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Old 9th November 2013, 01:34 AM   #1
cbfb is offline cbfb  New Zealand
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Join Date: Oct 2013
Default BEGINNER question about guitar amp issue

OK read no further if you can't be bothered with naive questions from a complete (but keen) beginner.... But if you do have a little patience I would be very grateful for some help.

My experience with electronics so far is I did study it in school 15 years ago but since then much has been forgotten. Just recently I started building a clone Boss BD-2 guitar pedal for something to do.

Well I got sidetracked when I discovered that the Aux input on my Acoustic guitar amp (Ashton AEA30) isn't working. I have tested it with multiple cables, sources, etc.

Next thing was rip the thing to bits to have a look. Tested the obvious e.g. that there was no resistance where it shouldn't be through the input socket etc.

Next I started looking at the circuit. Now I figure that the signal has got to get from the Aux In to the Preamp out. I started drawing a little schematic (below) which I am a little embarassed about displaying below but it took me ages and it's the best I've come up with so far!

Click the image to open in full size.

Now you're probably not going to be able to read that, so please click here for a full-size version.

This is the IC (I believe, it is the closest match I could find):
Click the image to open in full size.

Now at this point, I know that the signal should go from the Aux In to IC19. Although I haven't had chance to draw it on the schematic, IC19 is connected to the master pot which goes to the preamp out.

So I vaguely know the path of the signal. My question now is - how the heck do I test it? I have built stuff before but when it comes to diagnosing faults I am very green. Obviously I can test resistors, make sure earths are OK etc but I don't have an esr or oscilloscope, nor can I afford one. I am not really comfortable working with the AC on - is this necessary to test the circuit?

Any help would be greatly appreciated, I would love the satisfaction of learning to do this myself, but don't have the knowledge to decide whether or not it's possible.

Many thanks for reading.
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Old 9th November 2013, 01:47 AM   #2
diyAudio Member
 
Join Date: Mar 2007
Location: Canandaigua, NY USA
I've written a brief page on troubleshooting that won't tell you whats wrong, but might help with approach-
trouble

One of the first things is says is to search out all possible documentation. You can get the schematics for your amp at ElektroTanya | Service manuals and repair tips for electronics experts

The site can be a bit confusing at first, but it's not that tough to navigate. The trick is to watch for the words to change on the download page- after a few seconds it will say something like "get manual" and you can click on it. The pre section of your amp is the "A" schematic, I think the power section is "B".

Unfortunately, if you have minimal test equipment you'll probably have to do some power on testing, basically apply signals and see where they stop.
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Old 10th November 2013, 11:37 PM   #3
cbfb is offline cbfb  New Zealand
diyAudio Member
 
Join Date: Oct 2013
You're a legend, found the pre-amp schematic that I was painfully trying to replicate myself!!!

Stored it for posterity here if anyone wants it

Will have a read of your troubleshooting page now that I've got the diagram and get to work.

Bit nervous about power on testing I must say, but had a look at the section you wrote "Hey, I got no fancy test equipment, what about me?" and will do what I can first."

What would be the absolute minimum test eqpt if I was to buy enough to do it with the power off? Am guessing an oscilloscope for starters....

As I am interested in playing around with FX pedals for my guitar it might be worth investing in some eBay (or our equivalent in NZ) bits and pieces.

Thanks again.
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