Hi Peter, I think you said when we chatted that you couldnt tell the
difference with a test meter (ohm-meter) The reason ofr this is the battery
in an Ohm-meter is usually 1.5 volts. This is sufficient to turn-on a BJT
but not sufficient to turn on a power FET of the type used in the Decca. The
Audio FETs in the B&K may be different as they are used in linear audio
applications.....they probably start to conduct at much lower gate source
voltages. I suggest a little test rig with an ohm-meter across Drain-source
(I dont think it matters which way round as the channel is resistive). Then
use an external battery (PP3) or a PSU to put +9v through a 10k0
(series....i.e.current limit) resistor on the gate. Most of these power
switching FETs take about 6 volts gate-source to switch on....hence no
reading either way with an 'AVO'. The meter in d-s should be high
resistance and swing to very low resistance when the 9v is applied to the
gate..my IRF640s read about 2.6ohms on an Altai h/h DMM when on and
fullscale when off.
I think that any failure would mean that this sequence could not be
repeated. I am not sure how they fail, but the aftermath of the initial
fault could well be a ball of melted and recryatalised silicon. The high
resistor in the gate lead should ensure that if the gate has broken down
there will not be enough volts to switch the device on. A quick check could
be made directly across g-s to show that all the 9v was being dropped across
the 10k0 resistor. If the FET has suffered from over-current it will show
resistance between d-s with no gate drive I think. Maybe those who have
fryed fets will confirm.....I have not lost a single FET to my Class E
experiments !!
Cheers de Alan G3NYK
[email protected]
I hope this helps
|
