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Old 20th July 2011, 01:50 PM   #1
vingenz is offline vingenz  Italy
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Default Problem with Peavey amplifier

Hi, I have a peavey special 150 and I can hear a distorted sound in the
clean channel(like the lead channel distortion) in the background,
especially when I play very soft.
Why is it doing that?
I've turned down the lead ch. knobs but...nothing change!
I have also changed the speaker and relay switch but I still have got the same problem.
I put the jack in the power in socket and that noise wouldn't go away.
I think the problem might be the final section of the amp. Besides,
while I was looking at the circuit again, I found some transistors
that have been replaced, and they are different from the original
ones. Q5 has been replaced with a bd137 (original transistor MJE243),
Q10 has been replaced with a bd138 (original transistor MJE253) and Q7
has been replaced with a C33725 (original transistor MPS6531 or
2n3642)...here's the schematic http://www.mediafire.com/?wn5koz2cnyj and sample video ‪Peavey special 150 noise‬‏ - YouTube and test with oscilloscope ‪Peavey special 150‬‏ - YouTube
I even changed all the power transistors but the problem remains the same: noise at low volume.
Tnx!
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Old 20th July 2011, 05:37 PM   #2
Bone is offline Bone  United Kingdom
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Low level distortion like that is often crossover distortion from the output transistors. Changing those transistors has probably upset the biassing of the output transistors as these are their drivers.
Q7 shouldnt affect anything as it is normally turned off, and only turns on if there is an overload.
Unfortunately the circuit diagram doesnt give typical values of bias current through the output transistors, and with this determined by diodes (CR25,29,30,31,37 etc), there is no way to conveniently adjust it.
One thing you can check - what are the voltages across R73,78,93 & 98 and are they all the same?
Is there anyone with a similar amp who could do some measurements?
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Old 20th July 2011, 06:12 PM   #3
jaycee is offline jaycee  United Kingdom
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The use of the BD137/138's in place of MJE243/253 is a poor substitute.
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Old 17th October 2011, 10:46 AM   #4
vingenz is offline vingenz  Italy
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Hi, I find a resolution to this issue: replace the 1.5k resistor (R84 on schematic) with a 470ohm 1 watt resistor. Thanks!
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Old 17th October 2011, 01:07 PM   #5
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Quote:
Originally Posted by vingenz View Post
Hi, I find a resolution to this issue: replace the 1.5k resistor (R84 on schematic) with a 470ohm 1 watt resistor. Thanks!
I don't think that's a 'cure', it's just a bodge that's masking the real fault.
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Old 17th October 2011, 04:36 PM   #6
vingenz is offline vingenz  Italy
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Probably the the real fault is in bad biasing power transistor, maybe due to bad dual diode sz13886? (but the multimeter say that's good). I had the idea to swap the resistor by looking this schematic: marantz Photo Gallery by bob at pbase.com
There is a bias adj in order to eliminate the crossover distortion.
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Old 18th October 2011, 08:03 AM   #7
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The Marantz is a 'proper' design (with a Vbe multiplier), the Peavy circuit is really a pretty poor design.

You also haven't altered the bias as on the Marantz circuit, you've simply unbalanced the entire design.

The bias is set by the two diodes in series, NOT the resistors.
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Old 18th October 2011, 01:09 PM   #8
vingenz is offline vingenz  Italy
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Well I'm lucky. Tested on the oscilloscope, the sinewave is perfect and simmetrical. With a 4ohm load the amplifier puts out 128 watts. Porbably before swapping the resistor the power stage was unbalanced.
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Old 18th October 2011, 06:14 PM   #9
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Quote:
Originally Posted by vingenz View Post
Porbably before swapping the resistor the power stage was unbalanced.
Possibly - but because there's something wrong with it - you should repair the fault, not blindly bodge it until it just happens to work
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Old 18th October 2011, 07:02 PM   #10
vingenz is offline vingenz  Italy
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Quote:
Originally Posted by Nigel Goodwin View Post
Possibly - but because there's something wrong with it - you should repair the fault, not blindly bodge it until it just happens to work
as long as I can't afford to happily spend over 20 euros (Diode Dual Peavey Diode Dual SZ13886-DZ89052 30413886 - Stagebeat - DIODE DUAL SZ13886DZ89052 SZ13886-DZ89052- 70413886 30413886) to blindly try and solve the issue, I am happy the way it is
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