"Phil Allison" wrote in message
...
"Phil Allison"
http://www.users.on.net/~fzabkar/tem...ath_CP1000.jpg
** In the above schem, 2.7 ohm resistors R35 and R36 are each bridged by
the B-E junctions of three power transistors and their associated 0.15 ohm
emitter resistors. It is therefore not possible for the 2.7ohms to burn if
the connections are good.
In normal operation the standing voltage across each 2.7 ohm is about 0.4
volts - ie not enough to bias the output transistors into conduction. So
the two mosfets drive the load at low power and the other devices come in
to assist above about 100mW into 8 ohms.
Is that schem really correct ???
Hard to believe there are no zeners protecting the gates of the mosfets -
if the output is shorted, the gates can be overdriven and the mosfets
destroyed.
Also hard to believe the rail fuses connect DC to the *whole* amplifier -
cos if one rail fuse blows, the output looks bound to swing over to the
other rail resulting in relay and speaker destruction.
If instead, the rail fuses were only in the supplies to the 6 power
transistors and DC is applied permanently to the rest of the amplifier -
then burnt R35 and R36 resistors and dead mosfets are easily explained.
It happens whenever fuses blow and these parts then have to carry all the
current to the load.
Finally, 1 kohm resistors R30 and R 33 can burn if the associated mosfet
fails - cos the dead mosfet will have a low resistance from Gate to
Source so Q15 and Q20 will drive current through them to the load.
.... Phil
Thanks very much for the analysis.
Changed Mosfets thursday, but has now blown 4 out of 6 output devices on
power up. (no load)
Been away since then, will tackle this very carefully again wednesday.
The 2 fuses had not blown by the way, the front panel limit LED was lit
though. They are blown now.
It looks from your analysis and the schematic, that the amp should power up
into its quiescent state with no 2SC/2SA devices present at all. (with
R38/41 in circuit)
Is this correct?
Is this wise or helpful?!
Gareth.