sonosonny
Apprentice Shielder
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Post by sonosonny on Apr 20, 2008 20:05:58 GMT -5
I picked up a free Vox Valvetronix AD30. Yeah digital, but has one tube at least. No sound from speaker, but good sound from the line/phone output jack. The kid I got it from said someone told him it was the tube. I disassembled and replaced the tube, same problem. Kid said it was smoking at one time. On disassembly I found a paper clip in the chassis. All the digital stuff works, amp models and all effects, so that is not the problem,(headphones proved this). So the problem is not there. Fuse is good, or no power at all. It appears to be a power amp problem...
Any ideas on the fix? Poweramp transformer? The one 12ax7 tube is part of the power circuit I believe.
I did not see any blown components (amateur eyes) resistors, transistors, etc..
I just want it for a bedroom practice amp.
Used ones are about 100$ so there is my dilemma. Not worth fixing?
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Post by RJB on Apr 23, 2008 11:52:22 GMT -5
Never a good thing!
Looking at the schematic, if the amp works to the headphones that means the power supply section and all of the main board are working. The tube section is after the headphone/line out port. The 2 components I would look at first specifically are the power stage IC4 LM3886 and the protection resistor R48. IC4 runs about $7 for a replacement part, if you're handy with an iron. Other wise it's scope time!
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Post by sumgai on Apr 23, 2008 13:29:17 GMT -5
Ron, Since I don't have that particular schematic in front of me (want to blast a copy my way? Please?  ), I may be speaking out of turn here, but....... The 12AX7 is part of a power amp/speaker simulator circuit (including an inductive dummy load, etc.), and Vox makes a very big deal out of it in their advertising. If it comes after the headphone output, then how does one get to hear it, either during practice (using headphones), or if they are feeding it to a House PA system (Line Out function)? Seems to me that such a shortcoming would turn off many potential buyers, eh? sumgai |
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Post by RJB on Apr 23, 2008 13:46:26 GMT -5
Actually that has been a bit of a complaint about these amps. I guess the idea was if you're running into a bigger amp, you just want the preamp/effects part of the chain. But their usefulness as standalone seems to out way this shortcoming.
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Post by sumgai on Apr 23, 2008 17:55:19 GMT -5
Ron, Much Grass! ;D Yeah, that is a helluva disconnect between the Engineering and Marketing departments, eh?  I've always thought Korg was under-rated by most folks.... it would appear that Vox thinks the same way.  Thanks again. sumgai |
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Post by sumgai on Apr 24, 2008 13:25:49 GMT -5
sonosonny, I've received a copy of the schematic, thanks. After re-reading your initial post, I'd look closely at a couple of things. First, I'd harken to the second thing I ever learned about troubleshooting electronics: Close Visual Inspection is your friend. It's not often that something goes up in smoke (literally), and leaves no trace behind. Even after months or sometimes years, you can either see, or still smell, the part that went bad. Give this a try first.  After that, I'd take a gander at the Speaker connector itself, CN2. A slight wiggle to loosen it, and the house of cards comes tumbling down..... (And while that doesn't explain the alleged smoke, I'd still dot my i's and cross my t's before going any further.) Now we're into the nitty-gritty - get out your meter.  The muting circuit, found between the LO/Headphone jack and the power amp module consists of two terminals on that jack (which form a switch) and three transistors. If any of those parts become defective, then the amp will stay muted. Take them in order, from the jack upwards. I personally would give the jack a cursory inspection, then jump to Q6, I think that's the culprit here...... if the Speaker connector checks out as good. HTH sumgai |
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sonosonny
Apprentice Shielder
Posts: 33
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Post by sonosonny on Apr 28, 2008 22:35:34 GMT -5
Sumgai and RJB
I had a busy weekend with a gig and some family obligations. I will crack the AD30 open and this time take a good look. Liked your tip on the speaker connections and the other tips as well. Realize that I am a novice on schematics.... If the board is labeled with C2, C2, C3, R1,R2 etc I can get around but am nowhere near the component level knowledgebase as many others. Just looking for a the simplest fix that my skills can manage so as not to have to "take it in". All the help I get from the GNutz nation is really appreciated.
sonosonny
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sonosonny
Apprentice Shielder
Posts: 33
Likes: 0
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Post by sonosonny on Sept 17, 2008 21:54:57 GMT -5
Got this amp repaired finally. Three components were shot.
Inline Output - Resistor .047
Drive IC - BOJ206S
Analog switching IC - LM3886T
Oh yeah the speaker was blown -when all of these blew....
90$ for the amp repair and then I bought an Eminence RamRod 10". Sweet now.
Onboard models are pretty good, but having spent HOURS on my GNX modeler patches...
I plan on running the AD30 with my Boogie Mark IIb in stereo using my Digitech GNX modeler. I have a gig this weekend and am looking forward to the STEREO image onstage.
I must say, as my close compadre Runewalker also agrees to, that two amps run in stereo is HUGE.
Not sure who wins the troubleshooting contest, but that is the current status.
Highest regards,
Son O Sonny
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Post by newey on Sept 17, 2008 22:33:10 GMT -5
SoS- Glad you got it working. You said: We've missed your compadre hereabouts recently, so if you see him, please tell him to stop by and say "hey". |
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), I may be speaking out of turn here, but.......
The muting circuit, found between the LO/Headphone jack and the power amp module consists of two terminals on that jack (which form a switch) and three transistors. If any of those parts become defective, then the amp will stay muted. Take them in order, from the jack upwards. I personally would give the jack a cursory inspection, then jump to Q6, I think that's the culprit here...... if the Speaker connector checks out as good. 
