Area 51 - isolating / repairing surface mount issue?

DarrenF

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Area 51 - isolating / repairing surface mount issue?

I've got an Area 51. HDD is good. It boots up, all self-tests OK, and even plays fine. Only problem is the colors are wrong. Then I lightly press on the PCB... the colors change. Pressing a little harder, and suddently they're correct. Release... back to wrong colors.

I'm assuming it's a cracked surface mount joint... somewhere. Are there any tricks of the trade to isolating the location of the bad joint(s)? I can only narrow it down to about half of the board, by carefully determining where flexing matters & doesn't. Will it likely be visalbe? (i.e. should I spend an hour or two visually inspecting with magnification, or is that a waste of effort?) Can I just put it in the oven to re-flow everything?

SMT is kinda new to me... I'm normally more of an 80s PCB guy.
 
No on oven. You'll cook everything on the board by the time you get it hot enough to reflow things. Also, chips have to be slowly heated up and set at a lower temperature for 2 to 3 hours to drive any moisture out of the body of the ICs. (Yes, they get moisture migrating up the legs over time...)

If you don't cook 'em enough to get the moisture out then you'll popcorn the ICs. In other words, you'll hear a loud POP or Crack and the chip will give up the ghost in a small burst of steam - just like popcorn popping from the moisture inside it.

Use liquid rosin flux and a steady hand with an iron that has a small bit of solder on the tip and you can reflow them by hand. Use more flux and a clean tip to draw any solder bridges off the chip legs and onto the iron tip.

As for telling what chip is the issue, you can try magnification but that doesn't always let you see the problem. Take the boards off any metal mounting plate, set them on the bench and start tapping/pressing down on the chips until you isolate the issue.
 
Yeah, I was half-kidding about the oven thing. I didn't really think it was a viable option. Thanks for driving it home.

I'm not TOO scared of the actual re-flowing of the joints. (I'll get my wife to do it, she has steadier hands for uber-fine work). I'm more concerned with not wanting to just randomly resoldering stuff. I'll pull it off the metal plate and try pressing/tapping tomorrow. I'd really like to FIND the problem before attempting to fix it... Very incomplete schematics in the manual don't help, either.

Thanks for the advice!
 
Take the boards off any metal mounting plate, set them on the bench and start tapping/pressing down on the chips until you isolate the issue.

That seemed to do the trick. Certainly able to control the flexing of the board more sitting flat on the bench vice on the posts. Surprisingly, the issue did NOT turn out to be anything surface mount(!). I traced the issue to the (thru-hole) "blue & white block" resistor networks in the color DAC circuits. This is one of the few things that IS covered in the limited schematics in the manual. In any case, I resoldered the end pins, and it's MUCH better. Still not 100% immune to flexing, so I think I'm going to re-solder all 10 pins of all 3 of the SIPs.

Thanks again.
 
I traced the issue to the (thru-hole) "blue & white block" resistor networks in the color DAC circuits. This is one of the few things that IS covered in the limited schematics in the manual. In any case, I resoldered the end pins, and it's MUCH better. Still not 100% immune to flexing, so I think I'm going to re-solder all 10 pins of all 3 of the SIPs.

A (hopefully final) update to this repair:

Re-soldering all 10 pins of all 3 SIPs didn't fix the problem. So I removed one of the SIPs. There was what appeared to be corrosion where several of the pins went into the body of the SIP. I flux'd it, de-soldered, and re-soldered as best I could, then re-installed (all 3 of 'em). That seems to have done the trick. No amount of flexing the PCB now causes any color issues.

FWIW, the SIPs in question are designated RN1, RN2 & RN3, and their Atari P/N is 118015-001. They're 10-pin R2R networks with R/2R = 1K/2K. I think I may have found a some Bournes replacements to order, in case this fix doesn't last.
 
I'm sure I have some that I've pulled from other boards... if you need 'em in the future just PM me and I'll dig.
 
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