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I have a Nashville 400 amp that has low output power. I have been told it could be the DDT circuit. Unfortunately (for me) I don't even know what "DDT" stands for or where it's found in the circuit. I have plugged another power amp into the "pre amp out" and have lots of power. The output of the one I'm working on probably runs at about 20% max. Is the DDT circuit not in the pre amp section? And does it regulate +/- VCC to the op amps? All power transistors (70841140) test good with hfe's of 64 to 79. Also the 3531 and 3532 transistors test good. Q8,9,and 10 collector voltage is +52V and the voltage that should make it to the other three pwr transistor emitters is also -52V. The tone quality of the output is good.
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Link to schematic: PEAVEY NASHVILLE 400 Service Manual free download, schematics, eeprom, repair info for electronics
The DDT is a soft compressor circuit. (IC 8)
Is the Comp LED coming on? (that is part of the DDT circuit, IC 6)
Did you try jumpering Pre Amp Out jack to Pwr Amp In jack?
Did you try sending a signal into the Pwr Amp In jack? -
DDT stands for Distortion Detection Technique and is copyrighted by Peavey.
If your power supply filter capacitors are silver, you should replace them.
I had a similar problem with another Peavey amp, and I was told to replace
the filter caps if they were silver, I did, and it solved the problem.
Make sure the speaker is working, a quick test would be to see if it physically moves in and out.
I have found frozen speakers that still produce sound although diminished.Comment
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It could be the DDT, or it could be a million other things. Someone was guessing.
But if you want to know, power off, remove IC U7 from its socket, make sure to note which way it faces so you can put it back. Now power up and play, if the amp wakes up now, the DDT was involved. If it remains weak, the DDT was not.
You sent the preamp out to some other amp. That is only half the test. YOu also need to plug the guitar or some other signal source into the power amp in jack. I see Jazz already said that.Education is what you're left with after you have forgotten what you have learned.Comment
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SNOOKERED ONCE AGAIN!!!! Or as my father used to say, "Oh well, Back to the drawing board". Another tech had worked on this amp and let it sit for some time. He tried to tell me all about how the DDT circuit was bad. (He read about it on a forum). As it turned out he was looking at the wrong op amp. And I, in all my perfectness, proceeded to troubleshoot further into the matter utilizing all of my brilliant expertise. Now, I SWEAR. I DID CLEAN THE POWER AMP IN JACK HONEST I DID. I EVEN BURNISHED THE SHUNT CONNECTION, THROUGH WHICH EVERY BLESSED NOTE DOTH TRAVEL IN THIS AMP. BUT ALAS ... when I put a meter across the shunt it was found to have a gap ... maybe a thousandth of an inch or so. Replaced the jack all works well. Thanks for the input though. I know more about DDT circuits and how to isolate them.
Has anyone ever written an algorythm for troubleshooting ss amplifiers??Comment
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Peavey... snookered.... different words, same meaning.... have the TEE shirt.Comment
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