So the other night I had to drop some stuff off at the shop on the way home from a job, and outside our place I meet up with Sergio(custodian) on his way out of the building the dragging a box of junk down the ramp. I think nothing of it as I pass him when, out of the corner of my eye I spy something that doesn't belong with the others; some kind of gig gear? I say, "Sergio, what's that?" and he replies, "I don't know, I'm throwing it out."
That something turned out to be a Sunn Concert Bass amp head, of course. Not knowing anything in particular about Sunn in general, or this specific amps condition, I took it home and did probably not the smartest thing, I plugged it in and flipped the (broken) power switch. It came on and it sounded like I could hear some noise from inside the amp, kind of like quiet static, or maybe a cooling fan running? This was followed by the inevitable puff of smoke from one of the vents; and we unplug the unit hurriedly.
So, what to do. I found this site and a few other mentions of the amp around the internets and once I find the schematic on a google image search, I (whos knowledge of electronics is very limited) get the unrealistic impression that I might actually be able to fix this thing.
Fast forward to the present and I know a great deal more about the situation with this amp, but also have a great deal more questions to go along with this new-found knowledge.
I opened it up and found this...
OK, so four resistors on the board were obviously fried, I can change those out, but what caused it? I print out a large copy of the schematic and peruse it for some time, and slowly things stop making sense, and a few things tip me off that I have the wrong schematic, like a foot-switch were I have none, resistor #s on my unit's power board that show up on the preamp side of the drawing and other things, so I do an exhaustive search for a correct schematic and find a vast hole in the internet where this thing should be. I read all the posts I can find that are applicable to my situation, and some that are not. I learned a lot.
From the markings on the power board I trace out and mark the various transistor positions on the back of the case, starting with Q1 in the middle, Q2 & Q3 to the left, and Q5 & Q4 to the right. One post (on another site, I believe) notes how to check the transistors, so I remove each one and test it with an Ohm meter in both directions and find all are "closed" except the middle one, Q1 which reads 2ohms in one direction, so it must be bad. If the schematic I have applies at all to this amp, it would make sense that because Q1 seems to either be tied to, or controls the other four transistors, that it might explain why resistors which I assume are tied to each of the other four Qs are all fried, in stead of just one of them.
I know these are a lot of assumptions, but I'm doing the best with what I know.
So now on to the resistors which I have no idea what the values are because they are so scorched I can barely read anything of them...
(sorry for the poor image quality)
So I cut one lead and pry them up hoping for a number I can use, the top one reads R113 and the bottom one looks like R115, and the others are impossible to read. I look at the schematic, and again R113 is on the pre-amp side of it. Frustration. Other peoples' posts lead me to believe that they are 22ohm 2watt resistors, but the one that is the least damaged looks like red-yellow-black(?)-gold-yellow, wait WTF is the other yellow for? So that would be 24 ohms @ 5% with some weird yellow band. Maybe within 10% of 22 ohms, but not within 5%?
I pretty much gave up for the night and hoped my trip to All Electronics (
www.allelectronics.com) in the morning would help, I did find the transistors listed on their site, but no 2 watt resistors near these values (if they are 2 watts).
So the next morning I take a nice ride up to Van Nuys and prod the poor guys at All Electronics for answers, and their attitudes convince me that they are not paid enough to help noobs like me (I keed, I keed). They do have the 2N3055hv, an upgrade from the standard 2N3055, but I figure if I replace just one of these the others will not play nice; the guy there agrees, so I buy five (good salesman). At $1.25 each, who can complain. Now on to the real problem, the resistors. They have nothing close to 22ohms @ 2watts so I improvise, I think I remember that I can run two smaller ones in either series, or parallel and get the right value (in hindsight, I thought wrong). So I buy 8 22ohms @ 1watt and 4 27ohm @ 3 watts hoping I can make something work. When I get back I find another post that I should have re-read...
aaron: you would be my very own personal savior if you could get me shots from the inside. I'm mostly focusing on which wires go to which pin on the power amp board, I think I have it figured out after studying schematics and understanding the changes from the silverface to the blackface for about 5 hours, but I am not 100%. It's hard to tell since these seem to be like 2 power sections tied together, and I'm not sure if i have half a section crossed or anything.
isaac: would it be safe to put a pair of 5W 47ohm resistors in parallel in place of the 2W 24ohm? Am I right to understand that this would result in a 2.5W 23.5ohm resistance (close enough for me)?
This thread has been a major help for me, thank you aaron for starting it. If my amp gets to work again because of you two, I'll be forever greatful! I already sing your praises.
So they probably are 24 ohms after all, and the resistors I bought are useless, but I'm getting somewhere. I thought I would take care of this this weekend, but maybe I need to visit Mouser.com first.
The questions I have left are as follows:
What's the best way to test the filter caps before I turn this thing back on? Mine look fine, but maybe something else burned out Q1? (When I do get around to it I will build one of these(??)
http://sunn.ampage.org/sdp/index.php/topic,4685.msg17850/topicseen.html#msg17850.
What else should I look for when I do turn it on?
What can I do to help post a correct schematic. Can you list some changes I could make to the existing one? Some are obvious, like the two filter caps, update resistor values next to the 2N3055s, but the basic layout should remain the same, right? (are these already available from Sunn?)
Where would the date code be inside the amp? I can't see one. It would be nice to nail down some way to connect the correct schematic to the right model year/revision.
Has anyone ever found anything close a power switch replacement? I will look at mouser.
Thanks for reading all this, if you made it this far. This was mostly intended to help the next guy who might have this problem, but was also slightly cathartic for me as well.
Jay
