QdB 11/13/2017 5:04 PM
Please help with Sunn SL-260 repair. Exploded cap and blown output transistors.
Hi all.
Long time (anonymous) lurker first time caller. I want to start out by saying that this is an incredible community of people helping people with doing their repairs. Personally, I find it incredibly rewarding to be able to repair something with my own 2 hands, even if it does sometimes require just a little bit of guidance.

Now to the point of this, my very first post ever...

I have a Sunn SL-260 solid state amplifier. 2x12 combo I think it is rated a 60 watts. It doesn't work...entirely. Please let me give a little bit of history on the amp.

Chapter 1:
I have an older brother that went to college (OU) in the mid 80s, joined a band, bought a couple of amps, left the band, no longer needed the amps. I somehow ended up with them. One of those amps is the Sunn. So forward a couple of years, early 90s, I am in high school and get invited to join a band. It was a typical 15-year-old type of band, lots of attitude and ambition and plenty of as-yet undeveloped talant, in other words, another pissed off punk rock band. Oh to be 15 again!! nope forget that. not in a million years. So, back to the amp. We had a drummer with drums and a bass player with bass and some sort of amp and me with my guitar playing through a friend's little Marshall SS amp. and then for vocals, and I mean all vocals, we had a tiny mixing board, run the to HI input of the Sunn amp, and the Sunn was turned all the way up on every dial. The amazing part is that the amp (mostly) survived the abuse of 4 teenage boys. I say almost because the right speaker got blown out. But the amp continued on. So, as it is with all good things the band had to come to an end. The amp was in sort of serviceable condition but not completely dead, and I do not give up on things unless they are totally completely destroyed dead. So it went to the basement.

Chapter 2:
The Sunn ended up in the basement for a long time, like 25-26 years. I am the type to get around to things eventually. I decided on a whim to pull the thing out of the basement and dust it off and see if it still had a little bit of bark left in it. I inspect the thing and clean it up a bit, by all outward appearances everything seemed to be in tact and as it should be. Plugged it in, no fire, good. Plug in a guitar, and turn it on. Well whadya know! The power switch lit up the channel leds lit up and it was (kind of) alive. But good lord was it making awful noises, which I rather ignorantly fully attributed to old dirty pots. The volume was very very weak and scratchy and allaround ugly. But it was growing stronger, not prettier mind you, just stronger. The volume was finally coming up a little bit as I worked to knobs. And then Just as I was starting to feel good about the prospect of the potential rescue of this old dog. pop pop. Channel A, Channel B. Fuses out. The light stayed lit on the power switch. So that was at least a relief.

Chapter 3: seriously?!
I have always been decent with a soldering iron. If something just needed a part replaced, I do that. Cable needs fixed, I do that. Hell I even worked as bench jeweler for a time soldering gold and silver with a torch. That is fun. But I have always wanted to be able to repair electronics properly. Not just by guessing and hoping that replacing a certain component would do the trick. So began my quest. I am one to study and study well, I want to learn everything that I can. That is just me.

Now back to the amp, I pop it open, and surprisingly, it is as clean as you could possibly expect. No nests, no burn marks, no clearly violated components aside from 2 of the 3 fuses. So I replace the fuses and button it all back up. Fire it up, and about 15 seconds later the fuses go again. Ok. Something is amiss. Time to get researching.

Chapter 4:present.
I did some initial research. Sometimes blown fuses are trivial and clear cut. I read and read and read. I ask Google "why do my fuses keep blowing in my guitar amp?" After several articles, it seems that bad output transistors can cause fuses to readily and regularly blow. So I read more to find out about transistors, what they look like, where they will be, and how to test them. ( I forgot to mention earlier too, that this thing was still sporting only one pretty bad speaker, and there fore the wrong prescribed load impedance. I did replace the speakers when I realized that the amp could be saved.) I find the output transistors, and test them according to several articles and youtube videos and lo and behold they test out bad. Time to buy some new ones. Ok, so now I have to learn about equivalents, replacements, and datasheets. No worries. I got a 2955 and a 3055 and popped them in, and some new fuses. And the amp was good as new. But not. I played it for an hour at very bedroom volume then a one point I gave a good strong strum and Channel A, Channel B, leds both went out. Fuses gone again. Now I start thinking correctly. Did the transistors go again? Here is the important thought...it wasn't "I need to replace the transistors again", it was "I need to find out what is causing them to blow". So, research, heat kills transistors. DC in places that it shouldn't be can affect bias and make more heat and kill transistors. Another article, "Did you check for DC on the output before you even tried to plug in an input~?" Whoops, nope. First I'm hearing of it. So, I put the new transistors in, new fuses, leave the chassis out, unplug the speakers, I'm gonna turn on the amp and find out if there is DC on the outputs, because this one article said so, and if there is I am going to figure out why it is there and if that is what is blowing the transistors...

Turn on the amp, Immediately the fuses go then POP!!! nearly crapped my pants as a capacitor exploded.

That brings us to now. I have the boards out. A 3055 went bad again. A cap exploded.
Here are my questions;
1. What caused the cap to exploded (not HISS, but explode as in shot right to the ceiling explode)
2. What caused the output transistor to go and how does that relate to issue 1.

Here is what I have done;
1. More reading and research and I am an old rabbit hole pro but approaching all of electronics from an amp perspective back to basics (even a simple amp like this one) is insane.
2. Pulled and tested several components. ( I also forgot to mention, that somewhere between chapter 3 and 4 I also replaced the filter caps as that appears to be a go-to also)
test the TIP29 and TIP30 transistors. I don't fully understand what the do (voltage gain?) but they tested out fine.
I tried to trace on the schematic any components that are close to the cap that exploded and found a few things. First there appears to be a misprint between the schematic and the screenprint on the board. The schematic lists an r216 which I cannot find on the board but the board has an r261 which I cannot find on the schematic. And, if they are in fact one and the same, the resistor on the board is nowhere near what is prescribed in the schematic. Down the line a little further is a blown diode, under magnification it looks like '4148' Which I guess could be a 1N4148. Again the schematic lists all diodes as 'IN458A'.
3. Could these pieces be wrong? Can they be the cause of the issues?

Thank you in advance if you made it all the way through that terrible tale and also if you are willing to help. I really am trying as hard as possibly can to learn to repair these things correctly and to learn the ins and outs and whys of it all.

The other amplifier that I inherited by the way, is a '78 Peavey Musician mark III that I used for a lot of years. Even until probably 5 years ago or so. I turned it on a year ago and it also blew right through fuses. After I started my recent studying, I tried out the "test and replace the bad output transistors" method. And that amp works perfectly and sounds just beautiful now. And I have always loved tubes amps way more than SS (have a Mesa rect-o-verb dually) but man does that Peavey have a character all its own.
 
The Dude 11/13/2017 6:41 PM
Firstly, welcome to the place!

I want to stress that what follows is not meant to be criticism, but help. I don't mean anything derogatory. Nor do I mean to insult you.

That said, many of the members here, like myself, are techs at work. I check in from time to time during the day while waiting for the iron to warm up or something of the sort. It's best to keep your posts short and to the point. Speaking for myself, if I see a long post like this detailing information that isn't pertinent to the repair, it's unlikely I'll continue to read the whole thing.

I hope you take this in the spirit it was intended.
 
DrGonz78 11/14/2017 12:26 AM
Yeah that took me three smoke breaks on my phone to fully read the original message. Now that you have all that out reel it in a bit like the Dude so kindly suggested.

Research and build a light bulb limiter. I learned this lesson the easy way by following good advice that was given to me to make and use this device. For example, I replaced some transistors on a Yamaha PA system a few years ago. I checked for any other bad parts that might have been taken out in original failure, but did not notice anything else. Hooked the amp to light bulb limiter, no speaker or load, to turn it on the first time and the light bulb lit up bright. So I knew something was probably shorted drawing tons of current. If I did not use the light bulb limiter it would have blown the fuse and possibly burnt up the output transistors again. Opened it up and found a shorted out diode that I did not notice while testing components the first time. That thing saved me from wasting an hour or two of work that I spent on the amp. First time turning an amp on after installing faulty transistors I use the light bulb limiter and test for DC volts on the output. Once you know the light bulb goes dim and no DC on the speaker output, then turn amp on without the bulb limiter testing DC on output again. Then again with a speaker load connected. One nice thing about most solid state amps is that you don't need a load on the output when turning it on, unlike tube amps.

In your case Tip29 and Tip30 test fine. However, they drive the main power transistors and the 3055 failed at least twice. So rule of thumb is that these driver transistors (Tip29 and Tip30) are often changed out as they may have been stressed in the original failure of the amp.

Start with the light bulb limiter and then we can get going full stride with your repair.
 
Enzo 11/14/2017 1:17 AM
Hi QdB, welcome
I agree, save the life story for later. You basically have an amp that blows fuses, and you replaced output transistors, and they blew too. Yes?

Finding the shorted transistors is easy enough, but don't forget the resistors. An open resistor can be just as bad as a shorted transistor.
 
QdB 11/14/2017 2:53 PM
Hi Guys thanks for the quick replies. I hear you about giving the life story. I don't usually make long posts like this, but since it is my first I thought it would be rude not to introduce myself. Short and sweet from here on out.

First I have done what the DrGonz78 suggested and researched and built the light bulb limiter and tested it out with another amp plugged in and it seems to be doing what it should.

As far as replacing the tip29 and tip30, there are 2 pairs. Do you feel that I should replace all 4 just to be certain that they are 100%?
I have to order a capacitor (for blown c203) and a diode (diode for blown d201), if I should replace those transistors I will go ahead and order those as well.

Is it correct to guess that I also should not try to power thins thing up with missing components, even using the light-bulb limiter? I would think that that could risk damaging other components.

@TheDude. I hear you loud and clear. I appreciate your tact.

@enzo (incoming newbie question) are there particular resistors that can be more suspect than others? Or, as this is a small board with few components, should I endeavor to check them all?
 
The Dude 11/14/2017 4:09 PM
You are correct. It's normally not a good idea to power up with missing components. I'm not sure if it's been stated, but don't hook speakers up to the amp until he bulb limiter acts as it should and you measure no DC on the output of the amp. Also check the output transistors themselves (Q204 & Q205) and emitter resistors. They generally are first to go. Fortunately, there are voltage measurements on the schematic, so that will help. The TIP's are in different circuits (power supply and driver), so I don't think you need to replace all of them. It's certainly possible, but unlikely that all would be bad. Just check them to be sure.

Edit: Which of the TIP's did you replace, the ones in the power supply or the output drivers? In other words, are we more likely looking at a power supply problem or an amp problem?
 
QdB 11/14/2017 6:01 PM
@The Dude. I haven't replaced any of the TIPs as of yet. I have pulled them all, and tested them, and they checked out so I soldered them back in place. Q204 & Q205 tested bad so I replaced them both. After that I powered it up and capacitor c203 exploded. So I checked Q204 & Q205 again, Q204 (3055) went bad again, so I replaced It, and Q205(2955) tested fine, so I put it back in. I haven't powered it up again since replacing Q204 for the 2nd time.

I will have to see if I can track down which are the emitter resistors and check them out. r236 & r237 maybe?
 
The Dude 11/14/2017 6:11 PM
Yep. The emitter resistors are R236 & 7 (.33 Ohm 5W). Sometimes when output transistors short, those resistors go as full rail voltage goes right through the transistor. If C203 and D201 went bad, I would also check the bridge rectifier diodes (D202-205).
 
g1 11/14/2017 9:15 PM
Does the board have the cap polarity silkscreened? Or can you check how C203 was originally installed?
It looks to me like it's drawn backwards (polarity) on the schematic.
 
The Dude 11/14/2017 9:54 PM
Quote Originally Posted by g1 View Post
Does the board have the cap polarity silkscreened? Or can you check how C203 was originally installed?
It looks to me like it's drawn backwards (polarity) on the schematic.
Good catch! Hopefully it's just wrong on the schematic and not in the actual amp!
 
QdB 11/15/2017 4:54 AM
The polarity is silkscreened on the board. Unfortunately the capacitor vaporized when it exploded and the only thing left was 2 leads, some funky fur, and a ball of very shrunken shrink-wrap, so I can not say how it was installed.

After looking very closely at the board I can see that some work has been done on it in past, (there are some not so good solder joints and some flux muck left on the board in a couple of areas), but as far as I know it was the original cap in place.

What is it in the schematic that shows the polarity is backwards? I think these are the things I should learn how to notice.
 
QdB 11/15/2017 11:28 AM
Quote Originally Posted by QdB View Post
...What is it in the schematic that shows the polarity is backwards?...
I think I see it now.
 
g1 11/15/2017 12:12 PM
The only voltage that can get to C203 is a negative voltage, through R216. The other end of the cap is grounded. The negative end of the cap must go to the most negative point in the circuit, which is the R216 junction.
Have a look at the trace or do some resistance checks to make sure the silkscreened (-) connects to R216.
I mentioned it only in case there was no marking on the board, or if the marking is wrong. In that case, if you (or anyone else) replaced it according to the schematic, it would be reversed.
 
QdB 11/15/2017 2:26 PM
Quote Originally Posted by The Dude View Post
Yep. The emitter resistors are R236 & 7 (.33 Ohm 5W). Sometimes when output transistors short, those resistors go as full rail voltage goes right through the transistor. If C203 and D201 went bad, I would also check the bridge rectifier diodes (D202-205).
R236 & 7 check out at .03 Ohm. All of the rectifier diodes checked fine.
What are your thoughts on r212... what is installed on the board, according to its stripes is a 1.5k Ohm 5%, meters out at 1.487 Ohm. It looks like the schematic calls for 51k if I am reading it right.


@g1 thank you for clarifying that for me.
 
The Dude 11/15/2017 5:05 PM
If I'm reading the schematic correctly, it says R212 = 1.5K 1/2W, so it's fine.
 
QdB 11/15/2017 5:28 PM
ok.
I've got all the components installed. I replaced the bad diode and blown cap.
Plugged it into the light bulb limiter and turned it on for about 5 - 10 seconds. 100w light bulb lit up and stayed lit.
If I understand correctly it should have gone dim after a few seconds.
 
The Dude 11/15/2017 5:47 PM
It should indeed dim. Please, if you can fire it up again, measure DC on the speaker output and tell us what you get. If the bulb doesn't dim, it's best to leave the amp on only long enough to make the measurement.
 
QdB 11/15/2017 6:22 PM
Quote Originally Posted by The Dude View Post
It should indeed dim. Please, if you can fire it up again, measure DC on the speaker output and tell us what you get. If the bulb doesn't dim, it's best to leave the amp on only long enough to make the measurement.
Ok. Done.
~ -7.5vdc at output.
 
The Dude 11/15/2017 7:11 PM
Something is obviously still wrong. Next, I would check both of your supply rails and make sure they are balanced at +&-45V.
 
QdB 11/16/2017 4:38 AM
Quote Originally Posted by The Dude View Post
Something is obviously still wrong. Next, I would check both of your supply rails and make sure they are balanced at +&-45V.
I am not entirely sure on how yo check the supply rail voltages. So I took a shot a in the dark and measured from a terminal of each fuse to a screw hole in the chassis. ( I honestly have no idea if that is even anywhere near what you had in mind).
Regardless, the measurements that I got were ~ +8Vdc and ~ -8Vdc.
 
J M Fahey 11/16/2017 6:41 AM
OK, then your lightbulb is pulling down hard those voltages, (itīs doing its job) which means your amp is pulling too much current at idle.
Possible causes: either way over biasing or still bad parts.
Too busy now but later will suggest a couple tests.
 
QdB 11/16/2017 10:17 AM
Went home at lunch time and had a couple of minutes. On a whim, (actually something I saw in a different thread), pulled output transistors Q204 and Q205. Turned it on with the LBL and the bulb didn't light. Prompted me to check the transistors yet again. Q205 (2955) showed diode voltage drop ~5.7 for B-E and B-C, however, E-C and C-E the meter started screaming at me.
So, transistor is shorted E-C/C-E?
I didn't have time at lunch to put it all back together at lunch, so this is where I have left off for now.
 
The Dude 11/16/2017 4:02 PM
If any of the transistor leads read shorted to each other, the transistor is shorted and needs replacing. That is why your light bulb limiter is dim and supplies are low.
 
QdB 11/16/2017 4:14 PM
replaced the 2955. LBL is very near completely dark (tiny red glow). Showing no DC on output.
 
The Dude 11/16/2017 4:39 PM
That's good news! Next, fire it up without the LBL and recheck to confirm no DC on ouput. If that's good, you can hook up a speaker and try it. Bring the volume up slowly until you know it works. If that's good, the only thing left is to check/set the bias. Instructions are on the first page of the service manual you uploaded. Be sure to let us know how you come out and good luck!

P.S. Be extremely careful your probes don't slip when you bias it or you'll be back to square one!
 
QdB 11/19/2017 6:52 AM
Quote Originally Posted by The Dude View Post
That's good news! Next, fire it up without the LBL and recheck to confirm no DC on ouput. If that's good, you can hook up a speaker and try it. Bring the volume up slowly until you know it works. If that's good, the only thing left is to check/set the bias. Instructions are on the first page of the service manual you uploaded. Be sure to let us know how you come out and good luck!

P.S. Be extremely careful your probes don't slip when you bias it or you'll be back to square one!
Late response. Out of town for a couple of days.

Ok. Fired it up sans LBL. still no DC on output. Played a little bit through it with bedroom volume. Sounds nice

So here is where the newbieness kicks in, On checking/setting the bias, the manual says;

  1. put a voltmeter across 235 & 237
  2. With no input and no load
  3. wait a minute
  4. adjust for 6mvDC

Sounds easy enough. But, my inexperienced self is a little confused on the orientation of the +/- probes. When I check one direction I get ~6mvDC but if I flip the probes I get ~ -5mvDC. I know that I don't want to throw reverse bias into the thing. Seems to me that if I do that then there should no output? Best case, transistor doesn't turn on? Worst case, something more sinister?

EDIT: Per the instructions, I was able to get this thinged showing 6mvDC across r235 & r237.
~ 47Vdc and ~ -47Vdc on the +/- rails.
No DC on the outputs.

Everything looks like it is working as it should.

Thank you so very much to everyone here for helping me and more importantly for being patient with me.

...now onto the next one
 
g1 11/19/2017 11:54 AM
You are measuring DC across the resistor. So with probes one way it will say +, reverse the probes the meter will read - .
The same will occur measuring any DC. As you suspect, if the wrong polarity got to the base of those transistors, they would not turn on.

And as a side note, try not to use ~ symbol to mean "approx." when talking electronics. It also is used to denote AC rather than DC.