CV Flickering Garbage Screen

[jeff12345]

Hey,

after my last attempt to fix a CV (https://atariage.com/forums/topic/309666-colecovision-eating-vram-chips/), I had to pause the project for a while. Now I wanted to continue. The first time I plugged it in, it seemed to work nicely and I played my first round of Cosmic Avenger  But then the screen garbled up more and more, and now it's garbage directly after turning it on (even after letting it cool down for a night).

Other than in my previous problems, this does not look like a static memory problem anymore (both videos show what should be the static BIOS screen; no game inserted):
https://streamable.com/k9rbat
https://streamable.com/49y0ry

To make sure it's no a problem with the power supply, I bridged the switch and hooked up the CV to my lab PSU. I find that there is still a pretty significant voltage drop on the 5V rail when turned on:
OFF: 5.0V / 12.2V
ON: 4.5V (0.69A) / 12.1V (0.07A)

I checked components for heat with my finger, but the only thing that gets kind of hot is the VDP (not too bad though). I tried cooling it with a solid chunk of metal I put in the freezer for an hour, to see if there's maybe some heat-related issue, but it didn't seem to make any difference at all. Didn't see any reaction when adding/removing the cool metal while the CV was running.

Does anyone have an idea what I could check next? 

[+-^CrossBow^-]

Are you saying that your VDP didn't have a heatsink on it then? You must have a pretty early revision CV then because I believe there are more of them out there with later factory installed heatsinks on the VDP than without. 

 

The videos that you have linked to.... what exactly is supposed to be on the screens? What does it look like when you just turn on the CV without a game inserted?

 

 

[jeff12345]

51 minutes ago, -^CrossBow^- said:

Are you saying that your VDP didn't have a heatsink on it then? You must have a pretty early revision CV then because I believe there are more of them out there with later factory installed heatsinks on the VDP than without. 

 

The videos that you have linked to.... what exactly is supposed to be on the screens? What does it look like when you just turn on the CV without a game inserted?

 

 

It had one on it. I removed it at some point because it made it hard to measure the pins, etc. Console was never turned on for more than 1-2 minutes since I removed it though.

The videos I linked to are the BIOS "Insert cartridge" screen which appears when no game is inserted.

Edited August 22, 2020 by jeff12345

[ChildOfCv]

18 hours ago, jeff12345 said:

To make sure it's no a problem with the power supply, I bridged the switch and hooked up the CV to my lab PSU. I find that there is still a pretty significant voltage drop on the 5V rail when turned on:
OFF: 5.0V / 12.2V
ON: 4.5V (0.69A) / 12.1V (0.07A)

Your PSU loses voltage when you turn the CV on?  Those things are supposed to be better regulated than standard power supplies.

 

Or do you mean that there is significant voltage drop between the power connection and some point on the board?

[jeff12345]

59 minutes ago, ChildOfCv said:

Your PSU loses voltage when you turn the CV on?  Those things are supposed to be better regulated than standard power supplies.

 

Or do you mean that there is significant voltage drop between the power connection and some point on the board?

The voltage exactly at the PSU stays at 5.0V. I measure ~4.6V at the L6 choke next to the switch and ~4.5V at PIN9 of the 4164 chips.

The first measurement already confuses me a bit, because there is a more or less direct connection between PSU and the L6 coil...

[jeff12345]

I just desoldered and socketed the VDP. Without the VDP, the voltage at pin 9 of the RAM chips is ~4.8V, with the VDP inserted it's dropping to ~4.5V. That's not a good sign I fear...

[ChildOfCv]

What's the amp draw without the VDP?  The data sheet says it will draw an average of 250mA.
 

Where/how did you connect the PSU?  I suspect oxidation on the board/pins, so you may need to really clean your contact point, or maybe solder a wire to each power input point so that you can be sure to have a good wire connection.

[jeff12345]

I connected the positive lines to the corresponding chokes (even put some solder on the pin to remove oxidation, but didn't change anything) and GND to the ground rail via a soldered wire at the side. The effects were fairly similar with the original PSU connected via the official plug the last time I checked.

 

Voltage measurement at pin 9 of the VRAM chips, amp draw according to PSU:

With VDP: ~4.5V / ~0.7A
Without: ~4.8V / ~0.4A

 

 

[ChildOfCv]

So the amp difference is within bounds.  Try to follow the power supply lines up to the VDP area and see if there is an obvious via or through-hole solder joint where the voltage drops below tolerance.  Any TTL compatible chips want a minimum of 4.75V for guaranteed function.

[jeff12345]

9 hours ago, ChildOfCv said:

So the amp difference is within bounds.  Try to follow the power supply lines up to the VDP area and see if there is an obvious via or through-hole solder joint where the voltage drops below tolerance.  Any TTL compatible chips want a minimum of 4.75V for guaranteed function.

I tried to do that, but actually, the voltage drop from entry point to VDP area is not a lot (~0.05V). Rather the whole rail drops from ~4.8V to ~4.5V when the VDP is inserted (measured directly at the chokes).

[ChildOfCv]

So, you're saying that the power supply outputs 5V, but as soon as it reaches the board it's 4.5?  That means your clip wires are the source of the drop.

[jeff12345]

Yeah, I probably messed up something in the connection (had a switch inside there, etc). I tested this without the CV connected and it looked fine, but apparently I missed something. I removed all that stuff now to make the measurements easier, and now the voltages do actually look better. Unfortunately the screen is still exactly as in the videos uploaded above...

Here's the measurement photo story, all with CV turned on 
 

5.1V directly at the PSU:

 

5.1 directly after the cords:

 

4.9V at the board entry point:

 

4.8V at the VDP area: