Hello Chris and Bo

Thanks for your advise on both counts, i do not have variable power supply
to try and switch on these fets in order to measure them, but i have found
the idling current pre-sets and if i turn them fully anti clockwise the
oscillation stops, doesn't amplify much either, but by what you say about
idling current this could be the problem, also the gate of one fet has a 220
ohm resistor to it and the other 10 ohm resistor from the pre driver
transistors.
I am working tomorrow so i wont have a chance to look at it until tomorrow
evening or Monday (i will be glad of the break).
But i will get back to you.

Out of interest this is an old Hitachi HMA-7500 mk11 power amp and matching
pre-amp set up, that i have had sitting around for a very long
time..............then one day i thought i would have a look at
it...............*grin*

Thanks again
Gary


"Chris Morriss" wrote in message
...
In message , Gary
writes
Hi

Chris have downloaded PDF document for Exicon Mosfets and the pin out is
the
dame as the Hitachi ones, so i have just tried another brand new set of
Exicon Mosfets and the same problem exists, the power meters are showing
20
watts of output, i listened on some headphones with gain on the
headphones
turned down low and all i am getting is a raucous noise like instability,
if
i put the original Hitachi Mosfets back in it works perfectly.
Hmm i wonder if these Exicon Mosfets are as universal as they claim they
are, maybe the amplifier i am using requires some sort of modification to
it
so i can use them.

Thanks
Gary
"Chris Morriss" wrote in message
...
In message , Gary
writes
I tried replacing a set of Hitachi 2SJ-49/2SK134 mosfets with Exicon
MOSFET
Direct replacements from Maplins, i could not get them to work at all,
has
anyone else tried these replacements?

Gary



Check the pinout. The old Hitachi parts (as I remember) didn't use the
industry-standard pinout.

--
Chris Morriss


Does the amp use source-follower outputs? If so, you might be getting
RF oscillation of the output stage. This will really bugger things up.
I guess you don't have a 'scope so you can't check if this is happening.

You might be able to cure things by putting a 100 Ohms resistor in
series with the signal to the gate of each FET. Put the resistor right
up by the gate pin. This often cures this sort of problem.

The point of course is that the quiescent current is likely to be wrong
as a result of the threshold voltage and transconductance of the new
FETs being different from the old ones.
--
Chris Morriss