Maybe a dumb question...
Does a reply of mine seem to have disappeared ? It would have been after post#19 and detailed 3 steps to take. I also noticed the timestamp on my posts has changed.
Does a reply of mine seem to have disappeared ? It would have been after post#19 and detailed 3 steps to take. I also noticed the timestamp on my posts has changed.
This is the DC-protection. In later versions like the 405-2 it is on the main PCBs.
No, there was another where I posted three numbered steps. I also posted an hour before the time shown on all these posts i.e 8:28 pm and not 9:28 pm as it shows now.
May as well post again now 🙂 It went something like this.
1/ Unsolder the ground connection from the sockets and make certain the socket is 100% isolated from the chassis.
2/ Run a single ground wire from each input ground reference on the PCB to the appropriate ground tag on the socket. That is the point on the PCB that pin 3 of the opamp connects to.
3/ Connect the input signal wire from the PCB to each socket.
With shorting plugs fitted the amp should be silent.
With each shorting plug now shorted to the other channel it should still be silent. In other words you not only short each input individually but you then short then all to each other.
If it is not silent when doing that then we need to look at a different grounding scheme such as returning each sockets ground to some arbitrary 'clean signal ground' and also make sure that the PCB ground that goes to pin 3 of the opamp on the PCB is intact... in other words does this ground point have continuity via the PCB to 'ground' or did it rely on the input ground wiring for continuity. That's an important point... which may sound silly, but as always not having a specimen in front of you means you have to think of all possibilities.
May as well post again now 🙂 It went something like this.
1/ Unsolder the ground connection from the sockets and make certain the socket is 100% isolated from the chassis.
2/ Run a single ground wire from each input ground reference on the PCB to the appropriate ground tag on the socket. That is the point on the PCB that pin 3 of the opamp connects to.
3/ Connect the input signal wire from the PCB to each socket.
With shorting plugs fitted the amp should be silent.
With each shorting plug now shorted to the other channel it should still be silent. In other words you not only short each input individually but you then short then all to each other.
If it is not silent when doing that then we need to look at a different grounding scheme such as returning each sockets ground to some arbitrary 'clean signal ground' and also make sure that the PCB ground that goes to pin 3 of the opamp on the PCB is intact... in other words does this ground point have continuity via the PCB to 'ground' or did it rely on the input ground wiring for continuity. That's an important point... which may sound silly, but as always not having a specimen in front of you means you have to think of all possibilities.
OK thanks for the help guys. Just to clarify a few things with the input grounds are now connected and it is still silent when the input plugs are shorted. But when her actual input is plugged in you get a very loud buzz. Initially I had the input sockets isolated from the chassis.
I do not have the signal wire And shield wire interchanged, I’m not that much of a beginner LOL
I did get that board off the back of the speaker binding posts and there are two electrolytic set on there that are supposed to be 10 µF but test out at 3 nF. Right now I am hoping once a change those out it will resolve the issue. I should be able to do it by end of day tomorrow and will circle back to let folks know.
Again thanks for the idea so far
I do not have the signal wire And shield wire interchanged, I’m not that much of a beginner LOL
I did get that board off the back of the speaker binding posts and there are two electrolytic set on there that are supposed to be 10 µF but test out at 3 nF. Right now I am hoping once a change those out it will resolve the issue. I should be able to do it by end of day tomorrow and will circle back to let folks know.
Again thanks for the idea so far
Oh and also to clarify I got this amplifier very reasonably without hearing it working so we should not present was working fine before hand. But I did hear the music out of both channels underneath the loud chainsaw buzz so hopefully nothing too serious is wrong!
They are still connected at the chassis ground point beside the IEC. Independently run with 6 inches of wire to the same ground point on the chassis beside the IEC
OK I can try that. Honestly I don’t think this is a ground loop issue though. I have chased those before on other gear and from my experience it’s a much quieter sound, more like a hum. This to me sounds more like what I imagine a bad electrolytic cap which sounds like.
My previous experience has all being on tube Gear though, this is my first solid-state piece that I’ve worked on.
My previous experience has all being on tube Gear though, this is my first solid-state piece that I’ve worked on.
Have you another source you can try, or plug the CD directly into it, without pressing play of course?! What is the preamp?
It has nothing to do with the hum, but if I were you I wouldn't switch on the amplifier at all anymore until those capacitors are replaced. The QUAD-405 has a DC protection that bluntly shorts the output with a TRIAC, I wouldn't want to run any risk that it triggers unnecessarily because of the too low capacitance. By the way, those capacitors should be bipolar electrolytics.
Quick question. The input Stage has a 15 V one watt zener diode that I was looking to replace. The electronica supply store only has 15 V 400 Milla amps ones. Can I gang three of them up?
Does it not have two ? These are used as shunt regulators for the opamp supply.
Worst case dissipation with no opamp fitted would be 160 milliwatt. Why replace them though ?
Worst case dissipation with no opamp fitted would be 160 milliwatt. Why replace them though ?
15 V Zeners have rather steep voltage-current characteristics and usually a tolerance of +/-0.8 V or so. When you connect three of them in parallel, chances are that the one with the lowest breakthrough voltage will conduct almost all current.
OK so I replaced the last two electrolytic cops on the speaker output board. Still no change, its still buzzing loudly. Didn’t bother with the diode‘s. I went over everything again and can’t see anything that I possibly done wrong. As I said I did not hear it work before so maybe there was an existing problem. I’m throwing in the towel and going to bring it to my technician. I will circle back and let everybody know what the problem was. My tech does have a bit of a waiting period so it may take a while before I get the answer.
Really? Even after you were told and given suggestions? Any particular reason you have ignored the help offered?
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