blackoctagon
New Member
Hello,
This all started with a Plinius P7 power amplifier that had been sitting for the last 8 years at a local repair shop. Owner contacts me and asks if I can get it working again, I agree to take a look. Of the 4 main filter capacitors, 2 were original, one was missing and the other was a cheap one which had vented its top. Tested the transformer and the outputs, all good. Replaced the caps, bias trimmers and small capacitors. Got it working. Tested for days, bias held up, no excessive current draw, sounded great. Owner takes it back, runs it for a week, no overheating and then one day it blows all 4 fuses, two on each channel. Owner says it got really hot and then no sound on the left, followed by the right. Take it back, its blown one channel but the other is ok, replace the fuses and get sound and bias stable on one side.
While I was at the owner's place, he had replaced the power amp with a Pro-ject class D power amp. He still had the old Plinius M7 preamp hooked up. I noticed a loud buzz from both speakers. The buzz is volume dependent, so if the volume is all the way down, no buzz. Owner says the buzz has always been there, but the Plinius power amp never produced a buzz from the speakers 8 years ago. Apparently he noticed the buzz when the Plinius power amp died the first time and then he sent it to a repair shop and they changed the main filter cap. He got it back and the first time he played it, it vented the cap and smoke everywhere.
So I start removing all sources attached to the M7 preamp. No source connected, only cable is going to the power amp. Still buzzing. I pull all power plugs except the pre and power. Still buzzing. I pull the preamp out and measure the rca outputs. 140mv and climbing on both output RCAs in reference to RCA ground. This is with nothing connected to the preamp except the power cord. I bring the preamp back, hook up just the power cord and test, nothing. 0mv on both left and right RCA outs. Left it for a few hours, nothing. No DC on the outputs.
So we have the following:
140mv dc on the preamp outs with nothing but power cord connected
Power amp with all rail fuses blown so cannot be a channel fault. But one channel did blow two out of four output transistors. Also the amp got very hot before it blew the fuses. Was running fine for a week before that and never got hot. In fact the owner ran it very loud with the volume pot consistently around 11-12 o'clock for hours on end everyday after it got back and the amp barely broke a sweat. No heat.
With Plinius pre connected to Pro-ject class d power amp, loud volume dependent buzz.
I have checked his power sockets, they're all wired properly. I thought I would find a loose neutral at his distribution box. I found a few loose grounds and tightened them but no effect to the buzz. Preamp shows no symptoms when tested at my workplace.
Any ideas? I'm thinking excessive DC on his mains maybe?
Appreciate any thoughts on this. Thank you so much in advance.
This all started with a Plinius P7 power amplifier that had been sitting for the last 8 years at a local repair shop. Owner contacts me and asks if I can get it working again, I agree to take a look. Of the 4 main filter capacitors, 2 were original, one was missing and the other was a cheap one which had vented its top. Tested the transformer and the outputs, all good. Replaced the caps, bias trimmers and small capacitors. Got it working. Tested for days, bias held up, no excessive current draw, sounded great. Owner takes it back, runs it for a week, no overheating and then one day it blows all 4 fuses, two on each channel. Owner says it got really hot and then no sound on the left, followed by the right. Take it back, its blown one channel but the other is ok, replace the fuses and get sound and bias stable on one side.
While I was at the owner's place, he had replaced the power amp with a Pro-ject class D power amp. He still had the old Plinius M7 preamp hooked up. I noticed a loud buzz from both speakers. The buzz is volume dependent, so if the volume is all the way down, no buzz. Owner says the buzz has always been there, but the Plinius power amp never produced a buzz from the speakers 8 years ago. Apparently he noticed the buzz when the Plinius power amp died the first time and then he sent it to a repair shop and they changed the main filter cap. He got it back and the first time he played it, it vented the cap and smoke everywhere.
So I start removing all sources attached to the M7 preamp. No source connected, only cable is going to the power amp. Still buzzing. I pull all power plugs except the pre and power. Still buzzing. I pull the preamp out and measure the rca outputs. 140mv and climbing on both output RCAs in reference to RCA ground. This is with nothing connected to the preamp except the power cord. I bring the preamp back, hook up just the power cord and test, nothing. 0mv on both left and right RCA outs. Left it for a few hours, nothing. No DC on the outputs.
So we have the following:
140mv dc on the preamp outs with nothing but power cord connected
Power amp with all rail fuses blown so cannot be a channel fault. But one channel did blow two out of four output transistors. Also the amp got very hot before it blew the fuses. Was running fine for a week before that and never got hot. In fact the owner ran it very loud with the volume pot consistently around 11-12 o'clock for hours on end everyday after it got back and the amp barely broke a sweat. No heat.
With Plinius pre connected to Pro-ject class d power amp, loud volume dependent buzz.
I have checked his power sockets, they're all wired properly. I thought I would find a loose neutral at his distribution box. I found a few loose grounds and tightened them but no effect to the buzz. Preamp shows no symptoms when tested at my workplace.
Any ideas? I'm thinking excessive DC on his mains maybe?
Appreciate any thoughts on this. Thank you so much in advance.
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