Hello all,
I have an acoustasonic pro amp that I love. It has recently developed a significant buzzing sound, even when no instruments are plugged in. My first inclination was to think it was my filter caps, as I understand they will do this with age and have had this problem with another amp. However, I can get it to almost disappear by smacking the top of the amp. It will go away for a while, then come back. If I understand correctly, all filter caps do need replaced every 10 years or so. This amp is easily that old, so maybe I should just do this anyway. But, I've also read that transformers are another typical cause of buzz, and the fact that I can get the buzz to change with a light smack makes me think that it probably would not be the capacitors.

I can't really afford to take this to an amp tech right now, and I'm confident I can fix the problem with a little support. I have done plenty of soldering, including replacing the filter caps in another amp. Anyone have any advice? I"m attaching the schematic in case that helps.

Just an update here. I received and installed my filter caps, and all worked well on my second attempt (when I actually soldered them in correctly). I never received the schematic I ordered, but a moot point now. The amp is working good again. The really loud amp buzz is gone.

Though my amp buzz is now gone, I notice that I still get quite a bit of buzz out of one of my guitars--telecaster. It mostly goes away when I touch the metal control knobs or the tip of the cord--so it is obviously related to grounding. I guess before I had two sources of buzz--the old filter caps and the guitar. Now, after swtiching out the caps, I just have guitar buzz on this particular guitar. It is not noticeable on my other guitar. I'll have to figure out the guitar issue, but just wanted to close the books on the amp capacitors. Thanks for the comments and suggestions. I really appreciate forums like this where people help each other work through problems.

Geez. You are right. I had found some paperwork that told me it was a GV60, and I never actually double-checked the model number on the unit. Now the inconsistencies are making sense. (eg. references to tubes that didn't seem to exist). I actually have a different amplifier--the GS100D. Sorry to have posted the wrong schematic/model number. Honestly, I was pretty focused on simply replacing the two capacitors, so the schematic wasn't that critical to me, and I guess I wasn't looking closely enough at those details. I'm looking online for the right schematic. Again, sorry to have wasted folks' time looking at the wrong diagram. I should have double-checked.

Quote from: Roly on March 27, 2013, 08:11:33 AM

2200uF/63V is what I would expect to find in the power supply of a fairly hunky fully solid-state amp, but this one has a valve (toob) output stage and shouldn't need anything like that in either value or voltage for the preamps.  So where exactly are these new caps wired into the circuit?  Which caps do they replace?  C? and C?

We do have the right circuit for the right amp here I hope; this does have a couple of large valves (toobs), and not a big heatsink with transistors on it?

I've read people refer to tubes on this amp, but I don't see any. I'm going to try to post a pic of the bottom of the circuit board. To me this looks like what you describe above--a big heatsink with two transistors on it. The other side is just the circuit board with all solid state components. I'm squeamish about posting more than one pic here, or I would include images of both sides.

Quote from: Enzo on March 27, 2013, 12:38:14 AM
I don't see 2200uf filter caps in the schematic either.  Where did you get that value?

I've tried posting pics, but I keep getting an error message. On the sides of the original capacitors, the print reads: "1=2200uF (-10+50%), 5= -, Un 63V, -40/085/56 TYPE 1, IEFF, (100HZ) 5,3A (50 degrees C)"

Since numbers 1 and 5 terminals on the cap are the only ones used (and why they were designated in the printing), my assumption was that if I replaced this with a two-lead  capacitor with 2200uF and 63V, that I'd be fine. My new ones read "2200uF, 63VDC, HU3 105 degrees C" I suspect I would have been fine if I had not reversed the one replacement polarity first time.

Hello all, I'm new here. I'm trying to nurse my KMD GV60 back to health. It was producing a loud buzzing sound--but only when something (anything) was plugged into it. I suspect there is some kind of internal kill if nothing is plugged in. Given that the amp is about 25 years old, I decided that it couldn't hurt to change out the filter caps. I had trouble finding equivalent caps, but finally found what I thought would work. I tried attaching photos of them, but the post kept timing out.  I modified the old clamps to fit the new caps and soldered them in, being careful that positive and negagive were correct (I thought). When I turned it on, I got a lower hum right away and within about 10 seconds, one of the capacitors exploded. The line that connects the two capacitors together I thought should be connected to negative on both capacitors. However, looking closer at my photo from before removing the originals, it looks like the connecting line between the two goes to negative on the left and positive on the right. That is not how I wired. I wired negatives together. So, here are my questions. Would it ever make sense for the positive of one capacitor to be connected to the negative of another, as it seems is the case with the originals?  Assuming that I reversed the polarity and can replace and reinstall a new one, is there any likely damage to other components based on reversing on my first attempt? Finally, am I right to assume my replacements should work? The specs are matching--2200uf and 63V. Temp rating on new one is double.The originals had four leads, but only two were used, so I though replacing with a 2-lead capacitor would be acceptable. Am I off-track here? 

PS, I've also attached the schematic, but I couldn't even identify the filter caps in it. I'll try to post photos of the caps if I can get this thing to actually post this time.
Thanks for any help.