Yesterday while listening to some music this preamp blew a fuse. I pulled the covers , replaced the fuse and turned her on again. Resistors 81A&B started smoking so I turned her off. I pulled those resistors. Although they were a little scorched and swollen they tested good. However I went ahead and replaced them. As well I checked the FET transistor that was next to them in that circuit. It tested good. This is the circuit that provides +275 volts to the tubes. I pulled the tubes and then switched her on again. Same results. Somewhere I feel I have a short. I believe the transistor is being used as a Voltage regulator. Is it also regulating current? Is my problem in that Opamp ? Is it being used as a comparator to adjust current and output voltage of the transistor?
I am now wondering whether this is a common problem with the LS5 or whether anyone else has had a similar problem with this Preamp. Either way I definitely need some help. Any suggestions would be greatly appreciated. I have incuded the schematic.
Thanks Guys,
WILD1
I am now wondering whether this is a common problem with the LS5 or whether anyone else has had a similar problem with this Preamp. Either way I definitely need some help. Any suggestions would be greatly appreciated. I have incuded the schematic.
Thanks Guys,
WILD1
Since you have tried removing all the tubes during power up and resistor R81 (100 ohms) still overheats, you have virtually eliminated the entire audio section as a possible problem. This leaves only the power supply. The 275 volt source is a voltage regulated supply only. There is no current limiting or regulation.
I would check C39 first because it is directly across the output. Try lifting one end and see if the short disappears. Next I would recheck the series pass mosfet Q10 either by replacement or with a curve tracer. A simple DC ohmmeter test won't tell you very much unless it is shorted or extremely leaky. After that, check all the diodes. Shorted zener diodes are a common failure mode. You must use an ohmmeter with enough voltage to overcome the junction resistance. Some cheap digital meters won't. Then check U5 by replacemeent.
I would check C39 first because it is directly across the output. Try lifting one end and see if the short disappears. Next I would recheck the series pass mosfet Q10 either by replacement or with a curve tracer. A simple DC ohmmeter test won't tell you very much unless it is shorted or extremely leaky. After that, check all the diodes. Shorted zener diodes are a common failure mode. You must use an ohmmeter with enough voltage to overcome the junction resistance. Some cheap digital meters won't. Then check U5 by replacemeent.
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Well... that should keep me busy for a while. At least it is inside work where it is warm. I appreciate the input and will let you know what I find out. You have not only answered my question about exactly what that transistor is regulating but also steered me in a direction to solving the problem with the short. Thanks so much.
Gracias y lluego
WILD1
Gracias y lluego
WILD1
No SY I don't believe I did. But checking it in circuit when I put one lead on the source which they are using for the output and one lead on ground I get almost a dead short. 2.5 Ohms. So that is my problem correct? Or should I wait and pull it out of the circuit and check it before celebrating? Is this the correct way to check it? I thought you resided in Austin?
Sadly, I left Austin about 18 months ago and am now in the snow-bound North. I really miss the place, it felt like home to me.
OK, so if you have a low resistance from source to ground, that indicates a shorted output, not a shorted regulator. Capacitor C39 would be a likely culprit.
OK, so if you have a low resistance from source to ground, that indicates a shorted output, not a shorted regulator. Capacitor C39 would be a likely culprit.
One other thing to check- if you can disconnect the signal circuitry completely (not just remove tubes) and you still see a low resistance to ground, you've cut the problem in half.
Okay I am back. After reading your entry I realize now that all I have really accomplished by narrowing it down to the output side of the transistor is that it is somewhere in the output circuit. Without actually taking the output lead of the transistor loose [out of circuit] can I rule out the Transistor? Couldn't it be that the output lead of the transistor is shorted to ground? Should I forget about the transistor being the problem and proceed or pull it out and check it a little more. Before your first entry I had proceeded with taking resistor C39 out of circuit as Hollow State had suggested. That did not help.
IIRC The tab is the drain terminal of your mosfet and the output is taken from the source and if that is the case it is pretty safe to assume that the short is somewhere after the mosfet. The supply output capacitor would be the first thing to pull in order to check.
Yes it could assuming the heatsink is grounded. Removing the transistor will determine that especially if there is an insulating mica under the body. If you still have a short without the FET and no help from lifting C39, then you could have a short almost anywhere on the audio PCB B+ rails. The +275v only directly goes to V5, the output cathode follower. All other connections to the tubes are through resistors.
I once serviced one of these and found a short piece of resistor lead that was laying on the PCB. May have been there form the beginning never causing a problem. Then one day after moving the unit to roll tubes, it shorted out.
In servicing problems like this where you have a definite symptom it just comes down to a process of isolation and elimination. Keep on checkin' you'll find it.
Well... I have been studying the layout of the PCB board while looking at the schematic. Needless to say my eyes are weary and my Butt hurts. I had a total hip replacement a week ago. Anyways before I proceed any further I think the advice on disconnecting the audio circuit from the power supply is sound and might save me some time. But that will have to wait until tomorrow. I am tired and a nice fire with a good book [Steven Erikson- Dust of Dreams] awaits me. Until tomorrow.
Gracias
Gracias
Good Morning! I disconnected the signal circuitry from the power supply and the problem has disappeared in the power supply. Yes the problem is the audio stage or signal circuitry. I owe a debt of gratitude to whoever suggested actually disconnecting the signal circuitry. Thanks SY. That is the good news the bad news is that I still can not locate the short. I will keep looking.Thanks every one.
we would wish it would be this simple.....if the normal sate of the unit is to draw 50% of current at idle (making it up) and the power supply has a leak that results in 75% of power to be soaked then you could have what appears to be a defective audio circut becuase the fuse only blows when connected but the fact is all that is really happening is that the idle current of the audio circut plus the current that is being soaked due to a leak in the powersully exceeds the fuse rating whereas the same conditions which no audio circut board does not blow a fuse becuase the leak is less than the amount needed to trigger the fuse to blow.
my suggestion would be to re-test with the audio circut connected but first spray all the power supply semiconductors with co2 to see if that temorarily suspends the leak. if it does...now you know it's one of the semiconductors.
good luck.
Hey Screw the CO2. This isn't rocket science. I had a dead short after the Transistor. I am talking at the most 2-3 ohms resistance. I thought it was in the power supply but when I disconnected the audio circuit from the power supply and checked the power supply with a meter the short had disappeared. When checking the audio circuit with a meter the short was still there. Meters don't lie. However after combing the board I think I have found it. I will reassemble everything tomorrow. If this is truly the problem I will post and explain in further detail where and exactly how the short occurred. To early to celebrate but with any luck by tomorrow night the Tequila will be flowing.
Lluego
WILD1
Lluego
WILD1
Hmm, this is very interesting. There's no components in the audio circuit could short out the +275Vdc line to the chassis. May be there's some salty water, corrosion, or burnt that create the short.
Johnny
Johnny
Well... I believe I cured the short. I think it was as you had said HollowState. Just a piece of trash maybe some solder or something else metallic that shorted it out.
On the top of the board is a ground trace. It runs down the middle of the PCB board from front to back and then right in back of tube 2 it Ts off.Right there it curves under the resistor behind tube 2 and comes real close to the resistor's leads. I believe it is either resistor 17 or 18. Any ways I was checking that spot out and trying to lift the resistor up a little. When I flipped the amp over to check for the short it was gone. What is curious to me is that there was no arcing. No damage to the PCB board. When I turn her on no blown fuses or smoking so I believe that problem is taken care of.
However there seems to be some collateral damage. I put her back together and hooked her up in the studio. All I get is a hum. It was tough to haul her from my shop to my studio being that I am on crutches. As soon as I can get someone to move her back into my shop I need to do some testing. Until then any thoughts on what has happened would be greatly appreciated.
Thanks everyone
On the top of the board is a ground trace. It runs down the middle of the PCB board from front to back and then right in back of tube 2 it Ts off.Right there it curves under the resistor behind tube 2 and comes real close to the resistor's leads. I believe it is either resistor 17 or 18. Any ways I was checking that spot out and trying to lift the resistor up a little. When I flipped the amp over to check for the short it was gone. What is curious to me is that there was no arcing. No damage to the PCB board. When I turn her on no blown fuses or smoking so I believe that problem is taken care of.
However there seems to be some collateral damage. I put her back together and hooked her up in the studio. All I get is a hum. It was tough to haul her from my shop to my studio being that I am on crutches. As soon as I can get someone to move her back into my shop I need to do some testing. Until then any thoughts on what has happened would be greatly appreciated.
Thanks everyone
Hello everyone. I have the amp back in the shop. Measuring the output voltage after the Transistor I have 465 volts. The same as going in. It seems as though I am not getting any voltage regulation. Furthermore I have 465 Volts at the gate. From some of you that know a little more about these voltage regulating transistors and comparator opamps which do you think is my problem? I am contemplating pulling the transistor out of circuit and testing it. Does it need to be pulled out of circuit to test? Will testing it with a meter really tell me whether it is bad or should I just replace it? Am I barking up the wrong tree and should I be looking at the opamp instead? The short is definitely fixed. No more blown fuses so I feel I am making progress. However a lot of questions no on how to proceed. Any help would be greatly appreciated.
Thanx
WILD1
Thanx
WILD1
Pull the transistor. Measure resistance from emitter to collector- I'm guessing that's what fried.
Yes I think you are right SY. I have been trying to check it with my meter set on Diode however when I short the leads it is not giving me the 1/2 volt it should. It is giving me a little voltage and however I test the leads on the transistor it is showing leakage or a short and it is showing continuity between source and drain. So pretty sure it is fried. I will order another one tomorrow. Could this increase in voltage be the reason for the hum? I didn't have it on for long with the tubes in. Would this voltage to the plate hurt the Tubes? Stupid me I should of checked the voltage before installing the tubes and putting her back on line.
Thanks
WILD1
Thanks
WILD1
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