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Post subject: VOX AC30CC weak Master Volume
Posted: Fri Apr 14, 2017 10:21 am
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This is the second VOX AC30CC to come in with this problem. If I’ve seen two in this little backwater, I’m sure there are more out there.
It’s not often I see a systemic fault from a factory. It just happens to be Chinese this time.
The symptom is plain to see. The output is weak and variable. The Master control only works a little. You can turn it up, but little happens past 1 or 2.
The problem is a conductive residue on the control board from improper cleaning after it went through the flow-solder machine. The machines run batches of 50 to 100 boards at a time.
The first one I ran across I didn’t understand what was going on. I thought I was cleaning a problem in the control that wouldn’t go away with cleaning. After messing and cleaning it, it finally worked, but I didn’t really know why.
This time, I snuck up on it. I pulled con 4 and measured pins 5 and 6. They are marked in black in the photo. They connect to the 500K master pot. It would only get up to 256K and variable. I pulled the board and cleaned the backside, with no change. In the picture it’s the closest at the end of the board.
But when I pushed the shaft from side to side, it would go up to 500K. I wiped alcohol on a q-Tip across the rivets on top of the Master pot and the short went away.
There is a crusty residue on the board between components. I can’t really measure the leakage, but cleaned the entire board with lacquer thinner. The Master control is a very sensitive circuit that shorts the grids of the power tubes together. A little leakage between the pins seriously degrades the volume.
So I'm giving y'all a heads up to prevent headaches chasin this one down.
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Post subject: Re: VOX AC30CC weak Master Volume
Posted: Sat Apr 15, 2017 1:14 am
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Good to know TimsAudio, thank's


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Post subject: Re: VOX AC30CC weak Master Volume
Posted: Sat Apr 15, 2017 1:19 pm
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Thanks for kickin' this down TimsAudio.

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Post subject: Re: VOX AC30CC weak Master Volume
Posted: Sat Apr 15, 2017 11:23 pm
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Good detective work! Isn't it amazing when you look-&-look for a problem --- and finally find it in the most obscure places?

Pot rivets. Amazing.

:)


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Post subject: Re: VOX AC30CC weak Master Volume
Posted: Sun Apr 16, 2017 10:10 am
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It was truly a ghost in the machine.
I wonder if it was like that coming off the showroom floor or if it develops over time.
Its not a new unit. Tubes have been replaced on it.


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Post subject: Re: VOX AC30CC weak Master Volume
Posted: Sun Apr 16, 2017 10:54 am
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TimsAudio wrote:
I wonder if it was like that coming off the showroom floor or if it develops over time.


Vox never showed me squat.

An acquaintance of mine owned an AC30 for some five years and the reverb had never worked from the day that he bought it new from Guitar Circus during a "scratch and dent" sale. After a brief inspection I discovered that the "in" and "out" jacks for the reverb tank had NEVER been wired. Five minutes' work with my Weller soldering station had him happier than a hog in a cesspool. Great QC, that bunch in China!

:roll:

Arjay

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Post subject: Re: VOX AC30CC weak Master Volume
Posted: Tue Apr 18, 2017 9:25 pm
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Vox and Vox are two very different things.

I had an AC-15 for a while, and to be honest, it was just meh. No character.
But then I tried a British handwired AC-4, and it was just very sweet. Part of that is likely the simpler design of the true class A, but I suspect a little attention to detail also goes a long way.

Similarly with Laney. The first one I tried, I was thinking "What? No way Toni Iommi would play this [insert expletive]". Then I tried a British made Laney Lionheart, and changed my mind. Of course, it was twice the price for fewer watts, and you need a Sumo wrestler to carry it for you, but so much better. If you want British blues sound these days, don't get a Bluesbreaker, get a Lionheart.

And again, same with Fender, although to be honest, I don't think price always reflects the value with Fender. There are a couple of expensive Fender amps that play strangely, to put it politely.


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