pooge said:
PS measures fine. Regulator has fuses for each rail. I did not have the back end fused. I had hoped that the main fuse would take care of it. I may be wrong, but I don't think the component that failed is part of the back end. I have the back end power disconnected right now as well as T21 and T22 pulled and I still see the discrepancies in the front end. If the fuses are important for the back end I will add them. Before I started working on it the right channel played fine. Would it help if I listed the voltages for the transistors?
Blessings, Terry
Hello Terry,
I am very, very sorry to hear what happened to your Leach clone.
I am still stuck at work right now. I'll try and help you out when I can. In the meantime, let me make sure I understand the terminology here.
Front end = Input stage
Back end = VAS, Predriver & Output stages
This is based on the positioning of the fuses. I still don't get how T22 can short out to ground. It is on a little heatsink all by itself. Unless I suppose, the heatsink is somehow screwed to ground??
Statement: Back end is unfused. So if T22's tab (Collector) makes contact with ground, your entire output stage negative rail gets shorted, hence the flash.
Anyways, if you think the problem is with the Front end and you can power that separately, sure, listing the idle voltages might help. As usual, think safety first (for the sake of the parts at least, haha) when poking around. +/- 65V isn't funny.
Good luck.
I am very, very sorry to hear what happened to your Leach clone.
I am still stuck at work right now. I'll try and help you out when I can. In the meantime, let me make sure I understand the terminology here.
Front end = Input stage
Back end = VAS, Predriver & Output stages
This is based on the positioning of the fuses. I still don't get how T22 can short out to ground. It is on a little heatsink all by itself. Unless I suppose, the heatsink is somehow screwed to ground??
Statement: Back end is unfused. So if T22's tab (Collector) makes contact with ground, your entire output stage negative rail gets shorted, hence the flash.
Anyways, if you think the problem is with the Front end and you can power that separately, sure, listing the idle voltages might help. As usual, think safety first (for the sake of the parts at least, haha) when poking around. +/- 65V isn't funny.
Good luck.
Thanks guys,
I guess when I said front end I meant everything that is powered by the VS. Backend is everytnhing powered by the VSS. You may be right that that would leaver the feedback open and negate the voltage reading. I can power up the whole thing and take readings. Whatever smoked is done.
Yes about the heatsink. I had T22 bolted tothe heatsink and the insulator between the heatsink and the PCB. THe insulator got twisted and let the heatsink touch the board and it shorted out. I replaced T22 but it didn't fix it so I replaced T22 and T21 and still no change so I pulled both of them thinking I could trace the problem backward but I don't know enough to tell what's happening. Believe me that I did my best to find this before I posted something here. I really want to get this amp fixed. I have really been enjoying listening to it. If anyone can give me some things to check I would greatly appreciate it. I dod all of my testing through a variac and through a light bulb to try and not burn anything else up.
Thanks again, Terry
I guess when I said front end I meant everything that is powered by the VS. Backend is everytnhing powered by the VSS. You may be right that that would leaver the feedback open and negate the voltage reading. I can power up the whole thing and take readings. Whatever smoked is done.
Yes about the heatsink. I had T22 bolted tothe heatsink and the insulator between the heatsink and the PCB. THe insulator got twisted and let the heatsink touch the board and it shorted out. I replaced T22 but it didn't fix it so I replaced T22 and T21 and still no change so I pulled both of them thinking I could trace the problem backward but I don't know enough to tell what's happening. Believe me that I did my best to find this before I posted something here. I really want to get this amp fixed. I have really been enjoying listening to it. If anyone can give me some things to check I would greatly appreciate it. I dod all of my testing through a variac and through a light bulb to try and not burn anything else up.
Thanks again, Terry
Hi Terry,
check the track on the back of the PCB from Vs to T22 collector. It might not be there anymore.
The only earth connection beside T22 is the PCB mounting hole towards C27. All the ground plane is insulation covered with that (non pink) green stuff. Or did the heatsink puncture the can insulation of C27? Or could your T22 mounting screw be long enough to touch the main heatsink?
A better location for the drivers might still be the back of the board and use the main heatsink with insulators.
Remember what we said about mains fusing? How much current passes a fuse before it blows? It's there to stop the cable catching fire, not to protect any of your circuit. You also have +-42mF on Vss and the fault current from these will approach hundreds of amps.
check the track on the back of the PCB from Vs to T22 collector. It might not be there anymore.
The only earth connection beside T22 is the PCB mounting hole towards C27. All the ground plane is insulation covered with that (non pink) green stuff. Or did the heatsink puncture the can insulation of C27? Or could your T22 mounting screw be long enough to touch the main heatsink?
A better location for the drivers might still be the back of the board and use the main heatsink with insulators.
Remember what we said about mains fusing? How much current passes a fuse before it blows? It's there to stop the cable catching fire, not to protect any of your circuit. You also have +-42mF on Vss and the fault current from these will approach hundreds of amps.
I am not Jens but looking at the schematics, with Re = 0.47R, you can get aroud 380-400mA per output pair. Now you have 5 pairs........
BTW, does anybody think that the input stage differential pairs have excessive emitter degeneration? Dr Leach uses 300R, far from the usual 10-50R and I couldn't find any explanation.
BTW, does anybody think that the input stage differential pairs have excessive emitter degeneration? Dr Leach uses 300R, far from the usual 10-50R and I couldn't find any explanation.
AndrewT said:
Hi Andrew
If you look at board you will see that the ground plane is removed in the area where T21 and T22 bolt to the board but only slightly larger than the transistor itself. The little heatsinks I made are larger than that area. I didn't have any mica pads large enough to cover the back of the heatsink so I used two pads. I didn't use a pad between the heatsink and the transistor thinking that I had insulated the heatsink from the board. Well, one of the pads evidently moved and let the little heatsink come in contact with the board. It shorted through the green mask. So far I have replaced several transistors and still have the same problem. I was hoping that if I made a chart listing the voltages I have now that someone who knows circuits well could look at it and be able to tell where the problem is. I don't know how to go about testing things without current passing through them. I am probably a dope thinking that it can be done with only the front end powered up but I hated the thought of having rail voltage on the output. Anyway, If someone following this thread would be will to help me, we could handle it through email so everyone else doesn't have to listen to my whining.
still4given@yahoo.com
I appreciate all of the help I've received and I hope that someone will be able to benefit from my mistakes.
Blessings, Terry
Terry,
I don't mind reading your "whining". Feel free to post the voltages here. This way, someone can always check somebody's elses interpretation. I'm willing to take a stab at it.
It is ok to power the dead channel up if you replace the rail fuses with say 100R. If you don't have rail fuses, please install some. Please measure the rail voltages too. I am surprised that the power supply survived the short.
What about AndrewT's concern about the PCB trace leading to T22 collector? I think that should be the first check. Follow the point of failure back to the power source to ensure that there are no discontinuities. If you saw a flash, something had to give and not just dead active parts.
Pooge,
Thanks for pointing me back to Dr Leach's site. I read that countless times and even have a hardcopy printed next to me. Duh! Don't know how I missed that. Sorry.
I don't mind reading your "whining". Feel free to post the voltages here. This way, someone can always check somebody's elses interpretation. I'm willing to take a stab at it.
It is ok to power the dead channel up if you replace the rail fuses with say 100R. If you don't have rail fuses, please install some. Please measure the rail voltages too. I am surprised that the power supply survived the short.
What about AndrewT's concern about the PCB trace leading to T22 collector? I think that should be the first check. Follow the point of failure back to the power source to ensure that there are no discontinuities. If you saw a flash, something had to give and not just dead active parts.
Pooge,
Thanks for pointing me back to Dr Leach's site. I read that countless times and even have a hardcopy printed next to me. Duh! Don't know how I missed that. Sorry.
Regarding placement of the driver transistors I would use two TO-263 insulators under the heatsink to make sure there are no shortouts.
Terry I can make a version of the schematics where all ok DC voltages are printed. This will make your debugging work easier - let me know if 63V rails are ok.
\Jens
Terry I can make a version of the schematics where all ok DC voltages are printed. This will make your debugging work easier - let me know if 63V rails are ok.
\Jens
Jens - it is kind of you to offer to do this. I think this would be of benefit to many.
Really sorry to hear this bad news Terry. As Arius has said please don't feel like you are "whining" - I think alot of people, myself included, would benifit greatly from following you troubleshooting this problem.
Take heart - you will get to the bottom of it in the end and in the process have gained alot of experience. The first Leach I built - I had no problems - the second one I built I had rail voltages at output. After lots of tracing I found I I had swapped the input stage transistors around - what a twit - but in the process I learned alot about this amp.
Leach's recomended strategy for powering up the board initialy to test for faults is to have everything installed save for the output stage transistors and biasing diodes. With this configuration it should run fine - even with signal - but obviously don't connect up any loads to the output - test equipment only!.
Have you gone through Leach's checklist? I have shortened it to points I think may be relevent - if I have ommited any that should be here please somebody correct me.
Bad biasing diodes also exibited this negative rail voltage for me.
PS. I think you had better add "ensure adequete insulation" to your tagline - never leave that to God - he likes to test people
Really sorry to hear this bad news Terry. As Arius has said please don't feel like you are "whining" - I think alot of people, myself included, would benifit greatly from following you troubleshooting this problem.
Take heart - you will get to the bottom of it in the end and in the process have gained alot of experience. The first Leach I built - I had no problems - the second one I built I had rail voltages at output. After lots of tracing I found I I had swapped the input stage transistors around - what a twit - but in the process I learned alot about this amp.
Leach's recomended strategy for powering up the board initialy to test for faults is to have everything installed save for the output stage transistors and biasing diodes. With this configuration it should run fine - even with signal - but obviously don't connect up any loads to the output - test equipment only!.
Have you gone through Leach's checklist? I have shortened it to points I think may be relevent - if I have ommited any that should be here please somebody correct me.
Bad biasing diodes also exibited this negative rail voltage for me.
PS. I think you had better add "ensure adequete insulation" to your tagline - never leave that to God - he likes to test people
If I may comment a little bit more, I am no audio expert. My background is designing computer servers.
As a result of my work practice and also as good EE practice, safety and reliability are very important issues. So certain aspects such as mechanical mounting, exposure to high voltages, protection of power supplies, etc must be carefully considered.
Specific safety/reliability examples related to audio amps:
1) All power rails adequately fused. If not, a means of detecting overcurrent and shutting down. When it comes to protecting a certain rail, the question to ask is if that rail shorts, can it cause a fire? So low power DC rails can for some reason not be fused provided the AC side is.
2) All transistors/diodes that carry high voltage/current be mounted/insulated properly. Avoid voltages on heatsinks, even tiny heatsinks as they can accidentally come in contact with chassis ground.
3) Minimize routing of power rails. Keep adequate clearance from surrounding traces/parts.
4) Minimize routing of SPK OUT. They should go from output stage to relay (if any), to binding post. Minimum wiring here. Each connector/wire adds a point of failure. This is why I don't like bringing the Vq points (across Re's) to chassis for easy bias setting. Although it's at near 0 volts at idle, in operation, these nodes are at voltage and you can blow output transistors easily if the wires/chassis connectors fail. Amplifier SOA protection won't work here.
5) Use locking connectors to prevent wires from accidentally pulling out. Screw terminals are out. The wire strands can get weakened with excessive screw force.
6) Never mount resistors that may carry high capacity voltages vertically. Big no-no's are output Re resistors and power resistors for AC softstart.
7) PCB's that carry AC should not carry anything audio - i.e. don't add DC offset protection on the AC softstart board. It's preferable that these PCB's are not connected to the amp PCB in any way. Other people servicing your amp might mis-connect the wires.
8) Proper placement of PCB's, connectors and transformers in chassis. Optimize them to keep wiring short and neat. AC wiring, switches, connectors must be paid extra attention.
9) Likewise, anything that carries high voltages should be adequately insulated.
10) Quite often missed is proper derating of parts, especially capacitors. Running a 45AC transformer into a PSU made of 63V caps is asking for trouble down the road. DC rail will be 63V - no safety margin. Other parts are diodes and transistors. Both current and voltage ratings matter.
11) And the list goes on. Just imagine "What If" to evaluate potential risk areas.
Please keep in mind these are MY PERSONAL practices. If you practice them, they won't degrade audio quality. Cost impact is quite minimal. The downside is the time taken to think things through. If there are situations where best safety/reliabilty practices conflict with ultimate hi-fi, it's up to the DIY-er to decide how it's implemented.
As a result of my work practice and also as good EE practice, safety and reliability are very important issues. So certain aspects such as mechanical mounting, exposure to high voltages, protection of power supplies, etc must be carefully considered.
Specific safety/reliability examples related to audio amps:
1) All power rails adequately fused. If not, a means of detecting overcurrent and shutting down. When it comes to protecting a certain rail, the question to ask is if that rail shorts, can it cause a fire? So low power DC rails can for some reason not be fused provided the AC side is.
2) All transistors/diodes that carry high voltage/current be mounted/insulated properly. Avoid voltages on heatsinks, even tiny heatsinks as they can accidentally come in contact with chassis ground.
3) Minimize routing of power rails. Keep adequate clearance from surrounding traces/parts.
4) Minimize routing of SPK OUT. They should go from output stage to relay (if any), to binding post. Minimum wiring here. Each connector/wire adds a point of failure. This is why I don't like bringing the Vq points (across Re's) to chassis for easy bias setting. Although it's at near 0 volts at idle, in operation, these nodes are at voltage and you can blow output transistors easily if the wires/chassis connectors fail. Amplifier SOA protection won't work here.
5) Use locking connectors to prevent wires from accidentally pulling out. Screw terminals are out. The wire strands can get weakened with excessive screw force.
6) Never mount resistors that may carry high capacity voltages vertically. Big no-no's are output Re resistors and power resistors for AC softstart.
7) PCB's that carry AC should not carry anything audio - i.e. don't add DC offset protection on the AC softstart board. It's preferable that these PCB's are not connected to the amp PCB in any way. Other people servicing your amp might mis-connect the wires.
8) Proper placement of PCB's, connectors and transformers in chassis. Optimize them to keep wiring short and neat. AC wiring, switches, connectors must be paid extra attention.
9) Likewise, anything that carries high voltages should be adequately insulated.
10) Quite often missed is proper derating of parts, especially capacitors. Running a 45AC transformer into a PSU made of 63V caps is asking for trouble down the road. DC rail will be 63V - no safety margin. Other parts are diodes and transistors. Both current and voltage ratings matter.
11) And the list goes on. Just imagine "What If" to evaluate potential risk areas.
Please keep in mind these are MY PERSONAL practices. If you practice them, they won't degrade audio quality. Cost impact is quite minimal. The downside is the time taken to think things through. If there are situations where best safety/reliabilty practices conflict with ultimate hi-fi, it's up to the DIY-er to decide how it's implemented.
JensRasmussen said:
Hi Jens, I would love to have that list. It would be a great aid in finding the problem.
I found the main problem though I still have over a volt of DC offset at least it is not rail voltage. The main problem was the trace leading to/from the collector for T22 had been melted. I couldn't see it form on top of the board which I had been too lazy to remove. I am in the process of rechecking everything. I have tested all of the transistors for shorts between their leads and between ground and lead.
I have got to find some shoulder washers for these small transistors. The reason this happened is because I couldn't find a suitable way of insulating the transistor from the heatsink so I opted to insulate the heatsink from the board. Now I'm on a hunt to find all of the parts I ruined because of it. I'm out of time for today and will tackle this again tomorrow. I hope to have good news real soon.
Thanks so much, Terry
Terry, don't be in a hurry to replace transistors, only to blow them again. With transistors out of circuit, other components can be easier to check for shorts, because you don't have the transistor junctions acting as parallel diodes--conducting in one direction, but not in the other--causing all sorts of confusing measurements. On an unpowered board, clip an ohmeter lead to ground, and probe around the board with the other lead, looking for grounds that shouldn't be grounds. Do the same probing with a lead connected to each rail, and also to the speaker output. Measure the value of resistors, to see if they check out. It's easier to do this without transistors in place. In other words, keep the number of variables to a minimum, and then minimize those to make it faster to find the problem(s). I pluralized problems, because you could definitely have more than one.
You asked a while back about the need to fuse the boards as well as the transformer. The answer is a definite YES. There is plenty of energy in the caps to smoke things, as you've found out, even if the transformer fuse blows.
You asked a while back about the need to fuse the boards as well as the transformer. The answer is a definite YES. There is plenty of energy in the caps to smoke things, as you've found out, even if the transformer fuse blows.
pooge said:
Hi pooge,
Thanks for the advise. I will try some of the things you suggested. I've got a stinking suspicion that the bad trace may have been the whole problem but my attempts at replacing parts from the top of the board may have caused others. I am definitely adding fuses to the boards for the rails. Had I done that before, I most likely would not be going through this now. As I said before, hopefully someone else will benefit from my mistakes. Tomorrow I will go through the board testing for shorts. Hopefully Jens will be able to provide that voltage chart soon as I know that will be very helpful as well. Thanks to all of you for your help and patients with me. I really appreciate it.
Blessings, Terry
Byrd said:
OK I'll keep that in mind. I have been using a light bulb in series with the house current and it has been working pretty well. I also use a variac to slowly bring it up watching the light bulb. if the light stays bright I back it off and start looking for shorts again.
Blessings, Terry
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