Class A mosfet idle issue

[east electronics]

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]

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

 

[east electronics]

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

 

[east electronics]

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 .....

 

[east electronics]

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

 

[east electronics]

kind of you ...thank you very much zen

 

[east electronics]

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

 

[Zen Mod]

sorry - I recently made major cleaning of my workshop , but I didn't found that envelope

 

[east electronics]

thanks fro trying zen ...

will keep it as is and will see

 

[jacco vermeulen]

Take the basic Hitachi note, then add plastic MOSFETs and the typical British minimalistic approach.
Bias it to a few watts in Class A, name it MF something as usual for Englanski products and offer it for a Japanese retail price.

Many years later you expect a miracle to happen ?

 

[east electronics]

hello Jacco ... good to talk to you from time to time since it reminds of the wonderfull Dutch food ( or wonderfully stollen from the indonesian but it doenst matter does it ?? )

yes ...i find your approach very realistic personally i wouldnt mind to burn 250Wof power per chanel if it sounded better than my P3A ... but actually it doesnt ... so i dont bother i will have to limmit the operation only to biass it enough to be stable and thats it

thanks

 

[east electronics]

by the way all the efoort i made to locate some web or phone to talk with the company ended up in a total zero

 

[east electronics]

any body in for a joke ?????

the bias issue was fixed since secondary power supply that feeds the first stages of the amlifier was a mess but not 100%

the problem seemed to stabilize but not solved idoubled checked everything but it was impossible to find something wrong

to refresh the problem that if you install an A meter in the rails to monitor bias and set it up to 800ma for example ...eventhough the bias is stable voltage drop on the source resistors is much diferent from channel A to chanel B

just noticed ...source resistors in channel A are 0R22 and in channel B 0R33

this also is factory mistake ...the amplifier was never opened before somebody in this company was colour blinde

 

[jacco vermeulen]

but it doenst matter does it ??

Lack of QC appears typical for a number of GB products of that era.

The whole idea of stealing is that it is something wonderfull, or one would be better off with the local horrible food.
Sakis, every time i talk to you i'm reminded that some folks are 500 yards away from having snacks and ouzo at a sunny marina.
Moi, i still have to rely on invites for airconditioned coffee with the doors closed.
Shots of the latest aircon one i visited a few weeks ago, the 50m Sky, the yard uses the marina overhere as a docking during sea trials.