#1863282 - 12/28/07 02:31 PM
Vox AC-30 needs repair, can't figure out what went wrong
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netdotcom
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Registered: 12/28/07
Posts: 1
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hello,
I was wondering if anyone can help me diagnose a problem I am having with my Vox AC-30tb. It is from the 90's or early 2000. The amp has blue alnico speaker's, no master volume.
Lately, I can't get any sound to come out of it. This happened pretty suddenly. I went away for a weekend and came back to this problem. I'll turn on the power button and let it warm up. In 5 mins, I'll flip the stand-by switch but won't get any sound. Both of the switches light up. Then when I go to turn off the Stand-By switch, the light stays on. Even when I shut off the Power switch, the stand-by light will remain on. Whatever i do, i can't stop this from happening, and I get nothing to the speakers. I suspect it's tubes, but before i spent the money I'd like to find some advice.
I don't know too much about the internals so I was wondering if anyone knew what could cause this? And how i can go about fixing the problem. Thanks in advance for your time.
best,
willie
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#1867038 - 01/05/08 08:00 AM
Re: Vox AC-30 needs repair, can't figure out what went wrong
[Re: netdotcom]
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paully
Platinum Member
Registered: 05/25/04
Posts: 1028
Loc: Northern New Jersey
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Willie,
I could only find one schematic close to your amp's year http://www.schematicheaven.com/voxamps/ac30vrev.pdf . The fact that the sb lamp stays on can mean a few things worth mentioning:
1) a sb switch that is broken, but is fixed in the 'on' position. The amp however should still work.
* How long does it take for you to notice the lamp starting to dim, and how long till it is extinguished completely?
* With the amp 'on' and the sb switch left 'on', if you turn the amp 'off' with the power switch, how long does it take to extinguish?
2) more probable, a main supply point has either broken or become compromised at the power supply(caps). If a voltage dividing resistor in the supply is compromised, that would allow the lamp to remain on, force power to disipate slowly at standby, and cause an early stage of the amp(preamp?) not to function when sb is 'on'.
3) Most probably, the high voltage that feeds the output tube plates is compromised or non-contiguous. The fact that these tubes are constantly conducting power(bias), even when you're not playing, means that the supply will deplete fairly quickly when the sb switch is turned 'off' and the supply caps discharge. If the supplying wire or transformer's primary are open, no current can flow as normal to the power tubes, and no sound will be heard. Plus, it'll take a long time for the pilot light to 'burn off' the power stored in that first, large supply cap.
Unless you know how to trouble shoot these things, take it to a qualified shop. High internal voltages can seriously injure or even kill you. Good luck.
Best, Paul
_________________________
WUDAYAKNOW.. For the first time in my life, I'm wrong again!!
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#1871422 - 01/12/08 02:04 PM
Re: Vox AC-30 needs repair, can't figure out what went wrong
[Re: paully]
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GeoffB
MP Hall of Fame Member
Registered: 11/05/04
Posts: 6850
Loc: UK East Midlands via Scotland
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Have you tried posting this in the 'Feel Free to Ask Miles...' thread in the Guitar Forum?
G.
_________________________
"When the power of love overcomes the love of power the World will know Peace"
Jimi Hendrix
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#1900585 - 02/28/08 01:58 PM
Re: Vox AC-30 needs repair, can't figure out what went wrong
[Re: netdotcom]
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paully
Platinum Member
Registered: 05/25/04
Posts: 1028
Loc: Northern New Jersey
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Don't know if you got the problem resolved, but here's a link to an excellent Vox forum that might afford an answer.
http://www.vintageamps.com/plexiboard/viewforum.php?f=2
Best, Paul
_________________________
WUDAYAKNOW.. For the first time in my life, I'm wrong again!!
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