Pioneer m-4000 amp distortion

[Gary L. woodruff]

Howdy, I am new here and am wondering if anyone has a print or can help
me with this repair. Amp came to me with blown output chips. I cross
referenced the obsolete transistors and replaced all 4 outputs. I now
can power up and have a nice clean signal on the left channel. The right
channel is very distorted and attenuated. Any help would be greatly
appreciated.


Gary

 

[GregS]

Howdy, I am new here and am wondering if anyone has a print or can help
me with this repair. Amp came to me with blown output chips. I cross
referenced the obsolete transistors and replaced all 4 outputs. I now
can power up and have a nice clean signal on the left channel. The right
channel is very distorted and attenuated. Any help would be greatly
appreciated.

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

greg

 

[Gary L. woodruff]

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

 

[Arfa Daily]

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

 

[Mark D. Zacharias]

Howdy, I am new here and am wondering if anyone has a print or can help me
with this repair. Amp came to me with blown output chips. I cross
referenced the obsolete transistors and replaced all 4 outputs. I now can
power up and have a nice clean signal on the left channel. The right
channel is very distorted and attenuated. Any help would be greatly
appreciated.


Gary

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.

 

[Gary L. woodruff]

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

 

[Mark D. Zacharias]

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.

 

[Gary L. woodruff]

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

 

[Mark D. Zacharias]

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.