Musical Fidelity X-A50 monoblock amp Circuit

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Ianmac

Member
Joined 2007
#1
Recently aquired two Musical Fidelity X-A50 power amps and want to check them out.
I need a circuit diagram- Can anyone help?

Have searched only to find circuit info on the various clones of this amp, not the original.
Many thanks
 
#2
I've seen very few schematics for MF products that weren't just hand drawn on paper, probably by repairers who couldn't get access to technical details back in the day either. The XA50 model is pretty simple though - easy to reverse engineer, I would think. The PCB is probably directly photo-copied too, making it easier to compare using a light source behind the board.

As you already have a schematic of the clones that you can print out and verify, track by track, with a pen and notes as needed, it shouldn't take long by viewing the original board, to correct any changes due to using different semis etc.
 
#6
RM - many thanks for your help - saved me hours of work reverse engineering

One amp has a problem with random noise at switch on which disappears after 2 or 3 minutes. I can now have a 'structured probe' with the scope.

again many thanks for your help.
 
#7
Faults like that often respond well to judicious use of freezer spray. Practice being able to dispense the the stuff literally one drop at a time from the straw and try the input transistors and particularly TR6 and TR8 which look like they could run hot.
 
#10
Yes, there are DC issues with the amp which I have yet to resolve. On switch on it goes to +15v and takes a few minutes to drop to a nominal 0v.
On switch off the -ve power rail stays at -30v until circuit leakage discharges the power caps. Have bought some transistors to allow me to change suspects.
All were relatively easily obtained except the 152N thus my search for the recommended equivalent
 
#11
Interesting fault. The time while the output is at 15 volts is the golden opportunity to try and narrow the issue down.

The output voltage (whatever it may be) should be similar to the voltage seen on TR8 collector.

The voltage between B and E or TR8 should never exceed around 0.7V under any condition.
 
#14
dj_ I did not resolve the problem by component replacement. Issues with getting the exact replacement transistors and the costs were going overboard.
I bought two cloned boards from China remounted the power transistors to fit the MF heatsink assembly and used the original PSU. Measured its performance and put into use. Less than half of what I spent on replacement bits.
Been running fine since.

I concluded that I needed more info on the circuit set up and how to correctly break the various feedback loops to fault find. Issues with the replacement equivalent transistors ( mainly supplied by Cricklewood ) led to my over working the PCB and some of the tracks started to lift from the PCB. On both my boards TR6 and TR8 were getting rather hot for my liking.
Hope you get on better than i did.
 
#16
Ianmac said:
dj_ I did not resolve the problem by component replacement. Issues with getting the exact replacement transistors and the costs were going overboard.
I bought two cloned boards from China remounted the power transistors to fit the MF heatsink assembly and used the original PSU. Measured its performance and put into use. Less than half of what I spent on replacement bits.
Been running fine since.

I concluded that I needed more info on the circuit set up and how to correctly break the various feedback loops to fault find. Issues with the replacement equivalent transistors ( mainly supplied by Cricklewood ) led to my over working the PCB and some of the tracks started to lift from the PCB. On both my boards TR6 and TR8 were getting rather hot for my liking.
Hope you get on better than i did.
Click to expand...


I dont even want to go near it when switched on. I scared it might blow up or something...

This is the weird thing I have tested everything with a Multimeter and everything seems fine.

The easiest thing was to measure the resistors and they pretty much all checked out, the caps are fine also. So the only thing left is the transistors which also checkout in circuit.

What is going on? I dont fancy buying clone boards.
 
#19
dj_holmes said:
Yes, but all the caps measure just fine!
Click to expand...

Ok, you can try remove this cap from the board first, it is safe to run your amp without this cap.

Please prepare a Dim Bulb Tester to test your amp, don't connect speaker first, connect multi-meter to the speaker terminals for the problem channel and fire up the amp to see output voltage any change.
 

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