Class A mosfet idle issue

[sakis]

that is an issue i ve never seen before and i need some help

there is a Magnum class A amplifier that has unregulated power supply of 42+42 volts dc to drive 2pairs of mosftes per amplifier at 800ma idle about ...

first stages of amplifier work with regulated psu of 56+56 volts coming from a pair of 317 337

now biassing the amplifier using a ma meter at positive rail fuse and make sure that idle is at 800 ma at both chanels after a long warming up period was simple ...and bias was stable ..

But .... voltage drop on source resistors of chanel A was 80 mv and voltage drop on chanel B was 140 millivolt each and every 0R33 resistor had less than 2% tollerance so that means that semis are closelly matched

it is obvious that in the first measure something is wrong and there is current consumed elsewhere except output stages

have no schematic and seems that UK made machine is a pain in the a@@ to crack open ...

will see though
any input will be very usefull

[Zen Mod]

ask Magnum ;

they were pretty helpful and kind in past

[event horizon]

Originally Posted by sakis

it is obvious that in the first measure something is wrong and there is current consumed elsewhere except output stages

Are any of the PSU electrolytics fitted to the circuit board?

I'm simply asking as i used to find SMPS electrolytics that leaked onto PCBs tend to make the PCB rather conductive It might not be due to a large electrolytic either, the end result is a useless PCB.

Just a thought & i hope this isn't the case as the only cure is to cut away the area impregnated with electolyte

[eyoung]

cut the power to the outputs and see what the driver boards are pulling. I use a 1r resistor in place of the fuse and read voltage drop across so you don't have current meandering through your leads and meter...

Regards, Elwood

[sakis]

Originally Posted by event horizon

Are any of the PSU electrolytics fitted to the circuit board?

I'm simply asking as i used to find SMPS electrolytics that leaked onto PCBs tend to make the PCB rather conductive It might not be due to a large electrolytic either, the end result is a useless PCB.

Just a thought & i hope this isn't the case as the only cure is to cut away the area impregnated with electolyte

cant be since if leacky electros will 100% result to some voltage drop since its class A and there constand current draw

[sakis]

Originally Posted by eyoung

cut the power to the outputs and see what the driver boards are pulling. I use a 1r resistor in place of the fuse and read voltage drop across so you don't have current meandering through your leads and meter...

Regards, Elwood

thanks but no need to do that ...( its a lot of work also to isolate first stage and out ) it is obvious from voltage drop on 0.33 resistors that some ammount of power is consumed in a first stage .....

[sakis]

Originally Posted by Zen Mod

ask Magnum ;

they were pretty helpful and kind in past

yes i can also do that thanks ZEN but i actually cannot find any info on the internet regarding this company can you point me something ....

[Zen Mod]

I'll try to find envelope with addy

[sakis]

kind of you ...thank you very much zen

[sakis]

any news on that one ????

managed to solve the problem ( located in regulators and generally everything is boiling inside )

it would be very nice to have some info on the original bias ( now is set to about 700 ma ) and that is enough to rise temperature of 65 degrees c on tha main heat sink

other than that the amplifier is nothing that surprized me so much of class A i was expecting it be something magic but is not