Green Wagon Posted June 18, 2016 Share Posted June 18, 2016 WEEEEEEEEEEEEEEEEEEEEEEEE Here we go again.. Short version. Guy owns 7700 for decades. ~10 years ago juice was spilt into it. Takes it to a well know gent in the valley (brisbane) for clean up. He works on it, says, here, works now. All goes well for 10 years before one channel starts crackling, Original guy is no longer, takes it to another place in the valley. They charge him a quote fee and say, "Nah mate, shes full of corrosion, but we have this other second hand unit you can buy from us". Guy comes to me. Can it be fixed or is it time for the last rights. I open it up, have a look. um, what corrosion where ???? I can see its had a bulk load of small transistors replaced, copious amounts of ? lube ? sprayed into the switches. Minor bit of discoloration on some of the solder mask, And thats all. Wait, what the ??????? Why is the bottom panel on upside down ? On a general look over it's filthy and still has the original caps. Time to strip it to the ground...... And why yes Mr and Mrs eagle eyes, those are fuse holders on the input switch shafts. Now don't ask me, I don't know why. But I left them in place. Maybe one of the Sui wonder techs can explain why. Yes, it's had the odd part here and there replaced. Like the one output transistor, and a few other little bits here and there. Pull the amp board. Pull the input board (easy as the bracket screws were missing - Rolls eyes - ) Theres a hole in my amp dear martha dear martha, theres a hole in my amp dear martha a hole Hm, time to get the dremel and wire wheel into that. See if its just the mask or more. Turned out to be mostly harmless, but I did need to fix a few smaller tracks for long term self satisfaction. Yeah I think I'll just replace that tower resistor . I mean not like it's been arcing against the diode frame at all. Lovely clean board. Not. Nah those caps are still perfectly good for another 40 years. Wonder where those caps came from. Took two days, multiple solder sessions, and two cans of flux clean to get the amp and input boards done. Now to start on the front boards. Pull the knobs and front panel, remove screws, bit of jiggling and presto. Out come the next boards to work on. So obviously, previous missed a transistor and did a chop n swap rather than pulling the board out agin. Fine, I'll fix it properly. But, Why, would you pull it all apart, replace all the transistors, yet leave the (then) 30 year old caps ?? Ok sure, he replaced ONE cap. So now, it's all clean. So many cotton buds, so much chemical, so many hours of scrubbing and cleaning. All new caps and trimpots, one resistor, bit of wire and touch up on some tracks, full resolder. Oh so many bad joints. Full resolder was the only way to be sure. Adjustment in progress. Two meters can make life so much easier. Final bench test with signal and load. Gave the front panel and knobs a scrub while they were off. Another Sui owes it's life to me. Ungrateful little @#$%^ 21 Link to comment Share on other sites More sharing options...
Equaliser Posted June 25, 2016 Share Posted June 25, 2016 That is quite filthy, personally I would've just thrown the boards into a bucket of warm soapy water and let it degrease the dirty bottom traces, dust particles stuck on top of the pcb. I'm sure some of those resistors would be out of tolerance by now as well. Link to comment Share on other sites More sharing options...
Green Wagon Posted June 25, 2016 Author Share Posted June 25, 2016 Yes it was. I put it down to being ~40 years old, and the residue of whatever spray lube was used previously. I didn't even think of buckets etc. Prob would've saved me some time. Oh well theres always the next one It all measured ok. Bias and offset came up fine quickly and stable. So if any are out of spec, I would imagine their not that far out. Link to comment Share on other sites More sharing options...
Equaliser Posted June 25, 2016 Share Posted June 25, 2016 Maybe but I've been fooled before by measuring 30 year old + resistors in and out of circuits on the DMM that was within its whatever tolerance rating, it wasn't until the amplifier was under some load the resistors created problems for other close components etc. The bucket of warm soapy water will save you the effort going through the board with chemicals and the warm soapy water method also cleans it more thorough especially hard to reach areas, filling every nook and cranny. Link to comment Share on other sites More sharing options...
Green Wagon Posted June 26, 2016 Author Share Posted June 26, 2016 Cool. Two new things to try and remember for future reference. Ta Link to comment Share on other sites More sharing options...
skippy124 Posted June 26, 2016 Share Posted June 26, 2016 Nice work as always Nigel Re de greasing the dirty traces, I don't think the bucket of water would work in this case. The tracks underneath the green solder resist have started to oxidise, and the only way to fix these/prevent future corrosion is to remove the resist as Nigel has done and clean the tracks. Once they are clean of all contaminants and copper oxide, then a spray with acrylic circuit board protector will prevent further corrosion. I have had units with "upside down" PCA's where the complete track has turned to copper oxide - there was no track left after removing the resist and the oxidised copper. Cheers John 1 Link to comment Share on other sites More sharing options...
Equaliser Posted June 26, 2016 Share Posted June 26, 2016 (edited) Good points though it depends on the level of oxdisation thats occurred on the copper traces. I usually use a toothbrush to scrub the traces and they usually come off but my methodology would be to soak the pcb first which primarily cleans off the grease and dust build up followed by toothbrush scrubbing and a final 100% grade IPA cleanup. Edited June 27, 2016 by liteneasy Link to comment Share on other sites More sharing options...
Green Wagon Posted June 27, 2016 Author Share Posted June 27, 2016 Nice work as always Nigel Re de greasing the dirty traces, I don't think the bucket of water would work in this case. The tracks underneath the green solder resist have started to oxidise, and the only way to fix these/prevent future corrosion is to remove the resist as Nigel has done and clean the tracks. Once they are clean of all contaminants and copper oxide, then a spray with acrylic circuit board protector will prevent further corrosion. I have had units with "upside down" PCA's where the complete track has turned to copper oxide - there was no track left after removing the resist and the oxidised copper. Cheers John Thanks for the kind words. Coming from you I consider them high praise indeed. Good points though it depends on the level of oxdisation thats occurred on the copper traces. I usually use a toothbrush to scrub the traces and they usually come off but my methodology would be to soak the pcb first which primarily cleans off the grease and dust build up followed by toothbrush scrubbing and a final 100% grade IPA cleanup. How about a bucket of acid or alkali ? I'm sure that would remove a lot problems The input/pre board was the dirtiest with gummy residue. It was just that last bit of the underside of the amp board sticking out that had the 'corrosion'. I suspect it was from the juice spill. (acidic). Next shopping spree I have I should invest in some of those fiberglass pens. Should be a damn site gentler on the board/tracks than a dremel with wire wheel. Link to comment Share on other sites More sharing options...
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