musical fidelity synthesis

[latala]

I have a friend who has given me his amp for repair !
has any one got the schematic for this
problem is a distorted right channel dc offset etc seems to be ok
regards Trev

[latala]

I have opened the amp up Its a mess right channel has burned up drivers and burned out resistors It seams to be a mosfet unit.
The later hitachie plasic ones luckily i have a few sets of these!
Does any one out there have a shematic of the output stage ?
regards trev

[latala]

I am looking at this amp at the moment To be honest in my opinion it does not seem to be of a great build quality! The pc tracks around the driver /output pair seem to be very fine in fact too fine!!!
The output stage is mosfet driven by a mj350/mj340 pair in a source follower mode ?
Does anyone no of a similar output stage ?
I have contacted MF to be told they are all on a 2 week holiday
just my luck
regards trev

[latala]

Looks to me as I am all alone with this one all though i have received a leade fom an ex engineer from MF he tells me it a basically the hitachie mosfet circuit !
What a poor piece of equipment this is I have found that a neg rail decoupler capacitor has gone short and litterally blown the print off the board!!!
I am glad i never purchased one of these all though it did sound quite nice
the drivers seem to be in some sort of cascode configuration does any one have a drawing ?
regards Trev

[latala]

The circuit is derived from the hitachie application note
However the diif input pair are fed from a constant current circuit
The output from the diff pair go to a cascode connected set of drivers an improvement in the classic active sink source hitachie circuit ?
but to be honest it about the worst board I have ever seen !
I still would like a circuit though to see if my diagnoses is correct
Regards Trev
The adventure goes on
ps I still want to prove my high voltage driver but untill I get this out off the way I can not go forward

[anatech]

Hi Trev,
If it is the old Hitachi circuit, why not search for the app notes that may be on line? I don't have anything on it at all, sorry.

-Chris

[jaycee]

You could always e-mail MF and ask them if they'd be nice enough to send you the schematic.

Perhaps it is similar to AmpsLab Lm60 amplifier - this is very close to the Hitachi schematic. Take a look at http://www.ampslab.com/lm60_schematics.htm

[latala]

Thanks for your kindness Gentlemen
The schematic is derived for the Hitachie but has a lot of major differences hence the request !
I know the hitachie circuit very very well and I can draw it from memory !
I have contacted MF who tell me there tech dept is on Holiday for a few weeks so I suppose I will have to wait or press on the hard way
Regards Trev

[latala]

Well its done
The amp is now repaired and tests out well! The problems
faulty power supply decoupler at the board 100 uf 63 vdc short this then vapourised a piece of earth track this had the effect of diconnecting the Zobel network and the bottom leg of the feedback network.
A diode in the protection to the gate of the sk fet was also leaky causing the amp to be assymetric .
Also the string of 4 diodes (1n148 )that held the bases of the driver cascodes mje340 were either OC or sc this caused the mje340 to turn on fully and blow the bottom of the cascodes bc184 to catch fire
Schematic ?
Its a version of the Hitachi circuit but has the following changes
The first diff pair are fed from a constant current circuit that is also coupled to the preamp section psu in order to give a silent turn on/off.
The driver stage is a cascode formed from a pair of bc184and mje 340
On replacing the above components its a winner and now works well.
Conclusions?
Very nice sounding
Poor PC board
Typical of the amplifiers in its price range(£450) and time
I think esoteric is another way of overcharging for poor build quality'
Regards Trev