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 Post subject: M-3000 Mono Amp help needed
PostPosted: Sun Jan 15, 2012 10:57 pm 
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Joined: Sun Jan 15, 2012 10:00 pm
Posts: 2
Location: Howell, Mi.
I received a M-3000 Mono Amp with driver board #99309130. It has a shorted output transistor Q18 and a burned resistor R43. I replaced all of the output transistors, mounted driver transistors Q5 and Q10 on larger heat sinks, replaced the PS caps with 12000uF electrolytics and built a soft start for the power supply. I installed 8A time delay fuses on the +52 and -52Vdc leads to the driver board.
I slowly powered it up with out any input signal or load on the output using a Variac and at 80VAC it popped both of the 8A fuses but did not smoke anything. I replaced the fuses with 2A AGC fast blow fuses and disconnected P7A and P8A connectors from the driver board (disconnecting the output transistors from the driver board). I powered it up again and at 60VAC it smoked R43 and R51. Could leaving the output transistors disconnected cause both of these resistors to smoke? I am attaching a schematic for the M-3000 for reference. Any help would be appreciated.


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M3000 power amp schematic 615.pdf [954.27 KiB]
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 Post subject: Re: M-3000 Mono Amp help needed
PostPosted: Mon Jan 16, 2012 8:04 pm 
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Joined: Fri Sep 12, 2008 10:24 pm
Posts: 5913
Location: Lansing, Michigan
First suggestion. An amp like this didn;t fail because it was mis-designed. Altering the circuits and changing things adds more ways for the unit to elude repair. Always - in my opinion - make the amp work first, THEN do any modifications.

This amp is pretty much the 400BH power amp board, (used in MANY Peavey amps) by itself with enough stuff thrown in to make it a stand alone power amp. Solid reliable circuit.

Just in case, you do know that of the 8 power transistors, the end ones, Q11 Q15, are wired as drivers, even though they are all the same type. That makes Q5 Q10 pre-drivers.

If R43 burned, it is almost certain Q5 was bad as well. And as soon as I see that, I instantly suspect Q10, R51, plus R49,50. That while vertical stripe through the schematic. If you are replacing the whole output section, then no time to be cheap on the preadrivers for the couple bucks they cost.

You replaced all the outputs? 8 or 6? Usually just a couple fail, one per PS rail, but all is OK. WHat did you replace them with? Please have them all be the same part type. The schematic wants 6357. That part has a newer part number of 70484140. Those are interchangable. The generic for those house numbers is MJ15003. You can mix those as well. I hope we didn;t go to NTE parts. Never mix them. All 8 NTE is better than a couple NTE mixed in with Motorola. Of course NTE will cost mutliple times what MJ15003 cost.

The variac is a powerful tool, I keep mine next to me, but just as important as the variac is the current meter watching the current draw. The point of the variac is not so much the slow rise in voltage, rather we want to slowly bring it up WHILE WATCHING mains current draw. If the unit current starts to ramp up, back off immediately. DO not keep turning it up until fuses blow, that defeats the purpose. You need to watch current and back off before something burns.

No, do not operate the amp with P7 P8 disconnected. The circuit expects the current path through the outputs to be there. WIthout it, ther is nothing but those little 47 ohm resistors to complete the circuit. If R51 burnt, then expect Q10 to be bad

I buy MJ15003 from Allied or Mouser. But PV prices are not outrageous. The little 5331 and 5332 predrivers I always just buy from PV. You can sub them, but the real ones are not expensive, and the wire leads come preformed to fit the boards.

If enough current went through R43 and R51 to burn them, I'd assume Q5 and Q10 were stresed in the process. SOrry.


MY approach for this amp, and the 400BH and similar it resembles, would be to check each power transistor for shorts, verify all the cement power resistors as not open. Don't forget the R52, R53 in the middle there. Check Q5 Q10 and all the resistors in that vertical circuit. Work without a speaker load until we have the amp stable. I'd verify the extremely important dual diode that P6 connects from the power board to the main board. That is CR12. It should test like two diodes in series - about a volt drop instead of half a volt. Also make sure CR13 with 33 ohm resistor in parallel. If any of that diode-ness is open, then both polarities of output will turn on hard at once, blowing fuses. Above them are a couple power resistors 2k 5w R36, R37. Either open?


SOmething I used to do, when working on a blown up amp, was make a work copy of the schematic. Never draw on your real schematic, if you need to scribble, make a copy for that purpose. If I find a burnt part, I use a yellow marker and highlight the part. IN a well blown amp, this can lead to a number of parts colored in, and it reveals a failure current path. And that can suggest parts to check or replace. For example If R43 and R51 burn up and Q5 is shorted, coloring them in leaves a gap in our yellow line - Q10. SO we suspect Q10 in that case.

Start with that stuff and see where we get.


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 Post subject: Re: M-3000 Mono Amp help needed
PostPosted: Thu Feb 09, 2012 3:07 am 
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Joined: Sun Jan 15, 2012 10:00 pm
Posts: 2
Location: Howell, Mi.
I found the problem after checking nearly everything on the right side of the board. It turns out that U1 the TL074 was bad and putting out DC. I replaced the TL074 and now I have a good clean signal on the speaker outputs with a 1V PP input sine at 100Hz but I am only seeing 8.25V RMS at the speaker output with no load. When I place a 8 ohm load on the speaker outputs the voltage drops to 2.41V and the clip light comes on. The clip light comes on whenever I set the input level control to 5 or higher and the voltage at the speaker output is >2.24V. It goes out when I adjust the input level to 4 and the speaker voltage drops to 2.22 or less. I pulled U2 but it didn't make any significant difference in the speaker voltage. When I pull out the DDT defeat switch and turn the input level to full (10) the speaker voltage increases to 4.60V and the bottom half of the output sine is clipped flat but the top half looks good. I have pulled and checked every transistor on the right half of the board, checked every diode, resistor on the board. Checked every resistor and output transistor on the output board and all look good.
I am getting perplexed and could use some help getting this to put out full power. Any suggestions on where to start looking?


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 Post subject: Re: M-3000 Mono Amp help needed
PostPosted: Thu Feb 09, 2012 6:48 am 
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Joined: Fri Sep 12, 2008 10:24 pm
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Location: Lansing, Michigan
Progress is progress. The TL074 was a problem, if not THE problem anyway.

Pull U2 until we have the amp working, it can only get in the way at this point. The clip circuit controls it, not the other way around.

I'd feed the amp a nice healthy 1vRMS test signal, not the 0.35vRMS that 1vpp represents. That won't solve the clip problem, but we do need to see how high we can push the output. Line level is a volt.

I don;t recall here. Are both HV rails up to voltage and clean? DO they remain that way under driven load? Are both 15v rails up to voltage and clean? They do more than just the op amps.

Worry about the clip light later, without U2 it can;t do anything about it.

With no load, scope the output, test signal at input. I'd leave the volume control at max and control levels with the signal generator. How large a signal can you get on the output, and how large must the input signal be to get there? First thing we ned to do is get a full level output signal with no load. It looks like the rear panel says 28v RMS at ful out. You are getting 8v now with a third of a volt in. Triple that to 1v in as I suggested, and that ought to about triple the output, which would put you close to spec. SO we may not have a problem there, but find out.

ANy clipping or other anomalies?

When an amp makes a clean signal unloaded, but collapses under load, I usually suspect the output stage as being unable to source the current the load is demanding.


Geez, I forgot to use your schematic instead of my 400BH for a minute, the part numbers are different, and I got confused. OK now.

You tested all the transistors and they test OK? Did you test little limiter transistors Q6 Q7? Disable them by lifting CR14, 15 until we are done. Hell, if that makes the difference, replace those parts and see if the amp works. And verify resistors R44,45 etc down in there. Looking for ones burnt open.

Don;t convince yourself all problems are bad parts. An open connection or broken copper trace between two points is not a rare problem and can be just as deadly to an amp. For example the connection betwen Q10 and Q15. If it breaks, the amp can still make an output, but cannot drive the finals on that side, and they are current amplifiers for the load. Load clips.


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