Excess Solder Flux Phenomonon Again...

Paul Marossy · August 20, 2005, 02:27:07 PM

Maybe a little OT, but...

This has got to be the world's easiest tube amp repair ever! A friend of mine has a Mesa Boogie Nomad 45. It is a three channel amp and it was having trouble switching ever since he had it. It couldn't make up it's mind when you wanted to switch between channels 1 & 2 - the relay would just flip back and forth and try to switch between one channel or the other, never really switching as intended.

My intuition told me to start with the footswitch itself. I took it apart and took a look at the PCB. The first thing that I noticed is that there was a large amount of flux around all five of the connections at the LEDs. I got some alcohol and cleaned off the excess flux from the entire board. Then I turned the amp on and gave it a try, and now it switches perfectly!

It seems really weird that this could cause a problem like that, but this isn't the first time I have encountered this phenomonon where excess flux has caused a problem like this. Go figure. :?

The Tone God · August 20, 2005, 02:38:47 PM

Most flux is hydrophillic. After some time and exposure the flux can become conductive. This can also happen if you use certain chemicals (i.e. solvants) on flux.

I recently had a Mesa Rect-O-Verb in the shop. It kept blowing fuses. I popped it open to take a look see and I saw in the powersupply was a small cap in parallel with the larger filter caps with a big hole in it and carbon marks around the part. Replaced the cap with a new higher voltage cap and sucker fired up. I love easy jobs.

Andrew

Paul Marossy · August 20, 2005, 03:58:19 PM

Hmm... that's interesting. It's also a strange coincidence that yours was also a Mesa Boogie.

Peter Snowberg · August 20, 2005, 04:16:25 PM

I've seen problems in lots of CMOS digital circuits because the output drive for most gates is just sooooooooo tiny and the pull-up resistors are generally rather large. It's easy for a little soldering crud to stop things quickly.

The phone system at work was going bonkers a couple of months back. It's in the basement next to a couple of gas dryers and a washing machine. :? I pulled out one of the cards and saw a bunch of uncleaned flux around one chips that had been replaced. A little 99% isopropyl alcohol on a toothbrush and it was fixed.

The Tone God · August 20, 2005, 04:23:15 PM

Quote from: Paul MarossyHmm... that's interesting. It's also a strange coincidence that yours was also a Mesa Boogie.

Not really. I have a fair amount of Mesa gear coming through the shop. When the cap blew it went to short so it was effectively shorting out the B+ right on the transformer winding so hence why it was blowing fuses. The amp had an incorrect fuse in it (too high) so the amp probably took a surge hit and blew the cap. That and I think the stock cap was too close to it's max rating during normal operations anyways.

I have some "opinions" on Mesa. :evil:

Andrew

ninoman123 · August 20, 2005, 05:10:08 PM

Same thing happened to me. My Marshall Valvestate head wasnt working. I cleaned up the boards and resoldered the output jacks and it worked again.

Paul Marossy · August 20, 2005, 10:22:12 PM

Interesting stuff - I guess this happens a lot more than I previously realized! :shock:

Paul Marossy · August 22, 2005, 12:18:55 PM

OK, one last muse. If solder flux is hydrophilic, that would mean that it will attract & trap water and that is why it becomes conductive (I assume). The funny thing is that I live in the desert where the humidity is typically between 15-20%. And, on top of that, the solder flux was pretty hardened. That's what makes this whole thing rather perplexing...

sir_modulus · August 22, 2005, 01:20:31 PM

All the crap in the air can also be conductive. I've had an old stereo that had so much crap on it that the noise levels were through the roof, and a thorough cleaning revealed the real Hi-Fi amp inside. I've only recently learned the I should be cleaning the flux off of my boards, and have gotten two old projects to work really well now.

Cheers,

Nish

Paul Marossy · August 22, 2005, 01:36:31 PM

Quote from: sir_modulusAll the crap in the air can also be conductive. I've had an old stereo that had so much crap on it that the noise levels were through the roof, and a thorough cleaning revealed the real Hi-Fi amp inside. I've only recently learned the I should be cleaning the flux off of my boards, and have gotten two old projects to work really well now.

Cheers,

Nish

This thing with my friend's amp makes me want to clean every PCB that I ever made! Well, I guess if a pedal I made doesn't work properly all of the sudden for no good reason, I can clean off the solder flux and then it will probably work! :lol:

sir_modulus · August 22, 2005, 02:15:58 PM

That's basically my case exactly. I built a ross phaser (with mark hammer's INGENIOUS mods), and it just stopped working (unboxed and was left that way for about a year before testing... :oops: ) so I just cleaned it off and kaching!

http://www.mgchemicals.com/ These guys are great for any electronics related chemicals. I get their flux remover now, and it works wonders!

Cheers,

Nish

Paul Marossy · August 22, 2005, 03:11:43 PM

Wow. I never really thought about this subject much. I guess I need to pay more attention to it in the future! :lol:

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Excess Solder Flux Phenomonon Again...