I would greatly appreciate opinions on the following situation:
I recently took my bass amp head in for repair. It was blowing its circuit breaker when I turned it on. The technician fixed it, and reported to me that the amp had two output transistors "dead shorted" and that is what caused the circuit breaker to trip.
The amp is fine now. But I would like to understand WHY this happened.
The amp had worked fine the last time it was used (prior to the problem developing). I used it for band practice, shut it down and went home.
A couple weeks later I went to the band room (at someone else's house), picked up the amp and took it home. When I turned it on the circuit breaker tripped and the amp would not work.
The main point here is that nothing happend to the amp (or the output transistors) while I was using it - it worked just fine as it always had. All I did was move it to another location. The amp was NOT hot when it was moved. And the 120v AC at the new location was used for 4 other amps with no problems so I assume the power source was OK.
The situation brings to mind the following questions:
1) Is it possible for damage to occur to an output transistor when the amp is not powered up?
2) Can simple physical vibration caused by moving the amp cause 2 (of 10) of the amps output trasnsitors to become shorted? ( I would tend to think not, but that's why I seek opinions from more knowledgable people here on this board)
3) What are the most common ways to "blow" an output transistor AND when this occurs is it usually a "short" or an "open"? ( the repair tech did say that mine were shorted)
Thanks in advance for helping me solve this mystery...
I recently took my bass amp head in for repair. It was blowing its circuit breaker when I turned it on. The technician fixed it, and reported to me that the amp had two output transistors "dead shorted" and that is what caused the circuit breaker to trip.
The amp is fine now. But I would like to understand WHY this happened.
The amp had worked fine the last time it was used (prior to the problem developing). I used it for band practice, shut it down and went home.
A couple weeks later I went to the band room (at someone else's house), picked up the amp and took it home. When I turned it on the circuit breaker tripped and the amp would not work.
The main point here is that nothing happend to the amp (or the output transistors) while I was using it - it worked just fine as it always had. All I did was move it to another location. The amp was NOT hot when it was moved. And the 120v AC at the new location was used for 4 other amps with no problems so I assume the power source was OK.
The situation brings to mind the following questions:
1) Is it possible for damage to occur to an output transistor when the amp is not powered up?
2) Can simple physical vibration caused by moving the amp cause 2 (of 10) of the amps output trasnsitors to become shorted? ( I would tend to think not, but that's why I seek opinions from more knowledgable people here on this board)
3) What are the most common ways to "blow" an output transistor AND when this occurs is it usually a "short" or an "open"? ( the repair tech did say that mine were shorted)
Thanks in advance for helping me solve this mystery...
I am by no means an expert, but here's my take:
1) Yes (other power source, hammer, etc), but I don't think that's what happened.
2) No (unless it was the hammer from Q1).
3) All of the output transistors I have seem blow have resulted in a short. Not that that is always the case, just every one I have seen.
The amp I am working on now had the same problem. It worked fine for years. Took it out one day, turned it on, and smoke came out. All outputs on one channel were shorted.
Heat and time are usually the causes. I think that apart from extreme usage, turning an amp on is the most stressful part of it's day (like most heart attacks occur in the early morning). There is an inrush of current and weak devices fail. Probably only one device shorted because of this, but the short killed the second device before the breaker tripped.
-b
1) Yes (other power source, hammer, etc), but I don't think that's what happened.
2) No (unless it was the hammer from Q1).
3) All of the output transistors I have seem blow have resulted in a short. Not that that is always the case, just every one I have seen.
The amp I am working on now had the same problem. It worked fine for years. Took it out one day, turned it on, and smoke came out. All outputs on one channel were shorted.
Heat and time are usually the causes. I think that apart from extreme usage, turning an amp on is the most stressful part of it's day (like most heart attacks occur in the early morning). There is an inrush of current and weak devices fail. Probably only one device shorted because of this, but the short killed the second device before the breaker tripped.
-b
did you by any chance moved the stuff in cold weather?
if the gear is sitting in the truck in cold, once you brink it into the warm room, moisture gets precipitated inside
you need to warm the electronics before you turn it on
just the thought
i was working on a couple of hitachi amps recently which were blowing the fuses, both had output transistors shorted
if the gear is sitting in the truck in cold, once you brink it into the warm room, moisture gets precipitated inside
you need to warm the electronics before you turn it on
just the thought
i was working on a couple of hitachi amps recently which were blowing the fuses, both had output transistors shorted
Same with a amp i had, it was working but going into mute after a minute so i turned it off and did some measuring on the driver ics, turned it on and god dc on one channel, tuned out to be a power stansistor that failed during the off period, changed it and now the amp works flawlessy, that output transistor probobly was the cause of the amp misbehaving from the start as it finally descided to fail.
Oh, I have actually seen many output transistors that were open. It seems to happen with overdimensioned hi-fi amps when there is a single supply, usually in the order of 70-100VDC. I don't know the tecnical details of this.
These are the causes of breaking of outputs known (by me 🙂 )
1) shorted outputs (but sometimes intermittent contacts to speakers had made damage)
2) the 120V - 230V problem (I've seen this! the power transformer resisted, but the transistors...)
3) bad solder joints to the outputs, drivers, bias pots... The strangest in these faliures that I've seen was a bad joint on the filter cap. This made the supply reach higher voltages and pass the voltage limit of the transistors: they opened and shorted.
4) impedance mismatch on speakers (on cheap devices) like the one who put 2ohm speakers to his cheap mini-hi-fi...
5) driver transistor faliure by heat or something else (notice that these are in many cases not mounted on a dissipator, but they DO work and get hot)
Think of strange other causes, if you have an amp head the first thing that comes into my mind is a open/intermittent connection/short in the speaker cable to the cabinet. You may have stopped playing, removed the cable, broke it in some strange ways (stepping on it, usually, or by your bandmates), returned home, put it on, fired it up, and it died.
You may think: "but if the cable was broken, why do it works now?" because the short could be intermittent, as many cases I have seen on instrument cables. But I never seen this on speaker cable.
These are the causes of breaking of outputs known (by me 🙂 )
1) shorted outputs (but sometimes intermittent contacts to speakers had made damage)
2) the 120V - 230V problem (I've seen this! the power transformer resisted, but the transistors...)
3) bad solder joints to the outputs, drivers, bias pots... The strangest in these faliures that I've seen was a bad joint on the filter cap. This made the supply reach higher voltages and pass the voltage limit of the transistors: they opened and shorted.
4) impedance mismatch on speakers (on cheap devices) like the one who put 2ohm speakers to his cheap mini-hi-fi...
5) driver transistor faliure by heat or something else (notice that these are in many cases not mounted on a dissipator, but they DO work and get hot)
Think of strange other causes, if you have an amp head the first thing that comes into my mind is a open/intermittent connection/short in the speaker cable to the cabinet. You may have stopped playing, removed the cable, broke it in some strange ways (stepping on it, usually, or by your bandmates), returned home, put it on, fired it up, and it died.
You may think: "but if the cable was broken, why do it works now?" because the short could be intermittent, as many cases I have seen on instrument cables. But I never seen this on speaker cable.
N answer here but a couple of things to check:
A- is there any way the output could have shoeted? Not merely at the output connectors (binding posts) but out of sight some where that you can't see it. This could be something loose that moves a little and touches a ground/earth point.
B- Cooling. Heat can kill a transistor fast. If is uses a cooling fan, is the following scenario possible. Power is cut and the fan stops, the temperature of the heatsink and transistors go up, but the PS caps continue to supply current for a few seconds, SOA is breached and secondary breakdown occurs.
B- Alternative. This one really happened to me. Thermal protection is activated a couple of times but not fast enough. Damage to transistors is not fatal right away but it is cummulative. Finally you turn it on one more time and they "just can't take it any more" -- dead amp.
C- none of the above.
A- is there any way the output could have shoeted? Not merely at the output connectors (binding posts) but out of sight some where that you can't see it. This could be something loose that moves a little and touches a ground/earth point.
B- Cooling. Heat can kill a transistor fast. If is uses a cooling fan, is the following scenario possible. Power is cut and the fan stops, the temperature of the heatsink and transistors go up, but the PS caps continue to supply current for a few seconds, SOA is breached and secondary breakdown occurs.
B- Alternative. This one really happened to me. Thermal protection is activated a couple of times but not fast enough. Damage to transistors is not fatal right away but it is cummulative. Finally you turn it on one more time and they "just can't take it any more" -- dead amp.
C- none of the above.
I have to disagree. Damage on transistors is not cumulative on modern ones. Maybe with old Ge units, don't know... But with the moderns it is bad or good. No other choice. At least this is in my experience...
Possibly a voltage spike during turn-on. Or just random component failure... nothing lasts forever. Maybe the phase of one of Jupiter's moons is just right...
Just repeating what the Sony repair guy told me after charging more to repair the #$%^*&^ than it was worth.
I learned in school (wow, long time ago now) that generally solid state devices have a failure curve inverse to that of mechanical devices, meaning that mechanical devices tend to work well and wear out over time, solid state tends to either fail early, or last a long long time.
This is of course a generalization. I think a big exception is power devices that get hot. Heat kills just about everything. It really makes solid state devices mechanical devices.
-b
BTW: I remember now seeing an dead transistor that was open. It was also physically open (big crack where the flames came out).
This is of course a generalization. I think a big exception is power devices that get hot. Heat kills just about everything. It really makes solid state devices mechanical devices.
-b
BTW: I remember now seeing an dead transistor that was open. It was also physically open (big crack where the flames came out).
my 2c
I have fixed electronic failures caused by mechanical issues...namely thermal cycling of the output transistor mounts, ultimately resulting in a one or more transistors not having good thermal contact to the heatsink.
I have seen this occur more often in amps using self tapping steel screws in aluminium heatsinks, and as far as I can tell the disparity in thermal expansion makes the screws work themselves loose...at some point one of the transistors will 'pop' itself loose from the thermal pad/compound and from then it's a real short lifespan...
Stuart
I have fixed electronic failures caused by mechanical issues...namely thermal cycling of the output transistor mounts, ultimately resulting in a one or more transistors not having good thermal contact to the heatsink.
I have seen this occur more often in amps using self tapping steel screws in aluminium heatsinks, and as far as I can tell the disparity in thermal expansion makes the screws work themselves loose...at some point one of the transistors will 'pop' itself loose from the thermal pad/compound and from then it's a real short lifespan...
Stuart
Hi,
fatigue life of transistors that are asked to thermal cycle is finite.
After repeated junction heating the junction to die or die to case connection can open & then rapid degeneration will ocurr.
this could be one of the failure mechanisms that cause the reliability curve to start decreasing again as components age.
regards Andrew T.
fatigue life of transistors that are asked to thermal cycle is finite.
After repeated junction heating the junction to die or die to case connection can open & then rapid degeneration will ocurr.
this could be one of the failure mechanisms that cause the reliability curve to start decreasing again as components age.
regards Andrew T.
No one seems to have mentioned one of the most obvious possibilties:
Did someone else use the amp while it was sitting at the other fellow's house?
Of course, given that the head was blown, it may be difficult to get a confession...
Grey
Did someone else use the amp while it was sitting at the other fellow's house?
Of course, given that the head was blown, it may be difficult to get a confession...
Grey
Sometimes .....
Yes all of the above...
But sometimes s*** just happens. All components fit within a bell curve of reliability, so just one "random" component failure can avalanche into a major fault.
Sorry about your amp.
Cheers
Yes all of the above...
But sometimes s*** just happens. All components fit within a bell curve of reliability, so just one "random" component failure can avalanche into a major fault.
Sorry about your amp.
Cheers
Thank you all for your thoughtful and informative posts.
adason- In my original post I used the word "recently"...but the amp problem occured in early November and it wasn't very cold here in Indiana then...possibly 40-45 F. You bring up an excellent point about condensation when cold objects are moved into warmer air. But in this case the amp was moved from a warm house to a warm car then to another warm house....no sitting outside in the cold for an extended period.
WorkingAtHome- In it's prior life (previous owner) the amp did experience "extreme usage". But not while I was using it...the amp is a 475W @4ohm monster the was never turned above "3" during band practice. However, your comments about the turn-on being stressful makes sense...
Stuart and Andrew - great info regarding themal contact between the transistor and the heat sink, thanks. The heat sink did appear to be aluminum although I don't remember noticing whether the screws were the self-tapping type.
Giaimi- lots of good observations. All I can say is that I did a close visual inspection before turning the amp over to the tech and saw no apparent loose connections. The amp is a Peavey and that company's products are generally known to be put together well and very durable. Regarding the speaker cable: I made that cable myself and, although anything is possible, I believe it unlikely that there would be an intermittant short (or open) in the cable. I have had no problems with that particular cable and have used it with various other amp/speaker combinations.
-----------------------------------------------------------------------
I will address the comments of one other poster, but before I do I will say that everyone's comments are valid and the problem could have been caused, as I now realize, by lots of things - heat, bad connections, condensation... etc etc
But what concerns me is the timing of the problem...
GRollins- funny that you should mention about "other band members" because in fact, as sometimes happens in bands ( LOL, not that infrequently) there was a bit of , I will say, "tension" between myself and the owner of the band's practice house.
It's all OK now between us, but I can't help wondering if "something happened" to the amp in my absence. Again, it is the timing of the events that makes me wonder...
So I have another question for you all and anyone else who may care to pitch in:
Hypothetically - what would happen if the amp's output, the cable that is supposed to go to the speaker, was connected instead to one of the amp's inputs? ( the amp has 3 or 4 inputs for various purposes)
It would be possible to plug a guitar into one input, and plug the speaker cable into another INPUT and play a chord...what would happen inside the amp?
adason- In my original post I used the word "recently"...but the amp problem occured in early November and it wasn't very cold here in Indiana then...possibly 40-45 F. You bring up an excellent point about condensation when cold objects are moved into warmer air. But in this case the amp was moved from a warm house to a warm car then to another warm house....no sitting outside in the cold for an extended period.
WorkingAtHome- In it's prior life (previous owner) the amp did experience "extreme usage". But not while I was using it...the amp is a 475W @4ohm monster the was never turned above "3" during band practice. However, your comments about the turn-on being stressful makes sense...
Stuart and Andrew - great info regarding themal contact between the transistor and the heat sink, thanks. The heat sink did appear to be aluminum although I don't remember noticing whether the screws were the self-tapping type.
Giaimi- lots of good observations. All I can say is that I did a close visual inspection before turning the amp over to the tech and saw no apparent loose connections. The amp is a Peavey and that company's products are generally known to be put together well and very durable. Regarding the speaker cable: I made that cable myself and, although anything is possible, I believe it unlikely that there would be an intermittant short (or open) in the cable. I have had no problems with that particular cable and have used it with various other amp/speaker combinations.
-----------------------------------------------------------------------
I will address the comments of one other poster, but before I do I will say that everyone's comments are valid and the problem could have been caused, as I now realize, by lots of things - heat, bad connections, condensation... etc etc
But what concerns me is the timing of the problem...
GRollins- funny that you should mention about "other band members" because in fact, as sometimes happens in bands ( LOL, not that infrequently) there was a bit of , I will say, "tension" between myself and the owner of the band's practice house.
It's all OK now between us, but I can't help wondering if "something happened" to the amp in my absence. Again, it is the timing of the events that makes me wonder...
So I have another question for you all and anyone else who may care to pitch in:
Hypothetically - what would happen if the amp's output, the cable that is supposed to go to the speaker, was connected instead to one of the amp's inputs? ( the amp has 3 or 4 inputs for various purposes)
It would be possible to plug a guitar into one input, and plug the speaker cable into another INPUT and play a chord...what would happen inside the amp?
Ohh,Yea..Striking a chord with no load on the amp,and the overdrive/volume cranked up could blow the outputs.I've had an expensive accident with an un-loaded Hi-Fi amp once.
Also,connecting/disconnecting the speaker while the amp is playing is a bad idea..
My first thought when I read your post was that somebody may have used the amp,and doesn't want to admit that they broke it..
But,I suppose it is possible that something else went wrong.
Also,connecting/disconnecting the speaker while the amp is playing is a bad idea..
My first thought when I read your post was that somebody may have used the amp,and doesn't want to admit that they broke it..
But,I suppose it is possible that something else went wrong.
Yeah. I wasn;t even thinking about it being a PA Amp (obvious, just not thinking). Being a drummer, and watching (helplessly, usually) as guitar/bass players constantly mistreat their equiptment drives me nuts:
- plugging and unplugging instruments and speakers mid-play
- using instrument cables as speaker cables
- kicking bad cords to make them work while at half (or greater) volume
but I digress. The live of a PA amp is stressful. They die now and then. If they aren't built well, they will die all the time. I guess, hust be happy they play when you need them, and treat them well.
-b
- plugging and unplugging instruments and speakers mid-play
- using instrument cables as speaker cables
- kicking bad cords to make them work while at half (or greater) volume
but I digress. The live of a PA amp is stressful. They die now and then. If they aren't built well, they will die all the time. I guess, hust be happy they play when you need them, and treat them well.
-b
My shop is a factory authorized Peavey repair center (and Fender, and Marshall, and so on). Those are very sturdy and well made amps, quite reliable.
SOme thoughts: while we like to find a "reason" for everything, there really are random component failures in electronics. Maybe it is too many thermal cycles, or one too many static electricity hits, who knows. Not all failures are due to something in particular. Like the light bulb on the back porch. One day you flip it on and it is gone bad. There is no apparent cause, it was just time for it to go.
Everything works until it doesn't. I tell my customers that often. It means, there is not always some this happened therefore that went wrong chain of events leading up to the failure. We can worry that maybe there was a conspiracy to booby trap your amp, but it seems unlikely.
You turned it off when it was working, and now it doesn't, but it probably did not die while it was off. It probably failed the instant power hit it the day you found it bad. Just like the light bulb that blows when you flip the switch. If you were watching, there might have been a really brief flicker of the power light as it died.
All it takes is one part failure in the output stage to toast the whole circuit. WHen one output transistor shorts - and they rarely fail open, mostly short - then its opposite partner now faces opposing power rails at its either end. when it turns on it becomes a short across the power rails and so quickly dies too. But a driver transistor might have failed, which in turn stuck on an output, which then burned up hte others. A resistor could have opened which allowed both sides of the circuit to come on togehter, thus shorting across the rails and blowing it up. And so on
Bad cords and bad loads can damage an amp, but these amps are generally well protected against overload. Always use a real speaker cord and never a guitar cord to connect amp to speaker. If the cord is intermittantly opemn it won't hurt it. If the cord shorts it is not good. The conductors can be pierced, the insulation ground away along the cable, but the ends are the most likely failure. Make sure the plugs are not getting loose so the parts can move and touch. MAke sure none of the wire strands are loose and sticking out. Make sure the tip wire is not rubbing through the insulating sleeve in the plug cover.
The transistors are almost certainly on aluminum in there.
The solid state amp does not care if it is loaded. You can run this amp full out at clipping all day long unloaded if you want and it won't even get hot. That is because it is doing no work. The transistors put out a voltage, but until there is a load, no current is passing. No power is being developed. We routinely run solid state amps without load in service.
Tube amps MUST have a load or you risk frying the output transformer. But solid state is completely different stuff.
If you connect an amps input and output together, you don't need to inject a big chord into a second jack to start it. the inherent noise in the system will be more than enough to start anything. Any oscillation that reults would be limited by the power amp clipping anyway. Once it got to clip, that is as far as it can go. If you did that, you would probably burn something out in the front end. The power amp rails are maybe 35 volts or more depending on amp size. COnsidering the preamp op amps likely run on +/-15v you will exceed that which is not good for the op amp. Peavey and others usually add clamping diodes between the input and the 15v rails. But they are just little 1N4148, so they won't handle much current, so they blow with the chip. The tracery in the input circuit is not heavy, so a trace or two might burn open like a fuse. The outputs them selves may well survive this treatment. They don't realize it is their own preamp ramming the signal back through. They just see the preamp input as an uncomfortable load after it burns up. The current limiters in the power amp should protect it.
The amp has a phase relation between in and out. If positive input yields positive output, the amp is in phase and connecting in to out would indeed oscilate. If howver the number of internal stages is such that the input to the amp emerges inverted at the speaker pins, then the amp is inverting, and in to out connections would be self cancelling.
SO the bottom line is that the amp just failed. It was probably no one's fault, and you did not do anything wrong with the amp.
SOme thoughts: while we like to find a "reason" for everything, there really are random component failures in electronics. Maybe it is too many thermal cycles, or one too many static electricity hits, who knows. Not all failures are due to something in particular. Like the light bulb on the back porch. One day you flip it on and it is gone bad. There is no apparent cause, it was just time for it to go.
Everything works until it doesn't. I tell my customers that often. It means, there is not always some this happened therefore that went wrong chain of events leading up to the failure. We can worry that maybe there was a conspiracy to booby trap your amp, but it seems unlikely.
You turned it off when it was working, and now it doesn't, but it probably did not die while it was off. It probably failed the instant power hit it the day you found it bad. Just like the light bulb that blows when you flip the switch. If you were watching, there might have been a really brief flicker of the power light as it died.
All it takes is one part failure in the output stage to toast the whole circuit. WHen one output transistor shorts - and they rarely fail open, mostly short - then its opposite partner now faces opposing power rails at its either end. when it turns on it becomes a short across the power rails and so quickly dies too. But a driver transistor might have failed, which in turn stuck on an output, which then burned up hte others. A resistor could have opened which allowed both sides of the circuit to come on togehter, thus shorting across the rails and blowing it up. And so on
Bad cords and bad loads can damage an amp, but these amps are generally well protected against overload. Always use a real speaker cord and never a guitar cord to connect amp to speaker. If the cord is intermittantly opemn it won't hurt it. If the cord shorts it is not good. The conductors can be pierced, the insulation ground away along the cable, but the ends are the most likely failure. Make sure the plugs are not getting loose so the parts can move and touch. MAke sure none of the wire strands are loose and sticking out. Make sure the tip wire is not rubbing through the insulating sleeve in the plug cover.
The transistors are almost certainly on aluminum in there.
The solid state amp does not care if it is loaded. You can run this amp full out at clipping all day long unloaded if you want and it won't even get hot. That is because it is doing no work. The transistors put out a voltage, but until there is a load, no current is passing. No power is being developed. We routinely run solid state amps without load in service.
Tube amps MUST have a load or you risk frying the output transformer. But solid state is completely different stuff.
If you connect an amps input and output together, you don't need to inject a big chord into a second jack to start it. the inherent noise in the system will be more than enough to start anything. Any oscillation that reults would be limited by the power amp clipping anyway. Once it got to clip, that is as far as it can go. If you did that, you would probably burn something out in the front end. The power amp rails are maybe 35 volts or more depending on amp size. COnsidering the preamp op amps likely run on +/-15v you will exceed that which is not good for the op amp. Peavey and others usually add clamping diodes between the input and the 15v rails. But they are just little 1N4148, so they won't handle much current, so they blow with the chip. The tracery in the input circuit is not heavy, so a trace or two might burn open like a fuse. The outputs them selves may well survive this treatment. They don't realize it is their own preamp ramming the signal back through. They just see the preamp input as an uncomfortable load after it burns up. The current limiters in the power amp should protect it.
The amp has a phase relation between in and out. If positive input yields positive output, the amp is in phase and connecting in to out would indeed oscilate. If howver the number of internal stages is such that the input to the amp emerges inverted at the speaker pins, then the amp is inverting, and in to out connections would be self cancelling.
SO the bottom line is that the amp just failed. It was probably no one's fault, and you did not do anything wrong with the amp.
About that funny "amp output into its input"... I agree that this shouldn't be the case, because this would fry the pre, not the power stage....
But think about it: someone connects the output cable to the input. He plays a couple of chords. Then he hear you approaching (or he's simply fed up of playing) and quickly disconnects the cable from the input BEFORE turning the amp off. The tip of the speaker cable touches for an instant the ground connection of the input jack and the outputs, seeing a short, simply die.
Strange story, isn't it?
But think about it: someone connects the output cable to the input. He plays a couple of chords. Then he hear you approaching (or he's simply fed up of playing) and quickly disconnects the cable from the input BEFORE turning the amp off. The tip of the speaker cable touches for an instant the ground connection of the input jack and the outputs, seeing a short, simply die.
Strange story, isn't it?
I don't think unplugging like that would hurt the amp. I see guys do that all the time. It's not good for the amp, but they seem to be very tough in that regard.
- Status
- Not open for further replies.