Amp Repair
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Posted on Jan 28, 2012 06:11 pm
Runicen
Duke of Stewed Prunes
Member Since: Jun 01, 2007
Ok, I have an odd passion project I'm trying to undertake but I'm having a difficult time gathering good information to plan my attack.
I have an old Peavey Decade guitar amp. It was my first guitar amp and, as it happens, I also like the tone of the thing. Over the past few years, it has started losing output (but not power) while in use. At first, I could get an hour or two out of it... then fifteen minutes... and now, it lights up, but nobody's home.
I pulled the "head" out of the thing (it's a tiny 10w combo amp, for those not familiar) to see if anything looked obviously amiss. Right where the power cord meets the strain relief, I noticed that the insulation had become so brittle that it had split open. As it's still insulated from the metal AND itself, I assume this is just another thing I should fix while working on it and not the source of the problem.
Peavey was kind enough to provide me with a wiring diagram, but it's very needle/haystack trying to figure out WHERE to attempt to troubleshoot this thing. Could anyone point me to either a good book for this sort of thing OR a good procedure for pinning down the problem components?
Additionally, with regard to the power cord, I was inclined to strip and re-purpose an old PC tower (grounded) AC plug as a replacement, but this is where my novice/throw it at the wall approach shows - I have no idea if the grade of the wiring will cause me grief. I assume not given that PC towers are likely pulling much higher amounts of juice, but I wanted to appeal to you good people on that front as well so I don't accidentally create my first easy bake oven while I'm tinkering.
Just so it's clear, I've done minor circuit work, car stereo installation, and power cord replacement before. This is just a LEEDLE more next-level than I was anticipating when I decided to take it on myself.
As always, your help and input are worth their (metaphysical) weight in gold and much appreciated. =)
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Jan 28, 2012 07:39 pm Being that it seems to be losing output, I would start there.
If you have a way to run a 1k tone through the amp, do so. Measure the input to the output IC, and measure the output. Make sure signal is getting to the IC, and make sure something is coming out of it. You should see one volt or higher coming in, and five volts or higher on the output.
If the output is fine, then look at the speaker. If the input is not present, then start moving backwards. For myself, I tend to look at the input jack and IC if no signal is present on the output. And with PV, that is where I find about 80% of all issues.
Hope this helps.
RunicenDuke of Stewed PrunesMember
Since: Jun 01, 2007
Jan 28, 2012 10:51 pm I have an old signal generator (which I haven't yet used for anything, so it'll be a learning process) and a multimeter. I'm assuming these will do the job and no further equipment is required?
Jan 28, 2012 11:29 pm They should do just fine. All you're looking for is where the AC signal stops.
RunicenDuke of Stewed PrunesMember
Since: Jun 01, 2007
Feb 07, 2012 09:22 pm I haven't had a chance to get this apart and on the workbench yet, but I did have another question with regard to signal flow...
As this was a problem that got progressively worse over time, are there specific components that just age poorly on circuit boards? Everything looks clean and connected to the naked eye. My understanding is that resistors and the like don't "age" so long as nothing overloads or fries.
Additionally, while it does power up and the all-important red light goes on, when I look at the way it's wired, the on-off switch and the light itself are directly connected to the power cord, but a separate cord goes FROM that assembly to an enclosed brass box which I'm assuming contains some sort of voltage converter. Is it possible something inside of THAT is causing the problem and, if so, is there an easy way to open it? It looks like it's been folded and sort of "punched" into itself as a sealant, if that makes any sense...
Feb 07, 2012 10:06 pm Well resistors have been known to do strange things over time. They can suddenly open for no reason. Other than, they felt like it at the time.
The brass box could be the AC converter. Without seeing it, the description that you gave of where it's at in the chain sound correct. Both of these (resistors and converters) usually don't fade with time. They are more of an all or nothing at all type of failure.
This is not to say that there couldn't be some stray capacitor in the AC converter that is going soft. It could happen. It's just not a very common thing to have happen.
IC can go the way you describe, same with their filter caps. Input IC's, broken solder joints, other filter caps in line. These are more common to the issue that you are experiencing.
BeerHunterwww.TheLondonProject.caMember
Since: Feb 07, 2005
Feb 08, 2012 01:49 am VERY IMPORTANT: Rob and Runicen appear to have a great deal of experience working on amp and circuitry. Please do not attempt amp repairs unless you know exactly what you are doing. Several components of an amp contain a lethal amount of stored energy, even if the amp is unplugged.
Feb 08, 2012 02:39 am Good point BeerHunter.
We'll be charged with the PSA later. :)
pjkPrince CZAR-mingMember
Since: Apr 08, 2004
Feb 08, 2012 12:19 pm Actually, that's a good point to start looking. There are electrolytic capacitors, that often go bad, on solid state and tube amplifiers. These are usually easy to see (largish can looking things), but possibly smaller, since this is (probably) a solid state amp with low power requirements.
These are AKA the filter caps Rob mentioned earlier. They go after time, and I've seen a few times where the filter caps fade, and drop out power like this.
But, since you're probably not running high voltage (not a tube amp) the filter caps may be small, and possibly hidden inside the brass box thing. A look at the schematic would help, if you can link to it.
The brass box thing probably does convert the incoming AC over to some flavors of DC for the ICs to use. So there would be some sort of filter caps to smooth out the ripply DC before use in the converter circuit.
To your benefit, there may not be more than 12vdc in the amp, but I would want to make sure before I go poking my hands onto things. 200v in a cap can give you a right smack.
RunicenDuke of Stewed PrunesMember
Since: Jun 01, 2007
Feb 26, 2012 10:32 pm Sorry I've been remiss in responding to messages on here. First, I wanted to thank everyone who has responded. All the info I can get is appreciated.
Now, here's where I betray the fact that I never would have survived in the wild: this is the FIRST time I've attempted amplifier repair. I've done basic repairs on AC appliances, but this is the first time I've toyed with amps. Given that I haven't blown myself across the room YET, I assume the voltage carriers are inside the brass box I mentioned before.
Now, if all of you haven't turned away in shame, I was wondering if anyone knows of any good books or online resources on amp repair (which, ideally, include descriptions of what not to touch with bare hands, metal objects, soapy water ;-).
I got a tip on a relatively simple component replacement that may set me up, and I'll be chasing that down, but if this sort of thing comes up again, I'd like to be better armed with information on the basic workings of these horrid little gadgets (that make such joyous noise). =)
Feb 27, 2012 02:04 pm I don't have book suggestions at the moment. What I do have is a persons website that I have used for a few years now.
sound.westhost.com/troubleshooting.htm
This guy has some great information on his site. And has always given it out for free! Hopefully, he will be of service to you as well.
