Help with strange failure on Aleph
Amplifier is the amazing Aleph 5 (stereo amp in the chassis of one mono Aleph 2, same circuit design, half the mosfets per channel - only six).
After a few minutes of warm-up the right channel starts to fail - it drops a few dbs and sounds bad (under-biased). Left channel is OK. Tests were then made in idle, with no audio inputs/outputs.
If top cover is removed the problem is minimized (partial recover of right channel), thus making harder to measure possible origin of the problem.
Total AC line draw (240V) is 1.05A before failure and then drops to about 0.65A (sometimes oscilates between). After a while it is clear that dissipators related to right channel are cooler than the left channel ones.
By removing specific side panels I observed that the voltage on the IRF244-mosfets' source resistors on right channel drops from 0.6V to about 0.1V-0.3V after the failure occurs. It happens on all the six 1-ohm resistors.
Hard to measure with top cover removed as it recovers but no difference found between both channels - only minor differences from model specs.
Supply voltage: 39V (higher than 34V specs)
R11 voltage (DC offset adjust) - 5.8V on both channels (above 4-5V specs)
Q5 - 4.9V both channels (biased by R17, 26V and R18, 8V both channels). 26+8+5=39V
Power mosfets' source resistor at 0.6V on L-channel and oscilating from 0.1 to 0.6V on R-channel.
Any clew on what may be the cause of this problem?
- supply caps not original 50V 31000uF x 4 but 63V 47000uF x 4
- 220uF caps (3 per channel in an Aleph 5/2) not original 16V but Elna Serafine 25V (which are taller but keep clear from top cover).
The amplifier worked well for over a year with these mods.
it needs thorough check-up of all solder joints
same as - simplest way - replacement of all parts in front end - 3 mosfets and few diodes
Bad developments... The malfunction became more critical. Right channel went dead. It measured 0.8V on R17 (should be 26V), 33V on R19 (should be 12V), no current (0V) on R18 (should be 8V). Turned it off, waited a few secs, turned it on again, and the little MPSA18 just burned out - flame and smoke.
Is the bipolar MPSA18 prone to secundary breakdown?
As I recall, there are two of those npn devices. Neither is subject to
secondary breakdown, and both are operated in the 4 to 5 volt region.
Of course a failure in the amplifier could blow them out. The top one is the
control regulator for the current source, and the bottom one is a current
limiter. You can get along without the bottom one.
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