Ok, can you clarify a little? When you say the tabs you mean the back of the mosfet?
Thanks again
Jim
I do not have an open amp in front of me but yes the back of the mosfet is usually showing in the front where the bolt goes in or at the edges. This is from memory. The back of the mosfet is metal and should be insulated from the heatsinks. If not , dead short, smoking resistors.
I went to my parts bin and found a IRFP240 and on them what I am calling the tab from the front of the mosfet is the shiney metal part on both sides at the top of the mosfet. Probe there and ground and the should not be a reading with a ohmmeter. The middle leg of the mosfet shows continuity to the metal as well. It appears to be a not insulated mosfet or a shorted mosfet that is defective. Notice I said appears. You can check the middle pin and be sure it does not show continuity to the heatsinks and accomplish the same thing.
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A few things that I do before ever applying power to any PCB:
1) measure EACH AND EVERY part BEFORE installing it on the PCB - I've been shipped 220k resistors in a bag marked 220R on multiple occasions
2) check, double-check, and re-check ALL installed components, orientation, placement, values vs the schematic, etc. Use the data sheet to understand which pin is which on devices that have more than 2 pins. Match this to the schematic and to the PCB before soldering. Solder all of the same devices (same jFets, same value resistors, etc) on both boards before moving on to the next part. Parallelism helps reduce errors.
3) after soldering, take a fine flat-blade screwdriver and physically scratch between ALL solder pads on the back of the PCB (front, too, sometimes) to make sure there are NO errant solder bridges due to sloppy soldering
4) after mounting mosfets to the sink and BEFORE mounting them to the PCB, use your meter to measure each pin and make sure there isn't an inadvertent ground happening somewhere between the device and the sink. I've found problems in the past where a plastic collar was not seated properly, or when a tiny scrap of aluminum has pierced the sil-pad and grounded the device to the sink.
Hope this helps a little...
1) measure EACH AND EVERY part BEFORE installing it on the PCB - I've been shipped 220k resistors in a bag marked 220R on multiple occasions
2) check, double-check, and re-check ALL installed components, orientation, placement, values vs the schematic, etc. Use the data sheet to understand which pin is which on devices that have more than 2 pins. Match this to the schematic and to the PCB before soldering. Solder all of the same devices (same jFets, same value resistors, etc) on both boards before moving on to the next part. Parallelism helps reduce errors.
3) after soldering, take a fine flat-blade screwdriver and physically scratch between ALL solder pads on the back of the PCB (front, too, sometimes) to make sure there are NO errant solder bridges due to sloppy soldering
4) after mounting mosfets to the sink and BEFORE mounting them to the PCB, use your meter to measure each pin and make sure there isn't an inadvertent ground happening somewhere between the device and the sink. I've found problems in the past where a plastic collar was not seated properly, or when a tiny scrap of aluminum has pierced the sil-pad and grounded the device to the sink.
Hope this helps a little...
4) after mounting mosfets to the sink and BEFORE mounting them to the PCB, use your meter to measure each pin and make sure there isn't an inadvertent ground happening somewhere between the device and the sink. I've found problems in the past where a plastic collar was not seated properly, or when a tiny scrap of aluminum has pierced the sil-pad and grounded the device to the sink.
Hope this helps a little...[/QUOTE]
All excellent advice but #4 specially in his case.
Hope this helps a little...[/QUOTE]
All excellent advice but #4 specially in his case.
And always abide by Rule No "1" Never never release the smoke sealed inside any component.
Many have tried to put it back in without success. Usually when the smoke comes out you hear OH S**T next.
Possibly 5) stop, breathe, walk away, drink coffee/meditate/listen to kind of blue/etc then repeat steps 1 through 4, especially 4.
I sometimes forget step 5 in my haste to get results.
In all builds in recent years I am meticulous in steps 1 and 2. Every component in this build was checked and double checked. Sometimes as 6L6 says things just go wrong.
Thanks all for chiming in. I'm certain this one is not far away.
I sometimes forget step 5 in my haste to get results.
In all builds in recent years I am meticulous in steps 1 and 2. Every component in this build was checked and double checked. Sometimes as 6L6 says things just go wrong.
Thanks all for chiming in. I'm certain this one is not far away.
I am replacing all the devices on the troubled board.
My procedure will be as follows. Replace all the devices and compare resistances between the known good board and the troubled board.
I have two different sets of differential pairs, one J109 pair in the good board and will have a LSJ74 pair in the troubled board.
I'm guessing these will sound different and so I'll need to replace the J109s with the second LSJ74 pair ultimately but for testings sake I'll keep the good board the way it is.
My procedure will be as follows. Replace all the devices and compare resistances between the known good board and the troubled board.
I have two different sets of differential pairs, one J109 pair in the good board and will have a LSJ74 pair in the troubled board.
I'm guessing these will sound different and so I'll need to replace the J109s with the second LSJ74 pair ultimately but for testings sake I'll keep the good board the way it is.
bulb tester only with non- biased A class amp
considering that you can't have non-biased Aleph J , you need different path