Weird Takky Color Classic Issue

[Kevinlenane]

Hi I have a strange issue with a Color Classic w a Takky mods. Basically it boots when I have it tipped forward but when it's set upright normally it's almost like the screen shorts out after a few seconds. The LED and the screen power up and then immediately shut off. It seems to be related to the whole unit (CRT, logic and analog) settling and shorting. The logic board was just recapped and that seemingly fixed a low power issue - but this startup issue remains unless the unit is leaning forward. Any ideas?

 

[Nixontheknight]

Hi I have a strange issue with a Color Classic w a Takky mods. Basically it boots when I have it tipped forward but when it's set upright normally it's almost like the screen shorts out after a few seconds. The LED and the screen power up and then immediately shut off. It seems to be related to the whole unit (CRT, logic and analog) settling and shorting. The logic board was just recapped and that seemingly fixed a low power issue - but this startup issue remains unless the unit is leaning forward. Any ideas?

might be a broken trace or cracked solder joint on the analog board, try looking for it and fixing it, use a magnifying glass if you can't see it

 

[joshc]

How tight is the analog board sitting in its connector? Hit both ends with contact cleaner and reseat it a few times. Do the same with the logicboard. I’ve been repairing colour classics with @cheesestraws recently and the connectors have been the biggest causes of intermittent issues like this, after bad caps.

 

[cheesestraws]

Agreed - before you go peering at solder joints with a magnifying glass, do yourself a favour and just clean the heck out of all the connectors in the wiring harness.

Quick Q - is this with the back bucket on, or off?

 

[joshc]

Quick Q - is this with the back bucket on, or off?

This is a good question, and I know what cheesestraws is getting at here.

Basically, the grounding points on the logicboard rely on the metal ground loops touching various points of the metal cage under the machine. Depending on how you are testing, if the bucket is removed and the machine standing upright, the metal cage may be touching bits of the logicboard it shouldn't.

It's worth checking that those ground loops are making good contact, but also that the metal shield is not shorting anything on the logicboard as well. All in all, it's a really fault prone design unfortunately.

 

[cheesestraws]

Basically, the grounding points on the logicboard

A slight clarification - on the original logic boad. The little ground contact things in the shielding are in the wrong place for other boards, which is the problem, and the metal is bendy. So if you have it out of the back bucket, when you rest it on the shielding it's absolutely possible that it will just short bits of the logic board out, which probably isn't ideal.

Why, yes, @joshc and I lost a certain amount of time to this, and why, no, we're not bitter at all...

 

[joshc]

Why, yes, @joshc and I lost a certain amount of time to this, and why, no, we're not bitter at all...

Indeed. This + the AB edge connector as well which is just as problematic as the logicboard one. And depending on what is or isn't working, it could be either of them, or both to blame. Fun times indeed.

 

[Kevinlenane]

Thanks all - lots to try here - I did wonder about the metal shielding under the analog board. I removed the shield under the LB. What's really the point of the shielding functionally?

I'm going to nail the connectors on both.

 

[Kevinlenane]

Thanks all - lots to try here - I did wonder about the metal shielding under the analog board. I removed the shield under the LB. What's really the point of the shielding functionally?

I'm going to nail the connectors on both.

Scratch that the shielding is there - I do think it's related to these two shields under each boards given ifs propensity to fail when they'd be closest to them.

 

[cheesestraws]

What's really the point of the shielding functionally?

The shielding has two purposes in the CC - the first is to provide ground references to bits of the PCB. This was probably a bad idea.

The other, more important reason is to try to prevent RF emitted by the computer from screwing around with other electronics in the area, which may include your neighbours'. So it's best to leave it on - but I would suggest bending the little hook-shaped flaps away from the LB. When the back bucket is on, if the takky mod was done even remotely competently, the flat shielding will not short out the LB.

 

[Kevinlenane]

But wasn't the neighbor shielding function kind of debunked?

 

[cheesestraws]

No?

18 Months of Village Broadband Disruption Traced to Old TV Set

Residents in the tiny rural village of Aberhosan (Powys, Wales) have finally seen an end to 18-months of unusually slow and unstable broadband ISP connectivity

www.ispreview.co.uk

 

[Kevinlenane]

But wasn't the neighbor shielding function kind of

No?

18 Months of Village Broadband Disruption Traced to Old TV Set

Residents in the tiny rural village of Aberhosan (Powys, Wales) have finally seen an end to 18-months of unusually slow and unstable broadband ISP connectivity

www.ispreview.co.uk

Wow - looks like I need to reshield a whole bunch of Max IIS and IIFXs

 

[cheesestraws]

I love that news story, it's rare that you see an engineering principle demonstrated quite so conclusively :-D