I recently purchased a carver PM 420 from a local audio resale shop. It was listed AS-IS. When I asked what was wrong with it they told me that the connector from the input board only worked when it was plugged in half way at an angle. So I got it home and sure enough one channel wasn’t working. Since it was AS-IS I couldn’t return it. All of the guts looked fine powered up with no issues so I figured it should be an easy fix.
I proceeded to start by opening the case and removing the main PCB checking for cold solder joints on the connector. Everything looked fine but for my personal satisfaction I reflowed all of them. I buzzed out the connectors from the pins to the test points. Everything was good. It worked fine in bridged mono mode so I knew the audio circuits were ok and working, which took me back to the input board.
Here is where it gets odd, at least for me and what I can’t understand. This amplifier has balanced TRS inputs only. One channel works fine signal at tip ground at sleeve. The other channel only responds when the signal is present at the ring connection which is the negative inverting input. Which of course means the output signal is 180 degrees out of phase.
For the time being I reversed two wires between the input board and the main PCB and it works fine except for the out of phase thing. Took it up to my testing ground and ran it at a 4 ohm load for 45 mins not being easy on it. It ran cool and calm. So I know the circuitry is ok. This only accepting signal on the ring has me a bit perplexed.
I could easily reverse the output wires on the back of the binding posts so that it is in color coded phase but like I said I would rather either fix the problem or at least figure out what happened to this little beast.
When I opened it up I found jumpers soldered between the tip and ring on all 4 input jacks and the ring terminals were bent in a way to not make contact with the sleeve. I have worked on carvers before and this does not look like anything I have ever seen a repair shop do. The solder joints alone looked like something a first time solderer would do if he was just learning.
I proceeded to start by opening the case and removing the main PCB checking for cold solder joints on the connector. Everything looked fine but for my personal satisfaction I reflowed all of them. I buzzed out the connectors from the pins to the test points. Everything was good. It worked fine in bridged mono mode so I knew the audio circuits were ok and working, which took me back to the input board.
Here is where it gets odd, at least for me and what I can’t understand. This amplifier has balanced TRS inputs only. One channel works fine signal at tip ground at sleeve. The other channel only responds when the signal is present at the ring connection which is the negative inverting input. Which of course means the output signal is 180 degrees out of phase.
For the time being I reversed two wires between the input board and the main PCB and it works fine except for the out of phase thing. Took it up to my testing ground and ran it at a 4 ohm load for 45 mins not being easy on it. It ran cool and calm. So I know the circuitry is ok. This only accepting signal on the ring has me a bit perplexed.
I could easily reverse the output wires on the back of the binding posts so that it is in color coded phase but like I said I would rather either fix the problem or at least figure out what happened to this little beast.
When I opened it up I found jumpers soldered between the tip and ring on all 4 input jacks and the ring terminals were bent in a way to not make contact with the sleeve. I have worked on carvers before and this does not look like anything I have ever seen a repair shop do. The solder joints alone looked like something a first time solderer would do if he was just learning.
