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Electronics Repair (sci.electronics.repair) Discussion of repairing electronic equipment. Topics include requests for assistance, where to obtain servicing information and parts, techniques for diagnosis and repair, and annecdotes about success, failures and problems. |
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#1
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Pioneer m-4000 amp distortion
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 |
#2
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Pioneer m-4000 amp distortion
In article , "Gary L. woodruff" wrote:
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 |
#3
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Pioneer m-4000 amp distortion
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 |
#4
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Pioneer m-4000 amp distortion
"Gary L. woodruff" wrote in message ... 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 |
#5
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Pioneer m-4000 amp distortion
"Gary L. woodruff" wrote in message ... 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. |
#6
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Pioneer m-4000 amp distortion
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 |
#7
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Pioneer m-4000 amp distortion
"Gary L. woodruff" wrote in message ... 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. |
#8
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Pioneer m-4000 amp distortion
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 |
#9
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Pioneer m-4000 amp distortion
"Gary L. woodruff" wrote in message ... 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. |
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