Console Repair - steady beep on power-on

[Cory]

I know this topic has been covered multiple times, but was hoping to get some additional insight from the HW gurus around here.  

 

Symptom:  Console that produces the steady beep and no video on power up.  

 

So far, I have desoldered, removed, and tested the following chips on my known good test motherboard:

 

1) VDP - Good

2) 9901 - Good

3) 9900 - Good

4) Groms - Good

5) Sound Chip - Good

6) 2532 ROMS 1, 2 - Good

7) 8K 1, 2 - Good

8) Signal Generator - Good

9) Crystal - Good

 

So I've pulled out and validated all the main chips that normally are the culprits that cause the issue.  Now, I'm looking for what's next.  Is there a critical buffer or other logic chip that I should test?  I'm planning on re-watching a few YouTube videos out there that can help me use my scope to figure out where the issue is, but the two or three videos I know of found a bad ROM or RAM chips and those are known good on my board.

 

Any thoughts on next steps?

 

[Cory]

Referring to Matthew180's comments on this thread:

I suppose the next steps would be to test out the DRAM.  Can I use a scope to checkout the DRAM before going through the hassle of desoldering each of these chips?  The solder looks pretty oxidized and those can be such a pain to desolder.  But if I must I must.  I will have to do this on my test board.  My test board has sockets for all the chips listed above, but I've never had the need to socket the DRAM until now perhaps.

 

Thoughts before I embark on this next step?

[JJB]

Did you check voltages for proper +5V, -5V and +12V? Given the heat the power supply board produces inside the console there is a chance it has fried parts such as caps and voltage regulators.

Edited June 15, 2022 by JJB
clarity

[matthew180]

 

Specifically the 99/4A Servicing Manual, it has troubleshooting flowcharts.  This is the manual created by and for the TI technicians repairing 99/4A consoles.

[Cory]

19 minutes ago, matthew180 said:

 

Specifically the 99/4A Servicing Manual, it has troubleshooting flowcharts.  This is the manual created by and for the TI technicians repairing 99/4A consoles.

I’ll download it.  Maybe I can get a job with TI after I’ve mastered the process. ??

[Cory]

2 hours ago, JJB said:

Did you check voltages for proper +5V, -5V and +12V? Given the heat it produces inside the console there is a chance of fried parts such as caps and voltage regulators.

I have checked the voltages at the Power Supply, VDP Ram, 9900, 9901, VDP, Grom, and Scratch RAM and all voltage levels look solid.

[AwkwardPotato]

AFAIK bad DRAM can't cause this issue -- if it was the only fault, there wouldn't be a steady beep, and you'd likely get a corrupt title screen.

 

I'd first suspect the 16-to-8-bit multiplexer, it's made up of the three LSTTL parts above the CPU and next to the GROMs (74LS244, '245, '373). If any of those fails (mainly thinking of the '245 here), the CPU will be unable to read from the GROMs and the system will lock up before it ever touches the VDP RAM.