Stereo amplifier sqeals at idle, distorts on one channel?

nac134

Member
2008-09-04 1:47 pm
I have a pair of Luzman M120a stereo amps that I run as dual bridged monos. The Left channel on one of them has started to emit a mild squealing sound, even at idle, and even if no source is connected. It also sounds very distorted when I play music through it.
I pulled a bunch of transistors off of the input stage, and tested them - all fine. I put them back and the problem went away for a couple days, then suddenly came back. Does anybody have an idea what might be causing this?
 
Not easily. You could perhaps check a compensation cap by temporarily adding an extra one in parallel - about the same value or a little smaller. If this solves the problem then swap the cap for the correct value. You would need a circuit diagram and the ability to read it. Typically there will be Cdom across the VAS stage, and possibly a lead-lag network at the input stage collector and a Zobel network at the output.
 

nac134

Member
2008-09-04 1:47 pm
Thanks for your help. I do have the schematic and service manual. However, my knowledge of schematics is rudimentary. For example, I don't know what a Cdom is. It will probably be easiest for me to just get out the soldering iron and start pulling of caps and testing them.
 

nac134

Member
2008-09-04 1:47 pm
Here's part of the schematic, with some added artwork, showing the amp main boards for both channels - the one on the left is fine, the one on the right is not.

Highlighted are 4 caps which are all the same, being used in the circuit in the same way. With DMM set to the 2m ohm setting, all of the caps measure (in circuit) around 0.8 one way, and about 1.2 with the DMM leads reversed. The readout is stable, and does not move up or down.

However, the one on the far right measures differently. The impedance climbs as if you were measuring a medium sized cap by itself. With the leads place in either polarity, the impedance will eventually climb over 2m ohm. I have taken some of the nearby parts off the board to measure, (q202, q204, c201, c203) and they all check out.

Any ideas?
 

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nac134

Member
2008-09-04 1:47 pm
I traced the problem with the mystery impedance to a bad power supply connection. But the performance is unchanged - it still has the same horrible distortion problem. Im going to switch the connections from the input boards and see if the problem stays in the same channel or switches sides. That way I can determine if the problem is in the input section or the main section. I hope its the main - the input board is way tighter.