Yesterday, I was pulling an all-nighter (graduate school...) and at about 5:30 AM I heard a loud, but muffled, explosion off to my right followed by weird and terrible distortion from my speakers. In case you are wondering, I noticed no smoke from the amp or speakers. I have looked inside the Forte 1A and saw nothing noticeably wrong. The output transistors are all intact, no burned spots on any of the PCBs, the power supply caps all appear to be fine (no electrolytic blow-out). Fuses are all also fine. I only have a multimeter, so the only testing I could do was to check power rail voltages, and all appear normal.
As a test, I hooked the amp back up and turned it on. The speakers hummed and the woofer cones sucked in, so it is now throwing out some DC to my speakers. There was no smoke from my speakers, so I am guessing the amount is relatively small...
Does anyone know what has gone wrong with my amp? As I mentioned before, I am a graduate student, so I don't have tons of funds laying around for repairs if something is terribly wrong... Is it just one of the power supply caps? If so, what should I use as a replacement?
Thank you all for any suggestions you may have.
As a test, I hooked the amp back up and turned it on. The speakers hummed and the woofer cones sucked in, so it is now throwing out some DC to my speakers. There was no smoke from my speakers, so I am guessing the amount is relatively small...
Does anyone know what has gone wrong with my amp? As I mentioned before, I am a graduate student, so I don't have tons of funds laying around for repairs if something is terribly wrong... Is it just one of the power supply caps? If so, what should I use as a replacement?
Thank you all for any suggestions you may have.
disconnect speakers - and measure DC on speaker terminals
give us few pictures of inside
have you schmtc ?
Mighty Papa just wrote you invite .......... I'm bloody sure that he'll help too ;
remembering few times that he helped even to some pretty rude and thick headed ones .... this time must be piece of cake
give us few pictures of inside
have you schmtc ?
Mighty Papa just wrote you invite .......... I'm bloody sure that he'll help too ;
remembering few times that he helped even to some pretty rude and thick headed ones .... this time must be piece of cake
Before you go any further, and in case it happens again in the future, I will suggest that you go to Radio Shack and buy a couple of power resistors. These are in the ball park of 10 watt & 10 Ohm and cost less than a dollar.
When ever you need to diagnose or double check something, do not have the speakers hooked up. Rather, substitute the power resistors (wire them across the output terminals). This way you never need to rely on fuses or other protection circuits to protect your speakers.
When ever you need to diagnose or double check something, do not have the speakers hooked up. Rather, substitute the power resistors (wire them across the output terminals). This way you never need to rely on fuses or other protection circuits to protect your speakers.
I definitely agree WithTarragon. You can fix an amp easier than you can fix burned out speaker drivers.
As far as troubleshooting, I think a good first question to ask yourself would be whether both channels are humming or just one. If it's just one, you know it's probably a problem on just that one channel. If it's both, it's likely a problem in the power supply.
As far as troubleshooting, I think a good first question to ask yourself would be whether both channels are humming or just one. If it's just one, you know it's probably a problem on just that one channel. If it's both, it's likely a problem in the power supply.
I apologize for the delay in responding... I have been rather overworked this past week and am taking a break now (even though much remains to be done).
I spent a little time with the amp and a multimeter, and the problem is not in the power supply. Only the right channel hums. I adjusted the bias potentiometer slightly and the amp normalized... For a while, at least. Having thought I'd fixed the problem, I brought the amp home and hooked it back up. Everything went well that first listen. But, after trying it again last night, the same problem is back - the right channel hums terribly. DC offset across the right output terminals was sitting about 0.3V.
So, it looks like the bias potentiometer is bad, correct?
Thanks all
I spent a little time with the amp and a multimeter, and the problem is not in the power supply. Only the right channel hums. I adjusted the bias potentiometer slightly and the amp normalized... For a while, at least. Having thought I'd fixed the problem, I brought the amp home and hooked it back up. Everything went well that first listen. But, after trying it again last night, the same problem is back - the right channel hums terribly. DC offset across the right output terminals was sitting about 0.3V.
So, it looks like the bias potentiometer is bad, correct?
Thanks all
Only a couple volts is needed to "suck a woofer in", but you'd better keep that amp away from your speakers until you get it fixed. 0.3 volts can turn into full rail in no time, depending on what is wrong.
It would be a little more odd than normal for the problem to be just the pot. I'd look for bad solder, then wonder about semiconductors.
It would be a little more odd than normal for the problem to be just the pot. I'd look for bad solder, then wonder about semiconductors.
You will notice that Q11 is the bias transistor, providing
a DC voltage to the Bases of the output stage for bias.
R15 is the bias pot. If the wiper on that goes bad, then the
bias goes down without causing damage, and without
making noise or having offset.
So it is unlikely to be the bias pot. Also, don't adjust this
pot unless you know what you're doing - this is the quickest
way to blow up and output stage.
a DC voltage to the Bases of the output stage for bias.
R15 is the bias pot. If the wiper on that goes bad, then the
bias goes down without causing damage, and without
making noise or having offset.
So it is unlikely to be the bias pot. Also, don't adjust this
pot unless you know what you're doing - this is the quickest
way to blow up and output stage.
If there are no signs of physical damage to any components, could it be that the "muffled explosion" was really sound created by the loudspeakers responding to a transient caused by whatever fault in the amp?
You should check all of the output transistors individually, as well as look for intermittent connections. Sometimes it is easier to just reflow all the solder joints than try to find the bad one.
You should check all of the output transistors individually, as well as look for intermittent connections. Sometimes it is easier to just reflow all the solder joints than try to find the bad one.
It seems that all of the solder joints look nice... And I don't see any physical signs of damage to any component.
If it helps, my recollection of the muffled explosion: it was not like a 'boom' sound, but actually more like what a (large and sudden) fireball emission might sound like... Then followed by a sort of loud static-y oscillation sound until I shut the amp down.
Should I suspect a transistor in this case?
(by the way, thanks for attaching the schematic... I didn't have that one)
If it helps, my recollection of the muffled explosion: it was not like a 'boom' sound, but actually more like what a (large and sudden) fireball emission might sound like... Then followed by a sort of loud static-y oscillation sound until I shut the amp down.
Should I suspect a transistor in this case?
(by the way, thanks for attaching the schematic... I didn't have that one)
It is virtually impossible to tell if a solder joint is good by visual inspection. I agree with mightydub on re flow of the solder joints. It is also possible that you have a transistor that is getting flaky after warm up. A can of freeze spray will likely help you find it. Use a test speaker on the output (something cheap) so you can hear what is going on.
Cold spray...
"Freeze spray" is a product in a pressurized can resembling an old can of shaving cream. It has Carbon Dioxide under pressure that will "freeze" whatever you shoot it at....somewhat similar to a CO2 fire extinguisher.
It is used sparingly on PCBs' to locate hot spots.
I worked at Radio Shack & we had one clown who used it on energized vacuum tubes with predictable results. He filed a lawsuit because it did not say specifically not to use the product on vacuum tubes....we wound up pulling all the product off the shelves so it could be relabeled.
________________________________________________________-Rick.........
"Freeze spray" is a product in a pressurized can resembling an old can of shaving cream. It has Carbon Dioxide under pressure that will "freeze" whatever you shoot it at....somewhat similar to a CO2 fire extinguisher.
It is used sparingly on PCBs' to locate hot spots.
I worked at Radio Shack & we had one clown who used it on energized vacuum tubes with predictable results. He filed a lawsuit because it did not say specifically not to use the product on vacuum tubes....we wound up pulling all the product off the shelves so it could be relabeled.
________________________________________________________-Rick.........
actually it is a fluorocarbon, not CO2, but that's not so important.
Chemicals
What is important is the amount of mischief you can cause with the stuff. It comes with a long tube on the nozzle, like WD-40, so you can get the cold stuff in just the right spot. One time in college my lab partner was leaning way over the bench, his shirt pulled up, his jeans rode down, I had a can of freeze spray in my hand and applied a large squirt of freeze spray right in his butt crack. He jumped and howled curses at me while dancing around grabbing his ***, the rest of us were rolling on the floor laughing. We all kept our shirts tucked in and watched our backs after that.
Another favorite trick was to take the old plastic 35mm film cans with the snap on top, put in a squirt of freeze spray in liquid form, snap the cap on, then leave it on somebody's bench in the lab, ideally while they were concentrating on something. After a few minutes PV=nRT would do it's thing and POP! cap and can would blast in opposite directions.
sorry for the hijack...
on the topic of reflowing the solder joints, Steve is right. You can almost never tell visually. I have fixed a TV tuner, the FM tuner in my car radio, and a power amplifier that would randomly put out horrible scratchy noises, all by reflowing every joint with a little bit of flux. The TV and FM tuners were both very dense SMT boards inside sealed metal enclosures, probably there was a stress crack in a joint somewhere. Even with a lab microscope I couldn't find it visually. The power amp was probably long term oxidization caused by marginally clean component leads and manufacturing environment.
Chemicals
What is important is the amount of mischief you can cause with the stuff. It comes with a long tube on the nozzle, like WD-40, so you can get the cold stuff in just the right spot. One time in college my lab partner was leaning way over the bench, his shirt pulled up, his jeans rode down, I had a can of freeze spray in my hand and applied a large squirt of freeze spray right in his butt crack. He jumped and howled curses at me while dancing around grabbing his ***, the rest of us were rolling on the floor laughing. We all kept our shirts tucked in and watched our backs after that.
Another favorite trick was to take the old plastic 35mm film cans with the snap on top, put in a squirt of freeze spray in liquid form, snap the cap on, then leave it on somebody's bench in the lab, ideally while they were concentrating on something. After a few minutes PV=nRT would do it's thing and POP! cap and can would blast in opposite directions.
sorry for the hijack...
on the topic of reflowing the solder joints, Steve is right. You can almost never tell visually. I have fixed a TV tuner, the FM tuner in my car radio, and a power amplifier that would randomly put out horrible scratchy noises, all by reflowing every joint with a little bit of flux. The TV and FM tuners were both very dense SMT boards inside sealed metal enclosures, probably there was a stress crack in a joint somewhere. Even with a lab microscope I couldn't find it visually. The power amp was probably long term oxidization caused by marginally clean component leads and manufacturing environment.
Those pesky cracks on solder joints are really hard to see!
It's those circular cracks that develop..If I'm not mistaken the F5 produces copious amounts of heat...& heat expansion of PCBs hasn't really been addressed in the design stage.
I recently had a VCR/DVD machine go down for the likes of heat induced cracks on the PS...one culprit I noticed was the implimentation of barely adequate heatsinks....but the board expansion was still not calculated in the design.
_______________________________________________________Rick......
PS...What about those smoke emitting diodes?
I stand corrected on the freeze spray....It's been decades since I've seen a can of the stuff.
It's those circular cracks that develop..If I'm not mistaken the F5 produces copious amounts of heat...& heat expansion of PCBs hasn't really been addressed in the design stage.
I recently had a VCR/DVD machine go down for the likes of heat induced cracks on the PS...one culprit I noticed was the implimentation of barely adequate heatsinks....but the board expansion was still not calculated in the design.
_______________________________________________________Rick......
PS...What about those smoke emitting diodes?
I stand corrected on the freeze spray....It's been decades since I've seen a can of the stuff.
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Sorry to reopen an old thread...
I just had time to open the amp and take a more serious look. I reflowed one of the solder joints on a small TO-92 transistor (don't know which one) and the right channel now appears happy. I am going to reflow all remaining solder joints today as a precautionary measure for future trouble.
Unfortunately, while inside the amp, I noticed a problem with the left channel as well. One of the 0.47-ohm resistors shows no voltage across it, while the others sit at ~100mV. I am going to check all solder joints of this channel, as well, while I am inside the amp. But, does this allude to a more serious problem?
Thanks all
I just had time to open the amp and take a more serious look. I reflowed one of the solder joints on a small TO-92 transistor (don't know which one) and the right channel now appears happy. I am going to reflow all remaining solder joints today as a precautionary measure for future trouble.
Unfortunately, while inside the amp, I noticed a problem with the left channel as well. One of the 0.47-ohm resistors shows no voltage across it, while the others sit at ~100mV. I am going to check all solder joints of this channel, as well, while I am inside the amp. But, does this allude to a more serious problem?
Thanks all
That appears to be the case...
I couldn't get the left channel PCB off the heatsink - one of the MOSFET's is attached too well and the screw is nearly stripped after my efforts. I was able to resolder some things near the edge of the board and have fixed the no voltage across the resistor problem.
The amp is back to playing music now... And it sounds fine. Still have the slight hum/buzz in the left channel, but that was always present.
I know I've asked this before, but is there a simple way to isolate both channels from the earth ground? The right channel is isolated by varistors/resistors, correct? But the left channel is directly connected? Might this reduce the hum/buzz in the left channel?
I couldn't get the left channel PCB off the heatsink - one of the MOSFET's is attached too well and the screw is nearly stripped after my efforts. I was able to resolder some things near the edge of the board and have fixed the no voltage across the resistor problem.
The amp is back to playing music now... And it sounds fine. Still have the slight hum/buzz in the left channel, but that was always present.
I know I've asked this before, but is there a simple way to isolate both channels from the earth ground? The right channel is isolated by varistors/resistors, correct? But the left channel is directly connected? Might this reduce the hum/buzz in the left channel?
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