Acoustic 470 -- halp!!

[blackgrass]

My Acoustic 470 died last night!! Or so I thought, anyway. I was playing fine when all of the sudden the sound just stopped dead. Bummer.

I went through all of the simple diagnostics and have nailed it down to the A channel. The A channel no longer responds to input noise (i.e. plugging/unplugging cables) and there is no increase in speaker noise regardless of volume level. There is a very light yet steady sound produced from my bass which is similar in every way to when I have the bass's volume turned all the way down. I did notice that there would be an intermittent boost in volume which basically told me that something in there was shorting out.

It seems impossible that I could've pushed anything too hard. We haven't been gigging, so volume levels stay within 1-3 for private and band practices. The only pedal I use is a Russian Muff and the volume on that is only set at around 2 o'clock.

B channel works perfectly, so it certainly isn't the input jack. Switching channels was of course the last thing I even checked and, upon the discovery, I went ahead and banged around for about an hour. No issues whatsoever.


There aren't really any techs in my area, but I do have a brother-in-law that's handy with a voltmeter and soldering gun. Anyone have any clues on what I should check first?


EDIT: This should probably be in the Electronics sections --- ooops!

 

[N.F.A.]

I don't have any specific diagnosis, but I believe getting a schematic might be helpful. I bet one is fairly easy to find online.

 

[craig.p]

Don't discount a bad input jack. Check to see whether it's a shorting jack, i.e. shorts tip to sleeve (signal ground) when there's no plug there -- maybe something's messed up there. (Long shot, though, since you say it happened while you were playing.) Plug a cord into the "bad" jack but leave the instrument end unconnected, then measure tip to sleeve on the instrument end and make sure you don't read zero ohms. Should be at least 100K ohms, maybe a lot more. Use the highest range on the meter and work your way down. Try not to use the lower ranges because some low resistance ranges on some VOMs can wallop some small-signal circuits. (Just playing it safe.)

That what I'd try first.

 

[MrHeadfirst]

I have been inside a couple of those units, there is a lot of garb packed into those heads, just be careful when you open it up and pay attention. Things are a bit buried, look for all the simple stuff first, burned spots or popped caps. check all wiring point to point connections (ie. input jack as craig said) in those old solid state units, it pays to check the power transistors as well, they are very sensitive and a likely place for failure, if you dont find any obvious faults.

 

[blackgrass]

I discounted the input jack simply because I never switched inputs. The only difference between the A and B channel is a switch. A channel doesn't work regardless of what input I use, B channel works regardless.

My current assumption is that I've half-blown a transistor, but we'll see. Plan on opening it up in the next few days when I can rustle my brother-in-law over. The fact that you say the transistors are sensitive forwards the assumption.


Also the schematics for these things are surprisingly impossible to find -- at least if you want a free one. I did finally score one from musicparts.com though.

 

[MrHeadfirst]

if you dont see any really obvious faults, typically i start with the transistors. make sure you test them out of the circuit. when removing transistors, be sure to use a heat sink as they are sensitive to being overheated by your iron. good luck. those heads are built like tanks. side note, there is a switch inside the head for changing the power supply over to accept an overseas power input, dont accidentally switch that...lol.