Hello my friends, I have a Ming tube amplifier with 300B and 845.
Sometimes a loud noise appears on the speakers at 100 Hz as if someone flipped a switch. You hear the noise... as it appears in the same mode it disappears...
With the amp open during the problem with a plastic screwdriver, I touched all the parts without anything happening. Likewise when the audio is cleaned by touching the internal wiring, nothing happens. I attach the video below where the noise suddenly appears which in this case lasts 30 seconds and then disappears on its own.
Generally the noise at 100 Hz is corrected by adjusting the HUM balance, the trimmer connected between the 47 ohm resistors on the cathode of the 300B. In fact, by adjusting the trimmer when you don't hear any background noise you can flatten the output. Here too, however, a strange thing happens: to keep the two channels silent, I detect anomalous values. In the picture attacted the value.. with the measurements at the top without valves, at the bottom with the valves mounted, doesn't it look strange??
Thanks in advance for your precious help
Sometimes a loud noise appears on the speakers at 100 Hz as if someone flipped a switch. You hear the noise... as it appears in the same mode it disappears...
With the amp open during the problem with a plastic screwdriver, I touched all the parts without anything happening. Likewise when the audio is cleaned by touching the internal wiring, nothing happens. I attach the video below where the noise suddenly appears which in this case lasts 30 seconds and then disappears on its own.
Generally the noise at 100 Hz is corrected by adjusting the HUM balance, the trimmer connected between the 47 ohm resistors on the cathode of the 300B. In fact, by adjusting the trimmer when you don't hear any background noise you can flatten the output. Here too, however, a strange thing happens: to keep the two channels silent, I detect anomalous values. In the picture attacted the value.. with the measurements at the top without valves, at the bottom with the valves mounted, doesn't it look strange??
Thanks in advance for your precious help
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I have tryed with cold spray without results It's a very strange flaw and it does it as soon as it's turned on like after 10 minutes
I don't undestrand the difference from resistance internal of 300b
I have tryed first without 845 and 300c left, but the problem is the same and then without tubes to the other channel .. but the problem is the same
I don't undestrand the difference from resistance internal of 300b
I have tryed first without 845 and 300c left, but the problem is the same and then without tubes to the other channel .. but the problem is the same
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This is a mono amplifier? If so, does only one of the two amplifiers do this?
Seems like an intermittent power supply capacitor or connection, or else excessive current draw in the circuit.
It would be more helpful to have a schematic diagram of the circuit.
Seems like an intermittent power supply capacitor or connection, or else excessive current draw in the circuit.
It would be more helpful to have a schematic diagram of the circuit.
It's a stereo amplifier I don't found the schematic .. it's a ming mc 845-c12
intermittent interruption seems not
intermittent interruption seems not
BOTH i tryed without tube in right and then without in the left the problem appear the same
https://i.postimg.cc/tCNNjfjM/Total.jpg
https://i.postimg.cc/tCNNjfjM/Total.jpg
Ok, then it is likely to be a power supply problem.
Perhaps one of the HV filter capacitors, or the wiring has a bad connection.
You would have to track this problem down by careful examination and testing.
Maybe someone has a schematic.
Don't use speakers, just a resistor load, and scope the output.
Perhaps one of the HV filter capacitors, or the wiring has a bad connection.
You would have to track this problem down by careful examination and testing.
Maybe someone has a schematic.
Don't use speakers, just a resistor load, and scope the output.
Sure, I have look with attention and I tried tapping the connections lightly with a plastic rod but found no problems. I try it on the resistor load sure, the speakers were connected for a listening test. I hope in a service manual or schematic diagram.. Thanks for now.. Rayma
I meant, does the noise follow the switching on of the refrigerator?
It's not necessarily something in the amp.
Refrigerators, lamp dimmers, etc are known to have such effects.
Jan
I'm very sure of it because in the afternoon he played for un hour without problems, that is not a refrigerator
I started to back away from the 845: disconnected the grid capacitor, I still find the problem on both channels, disconnected the grid capacitor of the 300c and I still find the problem. I disconnect the decoupling capacitor of the 6SN7 and after the first amplification of the 6sl7 I see no problems. So it is located on 6sn7
I'm investigation ...
I'm investigation ...
Very strange situation...
I worked on it a lot I made many measurements with 6SL7 only on the left and only on the right, then 6SL7 and 6SN7 only on the left and then all 4 valves now the problem has disappeared... as if the problem was on a socket but it's so very strange... the soldering is strong... I tried tapping on all the connections again and there was no problem. However, I rebuilt and drawn the input diagram... it may be useful. It appears from left to right because it is the view directly on the left harness. If I made any mistakes please correct me.
I worked on it a lot I made many measurements with 6SL7 only on the left and only on the right, then 6SL7 and 6SN7 only on the left and then all 4 valves now the problem has disappeared... as if the problem was on a socket but it's so very strange... the soldering is strong... I tried tapping on all the connections again and there was no problem. However, I rebuilt and drawn the input diagram... it may be useful. It appears from left to right because it is the view directly on the left harness. If I made any mistakes please correct me.
