Pioneer m-4000 amp distortion

I would check all input devices driving the outputs and the bias, etc. Check DC levels.

greg

Thanks Greg, I have been out of the repair end for a while (25 yrs) so could you elaborate? I have checked the input signal of the channel and it looks clean. I do see the distorted output right at the output of the amplifier board.

Thanks, Gary

With faults like this, a good exercise is often to just compare channels using a simple ohm meter. With the unit off, and the main PSU filter caps fully discharged, just hook one lead to ground, and then probe corresponding points between a working channel, and the one that's bad. If the amp that you are working on has suffered blown output transistors, I would be looking for open circuit series resistors in the bases of the output devices. They are usually low value, and it's very common for them to fail when the outputs do. Also, check the output transistor emitter resistors carefully. Although they will usually fail open when the output devices blow, they can also go high, which can be difficult to spot by just doing a cursory run over them with your meter. When a nominal 0.02 ohm resistor has risen to 5 ohms, it still looks pretty much like a 'short', unless you check exactly what the meter is saying, particularly if you are using an analogue one.

Arfa

Look for an open 220 ohm (surface-mount) resistor across the emitter-base junction of each of the output transistors.

Be sure to bleed off the power supply voltages first! There are two sets of higher voltage rails on these. Measure from ALL of the output transistors' emitters to ground - these units apply B+ to the emitters, not the collectors.

Mark Z.

Mark, Thanks for the tip. I have not done too much component level repair in about 20 yrs. I am assuming by surface mount resistor you mean the ones that are so small I cant see them without magnification (20 yrs means I now need bifocals)!. If so, any tips for removing and checking such a small component? (other than don't drop it once removed!)

Thanks much,

Gary

Just measure across the emitter - base junction of each of the involved output transistors, and you can either trace back and find the bad one, or just solder a 1/4 watt or smaller flameproof type across the E-B junction of the transistor that is missing one.

I get them from Pioneer, but not so many these days of course.

Mark Z.

Howdy, Thanks again Mark for the advise. Am I going to measure resistance across E-B? If so, do I have to remove the transistor so I do not seen it in the circuit?

Thanks, Gary

The (good) transistor will not affect the measurement of the 220 ohm resistor.

Mark Z.