My Musical Fidelity Musical Fidelity XA-2 has suddenly lost some of it's depth and transparency of sound.
First is to check to make sure the power supply is putting out enough power.
From your description, it sounds like the classic signal "clipping" situation.
Since I tend to work from experience repairing a wide range of things, and I am not finding any references to your amp, not even in Russian sources, not even a picture, I have to make some assumptions for the moment, but this will help locate the problem.
Solder some test leads to an impedance matching transformer and the other winding to a plug that will fit your good headphones.
With chassis underside exposed, power up the amp, and apply a song you know well to the input. Use the test leads to check the 4558 IC's that are most likely to be found in the preamp stage(s) of the amp. and follow down the audio chain until you reach the power hybrids. If you happen to have the discrete transistors, those are better to work with and much easier to test.
If I am testing an amp in this way, I am listening for a dead spot, or a spot where the signal is very distorted. Being careful not to touch the power buss. Simply putting a test clip on one lead and attaching it to chassis ground is the easiest and fastest way to run through an amp.
if I am checking discrete transistors, I am just checking at the base lead of each one, or the gate if it is a FET until I find a distorted stage.
If you have power hybrid IC's, check at the website of the manufacturer of the IC for data sheets on that chip. Use those as a guide for checking voltages for the hybrid, as well as to help potential shorted components such as a capacitor or a resistor that may have failed.
If I know more details of the chips the amp uses, discrete transistors, or even if it happens to be a tube amp I can be much more specific.
Other things to check that are not always apparent- make sure there is a +15 colt supply and a -15 volt supply with respect to chassis ground when dealing with 4558 amp chips.
Feb 03, 2012 |