Sunn concert bass repaires

[brothercactus]

I bought this amp so i could get into repairs.

the problem i'm having is that the amp overheats after about 20 minutes and shuts off. theres a warning about this on the back, if the cab and the head are mismatched it'll fuck up. the cab and the head match though.

Inside there is a burnt resister that's where all the heats coming from. also there is a totally leaky filter cap in the pre-amp board and acouple that look like they started to leak in the power side. the amp works! though it overheats

my question is: could a leaky cap cause the resister to overheat?

I have a plan for what components i want to replace, where is a good place to get this sort of thing on the internet? the amp is solid state and i'd also like to replace the transistors.

[Jazz P Bass]

Sun Concert

Here is a link that queries the major semiconductor distributors.
Link:FindChips.com Search

[Enzo]

I wouldn;t just start replacing transistors. They don;t wear out like tubes do.

Of course if one is bad, then replace it.

Caps with stuff leaking out are most certainoly bad and need to be replaced. That may or may not be involved with your resistor.

WHich resistor?


And overheasting? We need to find out what is using too much current. A good place to start might be the four 0.27 ihm 10w reistors for the four power transistors. Is there the same voltage across each one?

[brothercactus]

the burnt resistor says 22ohm10% and i'm pretty sure it's the only one.

I checked to see if the resistors directly connected with the tranisitors (which say .27ohm10% )were all the same and they are. 1 of them however has a broken solder joint connecting to the transistor that probably kinda works kinda doesn't. good idea! could this cause the excess?

also, i'm having trouble finding parts. i'm total noob but i'm getting into it.

i'm trying to find caps online. the info i put into searches brings up results i can't tell if they match or not. the 2 big ones in the power amp side that look like their starting to leak say 1000 mfd 50vdc on them. what does this tell me? is radio shack gonna have this? other major places? i assume i'll have to rely on the internet.

i'm working other resources, these are just my questions at the moment.

schematic here http://sunn.ampage.org/site/schematics/concertbass.gif there are diferent versions of the amp but i assume it's mostly the same.

[brothercactus]

oh! dig, you said same VOltage!. ok, i have a multimeter and i'm figureing out how it works,

so one last nood question... if i turn this on is it safe to go around connecting things with a multi meter? for some reason none of the resources i'm reading mention if i'm going to get the shit shocked out of me.

[Enzo]

Is that 22 ohm resistor a 5 watt one? And does it connect directly to a 0.22uf cap? That is the stability circuit, and that resistopr is very important.

The broken solder could have caused trouble, resolder it for sure.

Those four 0.27 ohm resistors should measure the same resistance, but I am concerned about the voltage end to end on each one. That voltage will tell us the amount of currne through the transistor it is connected to, adn that in turn will tell us something about why they might be heating.

With that resistor burnt up, it is possible the circuit is basically OK, but is oscillating way above audio and the circuit is just hot from that.

Nothing is going to tell you it will shock the shit out of you, any more than your stove will warn you that the burners get hot. The highest voltage in this thing is the mains voltage coming in from the wall outlet. That is dangerous enough, but the point is not to stick your fingers in there. Your meter probe tips are the only thing that should touch points in the circuitry.


The parts in this are common, and most can probably be had from a large parts supplier like Mouser. Mouser Electronics - Electronic Component Distributor

Radio Shack will mostly have squat.

The old schematics sometimes wrote mf or mfd, as in 1000mfd, and some search engines will recognize that. But a more modern term for it is uf, as in 1000uf

Here is a typical search... 1000uf 50v cap By the way, you can always use a higher voltage cap than you need if the voltagwe you want doesn't come up. For example if I don;t find 50v caps, I look in the 63v list.

SO at Mouser I would enter "1000uf 50v" in the search window. it will come back with a couple of choices, but you will want "capacitors", then "aluminum electrolytic capacitor" Always a good idea to check that "stocked" box, that limits the search result to only what they have in stock.

The aluminum electrolytics will give your four choices
Aluminum Electrolytic Capacitors
• Aluminum Electrolytic Capacitors - Leaded (115)
• Aluminum Electrolytic Capacitors - Screw Terminal (117)
• Aluminum Electrolytic Capacitors - SMD (16)
• Aluminum Electrolytic Capacitors - Snap In (106)

Now you have to know what you want. What do the caps you want to replace look like? If they have screw terminals, we don;t want solder-in parts, and vice versa. SO the first choice is leaded. That simply means caps with leads - wires - coming out. Those would be soldered to a circuit board or to terminals somewhere. Screw terminals are self-explanatory, each cap has a couple screws on top. SMD means "surface mount device," which wasn;t even made in the day of your amp. SMDs are the super tiny parts that don;t even have leads. Your cell phone is full of them. And finally "snap-in" caps. Those are the kind with a couple short legs on one end. The legs are curves so they snap into holes on the pc board. I don;t recall the old Sunn amps using snap-ins.

SO I selected leaded. ( and checked stocked) I don;t remember them being screw terminal caps, but if they are, we use the same approach, but select screw terminal caps.

Now a lot of choices show up. We need to select a few search options. First the value of the cap. CHose by clicking on it, 1000uf. Move over a couple columns and click 50v. And look 0ver to column 6, axial or radial. This one is important. Axial caps are the ones with a wire out each opposite end - the wires come out on the axis of the part. Radial caps are the ones wher both wires come out the same end, and the cap is mounted standing on end on the circuit board. Select the type you need. I don;t recall what is in your amp.

I chose axial ( and rechecked stocked), I clicked the "apply filters" button, and it gave me 9 choices. These are the actual parts available. They range from $1.16 each up to some stupid amount. There are more filter options, but most of those don;t much matter. In some amps the dimensions might matter due to space limits. But almost always, a 1000uf cap of today will be much smaller in size compared to one from 40 years ago like this old Sunn.

That is how a typical search is done.

[brothercactus]

it's been a couple days now so i'll update the project on here.

i soldered the connection secure so thats cool.

I tested the voltages on the resistors directly connected to the transistors and they were fine across except one which didn't show anything. the dead one is also the closest physically to the burnt resistor on the board.
when i was trying to find what all it was connected to i finally found the actual problem. one of the resistors with the huge leads that hangs above the board has come loose from it's solder joint. i labeled and un-soldered all the connections to the board, when i got it loose i found it had already been repaired and broke again. whoever fixed it last time is worse at soldering than i am.

I tried to re-solder it but i need to practice. i'm wanting the project to go well. i watched a bunch of stuff on youtube and i've been doing all the typical shit....... which was causing more damage than good.

tomorrow is the first chance in acouple days to work with it again. found an electrical store in town i'll try tomorrow too.


I do you a question though!
are the brands different on the components like the caps? the price difference is huge on this shit, that gives me the impression that there my be a bit difference between parts. any to avoid? any to spring for?