Well, I just pulled all caps and tested them. I also pulled all transistors and tested them with DMM. Pulled transistors measure the same as new unused transistors. Caps measure fine, no shorts.
I went ahead and pulled VR 2k pot and measured it also, and it seems ok. I am now stumped, the only other thing left to do is to pull all resistors out
I went ahead and pulled VR 2k pot and measured it also, and it seems ok. I am now stumped, the only other thing left to do is to pull all resistors out
I am getting +36 volts on the output with 0 load. I just finished checking all resistors, pulled one side off and measured them that way. Replaced C2, C4, C6 with panasonic ceramic caps just in case. Replaced all transistors except for Q1 and Q2.
If I am getting +36 volts on the output, which is full rail voltage what is going on ? Is something on negative rail completely wrong or else ?
Edit < Thought maybe I got a blown fuse, both + and - rail side checked out ok >.
If I am getting +36 volts on the output, which is full rail voltage what is going on ? Is something on negative rail completely wrong or else ?
Edit < Thought maybe I got a blown fuse, both + and - rail side checked out ok >.
Hello Sangram,
Here is full schematic of the amplifier. Now keep in mind that one channel works just fine so schematic is not the problem.
Here is full schematic of the amplifier. Now keep in mind that one channel works just fine so schematic is not the problem.
An externally hosted image should be here but it was not working when we last tested it.
Hello Jean
Thanks for the schematic.
OK the problem if it is only in one channel means that there is a contruction issue (shorted solder joint, etc.) or a bad device. A lot of times the device may measure correctly but will not work correctly once full operating voltage is applied. It is imperative to test dynamically, with a load.
You're getting a full rail voltage on output means one of the output devices is locked into conduction. My guess is Q7, so a quick check on the voltage drop across Q7 (though I suspect it will be 0 volts) and R11 (if this is 0, check the condition of the BD139 at Q5) will be in order. So that you can connect load, I would second the suggestion of using a high-power resistor across the output. If you are scared of putting in too much DC current through the output stages, you can add a capacitor in series with it (this will allow you to connect a speaker without the attendant risk of blowing it up) for testing purposes.
Also check the negative rail side. In particular the bias voltage at the base of Q6, and the 0.7 volts between base and emitter of each transistor. I suspect the issue is a bias issue, and it has to be actively tested, I doubt passive testing can reveal the problem.
Thanks for the schematic.
OK the problem if it is only in one channel means that there is a contruction issue (shorted solder joint, etc.) or a bad device. A lot of times the device may measure correctly but will not work correctly once full operating voltage is applied. It is imperative to test dynamically, with a load.
You're getting a full rail voltage on output means one of the output devices is locked into conduction. My guess is Q7, so a quick check on the voltage drop across Q7 (though I suspect it will be 0 volts) and R11 (if this is 0, check the condition of the BD139 at Q5) will be in order. So that you can connect load, I would second the suggestion of using a high-power resistor across the output. If you are scared of putting in too much DC current through the output stages, you can add a capacitor in series with it (this will allow you to connect a speaker without the attendant risk of blowing it up) for testing purposes.
Also check the negative rail side. In particular the bias voltage at the base of Q6, and the 0.7 volts between base and emitter of each transistor. I suspect the issue is a bias issue, and it has to be actively tested, I doubt passive testing can reveal the problem.
Can you be a little more specific??
Hi,
I'm having exactly the same problem right now with one channel of my P3A. I know its not a construction error, because it worked fine before a malfunction in my preamp blew the one channel. I am at the "20vdc in the output" stage.
For the benifit of those of us just getting started in this hobby, what exactly do you mean by:
"put a 10-ohm high-power WW resistors in the collectors of the output transistors "
I mean, where exactly do I connect the resistor leads?? To the collector and ??
Thanks!!
Hi,
I'm having exactly the same problem right now with one channel of my P3A. I know its not a construction error, because it worked fine before a malfunction in my preamp blew the one channel. I am at the "20vdc in the output" stage.
For the benifit of those of us just getting started in this hobby, what exactly do you mean by:
"put a 10-ohm high-power WW resistors in the collectors of the output transistors "
I mean, where exactly do I connect the resistor leads?? To the collector and ??
Thanks!!
That's a good question!
I'm still trying to figure that out. The preamp routes the output from an ESP P88 preamp board to a 2P2T switch. One side of the switch goes to a pair of RCA jacks (for full signal out), and the other side goes to an ESP P81 active xover board that splits the signal at +/- 60 Hz, and goes to two pairs of RCA jacks for routing to the P3A and a sub.
The output from the P88 board works fine, but the xover started making random loud poping noises when the volume was adjusted (the pot is fine with just the P88), and the -60Hz signal was way too loud and distorted. One of the loud pops fried the P3A channel. I was trying to adjust the P88 sensitivity DIP's but it just kept getting louder no matter what setting I used. Finally it blew. I'm pretty sure it must be a bad opamp, so I have ordered more and will replace all 5 on the xover board.
The sad thing is that I got a taste of the P3A for a few blissful hours before it blew! Now I HAVE to fix it!
I'm still trying to figure that out. The preamp routes the output from an ESP P88 preamp board to a 2P2T switch. One side of the switch goes to a pair of RCA jacks (for full signal out), and the other side goes to an ESP P81 active xover board that splits the signal at +/- 60 Hz, and goes to two pairs of RCA jacks for routing to the P3A and a sub.
The output from the P88 board works fine, but the xover started making random loud poping noises when the volume was adjusted (the pot is fine with just the P88), and the -60Hz signal was way too loud and distorted. One of the loud pops fried the P3A channel. I was trying to adjust the P88 sensitivity DIP's but it just kept getting louder no matter what setting I used. Finally it blew. I'm pretty sure it must be a bad opamp, so I have ordered more and will replace all 5 on the xover board.
The sad thing is that I got a taste of the P3A for a few blissful hours before it blew! Now I HAVE to fix it!
Funny.
I mean, It couldn't be DC, could it, 'cause you've got a blocking cap in front of the first transistor of the power amp. I've yet to see an input-overdriven amp blow so spectacularly, so I dunno what it is, except if everything was connected it somehow affected the supply rails (or one of them) and pulled some high DC current through the transistors in the output. That is more voodoo than anything I've known, but whatever.
I assume your speakers are OK?
I mean, It couldn't be DC, could it, 'cause you've got a blocking cap in front of the first transistor of the power amp. I've yet to see an input-overdriven amp blow so spectacularly, so I dunno what it is, except if everything was connected it somehow affected the supply rails (or one of them) and pulled some high DC current through the transistors in the output. That is more voodoo than anything I've known, but whatever.
I assume your speakers are OK?
sangram,
Yes, there is 0 voltage drop on R11. I know the pot is not bad, I took it out and measured it while adjusting the screw to make sure its not faulty.
Like it was mentioned before about components measuring ok outside the circuit but not working when in the circuit, is there a good way to test transistors to know for sure ?
I am just using my DMM to test them.
Yes, there is 0 voltage drop on R11. I know the pot is not bad, I took it out and measured it while adjusting the screw to make sure its not faulty.
Like it was mentioned before about components measuring ok outside the circuit but not working when in the circuit, is there a good way to test transistors to know for sure ?
I am just using my DMM to test them.
Jean, the base - emitter voltage of all transistors should be 0.7 volts. That is the voltage across the silicon junction and is more or less an indication that it is OK. In NPN transistors the base should be 0.7 more positive and in PNP 0.7 more negative.
0 volts across any resistor means there is no current thorugh it, or it is shorted. If the resistor measures OK then the circuit has some problem, because the upper output transistor (Q7) is being driven to DC saturation, which is why you are getting full rail voltage at the output. OR Q7 is defective, and has a E/C short which is causing the rail voltage to 'pass through' the transistor and appear at the output.
My knowledge is extremely rusty but I can look at possible culprits here:
Q4, Q9: These are providing the bias voltage to the output stage. In some situations Q4 may be open. Its emitter voltage should be about the floating bias voltage, whatever that is as per circuit design. Q9 may not be able to sustain high voltage across itself and the junction may have broken, do not remember specs but I would suggest upping the transistor to something capable of 200 volts at collector. I may be talking through my hat here, but these are some pointers to start looking.
The process will be painstaking as I have not built this circuit and I have no reference troubleshooting experience with the particular voltage bias design, so you'll have to look. The moot point is that Q7 is receiving full saturation voltage at its base (Assuming it is OK). Therefore Q5 is not conducting. Therefore Q5 is not getting voltage at its base (assuming it is OK). And so on.
0 volts across any resistor means there is no current thorugh it, or it is shorted. If the resistor measures OK then the circuit has some problem, because the upper output transistor (Q7) is being driven to DC saturation, which is why you are getting full rail voltage at the output. OR Q7 is defective, and has a E/C short which is causing the rail voltage to 'pass through' the transistor and appear at the output.
My knowledge is extremely rusty but I can look at possible culprits here:
Q4, Q9: These are providing the bias voltage to the output stage. In some situations Q4 may be open. Its emitter voltage should be about the floating bias voltage, whatever that is as per circuit design. Q9 may not be able to sustain high voltage across itself and the junction may have broken, do not remember specs but I would suggest upping the transistor to something capable of 200 volts at collector. I may be talking through my hat here, but these are some pointers to start looking.
The process will be painstaking as I have not built this circuit and I have no reference troubleshooting experience with the particular voltage bias design, so you'll have to look. The moot point is that Q7 is receiving full saturation voltage at its base (Assuming it is OK). Therefore Q5 is not conducting. Therefore Q5 is not getting voltage at its base (assuming it is OK). And so on.
Jean said:
What about Q9...
Now since R6 is also ) volts, there may be something in the output stage. Jean, try and look at the transistors one more time, there could be a problem with the input stage, Maybe. I would suggest build the circuit upto the output stage first, do not connect load, and then test. Maybe that way you can get some idea of the problem area.
And if it is too much trouble, garbage bin it is...
Sangram, I am giving it one last try. Today I removed ALL components out , measured them, put them back in. All resistors and caps measured just fine. I am waiting for a new batch of transistors to arrive, then I'll put all new ones in and try it again.
The board is still in pretty good shape. I am thinking maybe tip2955/tip3055 keeps failing on turn on every time ? My rail voltage is 36 volts, I remember reading with over 35Volts tip2955/tip3055 is not recommended.
The board is still in pretty good shape. I am thinking maybe tip2955/tip3055 keeps failing on turn on every time ? My rail voltage is 36 volts, I remember reading with over 35Volts tip2955/tip3055 is not recommended.
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