OH no this amp is so clean!!

[nsubulysses]

Oh no I have received a SUnn Concert Bass and it's so clean and appears to be mint, but when you look inside the PCBs seem to be dissolving.

The tolex on the enclosure has been sprayed with some type of automotive cleaner or armor all or something. It is very, very slick, clean, and dark black. I guess this is why the PCBs are dissolving. At this point it works but sounds very bass-less and weak. Even if I put lots of time into re-soldering and fixing this thing up, what are the odds of it being reliable, if anyone would wager a guess???

[pdf64]

Isn't it just that the solder resist coating on the PCB copper side is degrading?
As long as the pcb base material is still insulating, it may not be a problem.

[The Dude]

Isn't it just that the solder resist coating on the PCB copper side is degrading?
As long as the pcb base material is still insulating, it may not be a problem.

Agreed. I wouldn't worry about it, even a little bit.

[52 Bill]

Isn't it just that the solder resist coating on the PCB copper side is degrading?
As long as the pcb base material is still insulating, it may not be a problem.

I think that he is referring to the wrinkled copper foil on the boards. I think that is caused from overheating the boards when wave or dip soldering. I've seen it before on older stuff even when it was new.

[nsubulysses]

Well, it seems like the section with resistant coating is flaking off because the whole ground plane is bubbling and flexing. It made me wonder if other traces on the board would bubble and flex and eventually break.

Also in that section the solder has somehow evaporated away from the component leads. They are just sitting in the PCB, not soldered in anymore. How is that possible? If this is a production era as Bill said that would mean it's been that way, probably performing poorly since 1975?

[52 Bill]

Well, it seems like the section with resistant coating is flaking off because the whole ground plane is bubbling and flexing. It made me wonder if other traces on the board would bubble and flex and eventually break.

Also in that section the solder has somehow evaporated away from the component leads. They are just sitting in the PCB, not soldered in anymore. How is that possible? If this is a production era as Bill said that would mean it's been that way, probably performing poorly since 1975?

Is the copper foil loose and flexing?

I can't think of anything that would evaporate solder, but I suppose anything is possible. I've never seen anything like that before.

[tedmich]

Many solvents will remove the solder mask, or indeed it can flake off with age, but it would need to be an acid to dissolve solder, usually HCl (muriatic) but I think it turns dark not all shiny...

I'd resolder and then overlay with acrylic lacquer, which is a pretty good mask but easily removable with solvents. I've seen wrinkly copper on old PCBs pretty frequently but they seem just fine and have intact solder joints. I think it may have to do with poor substrate material and or bad copper adhesion/epoxy.

[loudthud]

Well, it seems like the section with resistant coating is flaking off because the whole ground plane is bubbling and flexing. It made me wonder if other traces on the board would bubble and flex and eventually break.

That looks like the copper was solder plated under the solder mask and under the heat of wave soldering, it re-flowed.

Also in that section the solder has somehow evaporated away from the component leads. They are just sitting in the PCB, not soldered in anymore. How is that possible? If this is a production era as Bill said that would mean it's been that way, probably performing poorly since 1975?

That looks like an attempted repair. Those connections need to be touched up.

[Enzo]

Solder and copper don't dissolve. I see nothing other than the solder mask coating flaking off.

[J M Fahey]

I have seen many such boards, no big deal, very popular in the 80's.
Yes, those are pretinned boards where later solder mask was silkscreened.
Ugly but functional.
Example: the blue G&K PCBs, and many home studio processors (compressors, etc.) .
here's a green one,where it's not that noticeable, but still visible on the wider diagonal tracks:


I remember when a "new - improved" technique was introduced: SMOBC : solder mask over bare copper ... so called because before copper was not bare but tinned:

Re: Solder Mask over Bare Copper (SMOBC) question

Many years ago you got the solder mask applied over the tin/lead coating used for etch resist, this would re-flow causing bumps etc .... then they decieded to put the solder mask over bare copper to combat this .... Again I've always called it solder mask... This term is rather historical these days, the term and practice of putting solder mask over plated copper went out soon after SMD became more popular .... Once the board was etched the electroplated tin/lead coating was fused, or stated differently was reflowed. It was after this point in the process when the soldermask was applied to the board. This was the standard board process for plated through hole technology. ...... later the board fabrication process was changed to remove the tin/lead etch resist materials and leave the board simply as a copper board.

The soldermask was then applied over the bare copper, which became known as SMOBC, Soldermask over bard copper.

[nsubulysses]

This amp is all greasy. I am glad to hear it is not dissolving.

I think tedmich is right to say, "it may have to do with poor substrate material and or bad copper adhesion/epoxy."

I will proceed with the repair thanks everyone

[MarkusBass]

There is nothing wrong with the board. As Juan wrote, this is tin melted under the silkscreen paint during automatic soldering process. I also saw this many times.
Just fix it and don't worry about how it looks like.

Mark

[gbono]

[ATTACH=CONFIG]34949[/ATTACH]
[ATTACH=CONFIG]34950[/ATTACH]

At this point it works but sounds very bass-less and weak.

Are you getting the rated output level from the amp at 4 ohms?

[nsubulysses]

Yeah if you give it a decent enough (big) input signal you can get abbot 140W out of it into 4 ohms. Through a speaker it sounds terrible though.

[guitician]

If that slick stuff was silicone based and it's penetrated into components it may have caused some damage, maybe to the electrolytic caps.