Those of you who have read my whole blog, may remember the BLA 350 in for repair some time ago.
If not, read the article here.
In the mean time, the amp has changed hands and moved to a new owner. The Old Man brought it in complaining that it only had a max. output of 45 Watts.
The case was opened again and the fuse removed, so that the MOSFET bias current could be measured. The current at idle was 125 mA. Way too low for this amp. It should be 500 mA in total, with 250 mA per MOSFET.
Therefore I reduced the bias voltage on MOSFET1 and it immediately went to zero. This meant MOSFET2 was not drawing any bias current.
MOSFET2 was removed an put on the tester. It showed up as two resistors in parallel. Obviously, the magic smoke had escaped once again. What caused it? Maybe the Chinese components that the first Old Man bought as replacements were not as reliable as the originals? Who knows...
A new set of MOSFETs was procured and soldered in place.
Once the bias was set on both MOSFETs, the amplifier came back to life again.
So why did the MOSFETs fail? The last ones were procured from China at a fraction of the price of originals. Were they badly built? Were corners cut? Who knows. It may also have been due to operator error.
