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Author Topic: Lab Series L5 issues... (recording included!)  (Read 3471 times)

kvandekrol

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Lab Series L5 issues... (recording included!)
« on: July 03, 2010, 06:13:48 PM »
OK. I've posted on here a couple of times before about a Lab Series L5 that's been giving me problems. I worked on it off and on for about six months without really making any progress, and finally about a year ago I gave up and put down my soldering iron, and just used the amp as an external cabinet.

But it's not in my nature to just let broken things sit around, and a week ago I was doing some unrelated soldering when I decided to have another go at it.

The good news: I fixed one of the problems, an inexplicable constant crackling sound that was unaffected by the master volume control. I replaced CR101 and CR102 (1N4004 diodes) and it went away. Very cool.

But there was still another problem, a bad compressor that made a loud popping sound every time it kicked on (i.e. when the signal went over the compressor's set threshold). That was detailed in this thread from last year. (I said at the end that it was fixed, but the problem came back about a day after I posted that, which was right about the time I gave up on it.)

All symptoms pointed to the power supply, since I replaced just about every part in the compressor's circuit on the preamp board and nothing changed, plus the LED flickers off when the compressor comes on, like there's insufficient power to supply both the LED and the compressor itself. So I replaced all the 1N4004 diodes on the power supply (couldn't hurt, and I had a bunch of spares), and I strengthened a torn trace coming out of one of the TIP30 transistors. (I had fixed it before, but the two components were only connected by solder, so I put a copper wire over the trace between the parts.)

That's all I did. But I somehow created more problems. The amp makes a loud crack when it first turns on, then it makes a steady, rhythmic popping sound all the time that it's on. And when I turn it off, it makes a very loud pop. I've only turned it on and off three times since messing with the power supply board, but it seems to me that this pop has gotten louder each time. I'm now pretty concerned about turning it back on again at all.

Anyway, I made a recording of it this last time I turned it on, which I've attached to this post as a zip file. This was recorded using an iPod, and it was a bit too close to the speaker, so the crack when it turns on and the crack when it turns off are heavily clipped, but the steady pop is clear.

Isolation details: The cracks when the amp turns on and off are unaffected by the master volume. Volume was at 0 when the amp turned on, then I raised it to listen to the rhythmic popping, then I turned it back to 0 before shutting it off. The rhythmic popping sound is affected by the master volume, but not by the channel volume of either side. Both of these were at 0, and I just raised and lowered the master volume for the sound recording. The loudest I turned it was about 3.

WHAT IN THE WORLD IS WRONG WITH THIS AMP? I am pretty good with a soldering iron so I don't think it's due to a bad soldering job anywhere. I very meticulously researched replacement parts and only used the exact part number, never even an equivalent, and I was very careful that every replacement went in the same way its predecessor came out. I took plenty of pictures before I removed any parts to ensure this. This is just baffling, as well as frustrating, because I feel like every time I do something, something else goes wrong.

For reference: board layout and schematic can be found here -
http://www.netads.com/~meo/Guitar/Amps/Lab/schem.html
« Last Edit: July 03, 2010, 08:45:27 PM by kvandekrol »

kvandekrol

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Re: Lab Series L5 issues... (recording included!)
« Reply #1 on: August 05, 2010, 01:57:35 PM »
So I found out the problem while working on another unrelated project:

Conductive flux.

I had just switched brands after running out of my Radio Shack water-soluble flux. I was at Home Depot that day and decided to pick some up from there. After I modded a Boss SD-1, some crazy stuff started happening that made me doubt my skill with a soldering iron. It took me a couple of days to track down the problem, but I learned the magic of an audio probe along the way. Turns out I had some leftover flux that was passing signal between unrelated solder points, and when I cleaned up the flux all the problems went away.

So I took a Q-tip to the power supply board on the Lab amp, then flipped it on to test it out, and it was back to normal. (Compressor's still broken of course...) I didn't do a sloppy job with the flux but I guess there was just enough left over to cause the crazy problems I was having.

Anyway, tuck that one away: Home Depot's water-soluble flux is conductive. I'd love to meet the guy who came up with that formula.

joecool85

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Re: Lab Series L5 issues... (recording included!)
« Reply #2 on: August 05, 2010, 02:06:04 PM »
So I found out the problem while working on another unrelated project:

Conductive flux.

I had just switched brands after running out of my Radio Shack water-soluble flux. I was at Home Depot that day and decided to pick some up from there. After I modded a Boss SD-1, some crazy stuff started happening that made me doubt my skill with a soldering iron. It took me a couple of days to track down the problem, but I learned the magic of an audio probe along the way. Turns out I had some leftover flux that was passing signal between unrelated solder points, and when I cleaned up the flux all the problems went away.

So I took a Q-tip to the power supply board on the Lab amp, then flipped it on to test it out, and it was back to normal. (Compressor's still broken of course...) I didn't do a sloppy job with the flux but I guess there was just enough left over to cause the crazy problems I was having.

Anyway, tuck that one away: Home Depot's water-soluble flux is conductive. I'd love to meet the guy who came up with that formula.

Good to know you got it figured out.
Life is what you make it.
Still rockin' the Dean Markley K-20X
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J M Fahey

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Re: Lab Series L5 issues... (recording included!)
« Reply #3 on: August 05, 2010, 08:17:12 PM »
Electronics solder flux and water soluble do not come together in my dictionary.
I still use old reliable pine tree rosin in alcohol.