange behaviour happens:

   The string is not properly tokenized. I get some some replaced characters on screen and cannot run the program.

 

All of this kinda makes me believe there is some VRAM problem.

But here is the catch, it has an F18a onboard so my assumption is that the still mounted VRAM memory chips are not used anyway. Is that assumption correct?

 

I will try to do a video in the next few days.

 

So is this a VRAM issue or is this something completely different?

[Gary from OPA]

What type of f18a? Maybe it has become a little loose in the vdp socket.

 

Does just ti basic screw up? What about a normal cartridge like XB or EA

[+retroclouds]

1 minute ago, Gary from OPA said:

What type of f18a? Maybe it has become a little loose in the vdp socket.

 

Does just ti basic screw up? What about a normal cartridge like XB or EA

Mainly TI Basic. Also gave parsec in the strangecart a try and it works but the parsec sprites are not acting properly and patterns are scrambled.

I did not try XB or EA yet. Will give that a try tomorrow.

[+retroclouds]

One idication, is that after typing a command in TI Basic and doing enter “6” appears above the TI prompt. This is reproducable behaviour.

[Gary from OPA]

2 hours ago, retroclouds said:

One idication, is that after typing a command in TI Basic and doing enter “6” appears above the TI prompt. This is reproducable behaviour.

do you still have your old TMS9918 VDP chip, I would remove the F18A and place it back in, and make sure the console is working ok. Then you narrow down what could be the issue.

 

Try removing the 3 grom chips and placing them in, after cleaning the pins, that can be issue as well.

 

I still leaning towards the f18a is loose in the socket, not making contact fully.

[WhataKowinkydink]

7 hours ago, retroclouds said:

But here is the catch, it has an F18a onboard so my assumption is that the still mounted VRAM memory chips are not used anyway. Is that assumption correct?

Yes

[Tursi]

It's pretty easy for the F18A to come loose if the machine gets bumped, so yeah, that really should be your first check. Behavior seems a bit odd, but maybe its making intermittent contact.

 

[WhataKowinkydink]

1 hour ago, Tursi said:

It's pretty easy for the F18A to come loose if the machine gets bumped, so yeah, that really should be your first check. Behavior seems a bit odd, but maybe its making intermittent contact

Hopefully it's something that simple and Retro can get back onto what he was going to do in the first place.

[+retroclouds]

Here's a short video of the behaviour. In there I try a short program in:

 

1. TI-Basic 

2. Insert TI Extended Basic cartridge and repeat

3. Use the FinalGROM to load TI Extended Basic and try again

 

All of this is done without the PEB box being connected.

 

IMG_1318.MOV

 

All of the above is repeatable behaviour. I am confused 🙃

 

EDIT: If I leave the console powered on long enough, everything starts to work normally

[Hans23]

The console's DRAM is not used when the F18A is in use.  From looking at your movie, it appears that the problem occurs when the CPU writes multiple characters from BASIC to the screen.  What's interesting is that this only seems to happen with BASIC, as other programs seem to have no issues writing to the screen.  I would also try replacing the F18A with the original VDP or another F18A first.  Given that the problem goes away when the machine is warm, using some cold spray to cool down chips may help pinpoint the problem.

 

I would not be surprised if the issue is with one of the TTL chips that drive the 8 bit bus, given that the machine completely fails when the PEB is connected.  Still puzzling that only BASIC exposes the issue, though.

[+arcadeshopper]

If it only does it with the peb connected..

 

remove all the cards except the flex cable..test

if that works without issue

add each card one at a time.. test

 

makes me wonder if your power supply (internal) is reaching it's eol?

 

Greg

[Asmusr]

1 hour ago, arcadeshopper said:

If it only does it with the peb connected.

4 hours ago, retroclouds said:

All of this is done without the PEB box being connected.

 

[+arcadeshopper]

3 hours ago, Asmusr said:

some stuff

gotcha..sorry  anyway power supply or cpu is getting errors on the bus 

 

 

[Gary from OPA]

Power supply is good bet. Should open it up and test the voltages and also look over the motherboard in regard to some caps that could be dying.