Once the rebuild is complete then the testing begins. Those who are fortunate enough to have the test gear check S/N, THD, frequency response etc. Something I don't hear much about is current sharing in the output stage. The test is pretty simple. You drive the amp into a load and check the voltage drop across the emitter resistors. If all of the outputs are functioning properly then the voltage drop is the same. The procedure is well defined in all of the P/L manuals. Another test is the short circuit (shorted speaker or cable) that doesn't get much attention either. This is also defined in the manual.
With a complete rebuild and all new parts, it may seem safe to assume all is well. If all of the new outputs come from the same batch there's a good chance they are all the same. Although I tend to agree, experience has taught me to verify everything.
On a recent rebuild of a 700II I was tempted to install all of the outputs and then do the load share test. I've probably changed out a couple thousand outputs and never seen a bad one. Never the less I put them all on a curve tracer just to verify operation. To my surprise one was open. I tested it probably 8 times in the test set with the same result. Each time the oscilloscope indicated an open transistor. Finally I changed the base drive setting on the tracer and realized it wasn't open, but instead had a gain of only 4 which is less than 1/10 of the normal gain. I marked it so that it would get used and set it aside.
A few days later a friend stopped by and in the conversation we cut open the transistor to see if anything seemed out of place to the naked eye. Nothing did seem out of place but then we got to thinking what do other outputs look like with their lid removed.
The photo shows several different outputs. From left to right is XPL909, FPL909, MJ15024, MJ15025, and MJ21196. The FPL909 and MJ15025 are both shorted but the naked eye couldn't tell any difference. What is interesting is the size of the die has increased from the XPL to the MJ series. Also the later devices have a copper heatsink taking up as much area inside the can as possible.
What does this mean to us? Probably not a whole lot. It was fun looking under the hood.
With a complete rebuild and all new parts, it may seem safe to assume all is well. If all of the new outputs come from the same batch there's a good chance they are all the same. Although I tend to agree, experience has taught me to verify everything.
On a recent rebuild of a 700II I was tempted to install all of the outputs and then do the load share test. I've probably changed out a couple thousand outputs and never seen a bad one. Never the less I put them all on a curve tracer just to verify operation. To my surprise one was open. I tested it probably 8 times in the test set with the same result. Each time the oscilloscope indicated an open transistor. Finally I changed the base drive setting on the tracer and realized it wasn't open, but instead had a gain of only 4 which is less than 1/10 of the normal gain. I marked it so that it would get used and set it aside.
A few days later a friend stopped by and in the conversation we cut open the transistor to see if anything seemed out of place to the naked eye. Nothing did seem out of place but then we got to thinking what do other outputs look like with their lid removed.
The photo shows several different outputs. From left to right is XPL909, FPL909, MJ15024, MJ15025, and MJ21196. The FPL909 and MJ15025 are both shorted but the naked eye couldn't tell any difference. What is interesting is the size of the die has increased from the XPL to the MJ series. Also the later devices have a copper heatsink taking up as much area inside the can as possible.
What does this mean to us? Probably not a whole lot. It was fun looking under the hood.
