Tweet
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.
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.
Now that you have all that out reel it in a bit like the Dude so kindly suggested. 
Comment