Hi Y'All,
James again with my next little project. I scored a clean Carver M500t that when I tested at the seller's house sounded great. Still does but here I noticed that there is an significantly noticeable transformer hum that occurs even when nothing is connected. I have done some research and downloaded the service manual.
I know there is a mag coil that fires up and then subsides and that sounds normal but there is a hum that remains after that. It is definitely the transformer area. That said, one thing I read said that if the bias is high, the mag coil can hum. So, I am testing the bias settings per the manual and I am getting 6.8mV on my first test. That is close per the service manual.
However, the testing point ceramic resistors have additional resistors mounted on top of them. Does that mean I should be looking for the modified settings of 3.5mV or so?
James again with my next little project. I scored a clean Carver M500t that when I tested at the seller's house sounded great. Still does but here I noticed that there is an significantly noticeable transformer hum that occurs even when nothing is connected. I have done some research and downloaded the service manual.
I know there is a mag coil that fires up and then subsides and that sounds normal but there is a hum that remains after that. It is definitely the transformer area. That said, one thing I read said that if the bias is high, the mag coil can hum. So, I am testing the bias settings per the manual and I am getting 6.8mV on my first test. That is close per the service manual.
However, the testing point ceramic resistors have additional resistors mounted on top of them. Does that mean I should be looking for the modified settings of 3.5mV or so?
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Hi,
Thanks Tauro. I noticed that and C103 and C104 too are a bit "rounded". Definitely going to be replaced. Not sure it has anything to do with transformer hum though?
Thanks.
I believe my question is answered by a service bulletin I saw somewhere and I have adjusted the bias to 3.5mV. The hum has diminished some but not quite enough. I suspect that the PS caps are old and need replacing.
Thanks Tauro. I noticed that and C103 and C104 too are a bit "rounded". Definitely going to be replaced. Not sure it has anything to do with transformer hum though?
Thanks.
I believe my question is answered by a service bulletin I saw somewhere and I have adjusted the bias to 3.5mV. The hum has diminished some but not quite enough. I suspect that the PS caps are old and need replacing.
The M500t's power supply is not a regular power supply transformer - it uses a mag coil with triac control to regulate the power, a bit like an old fasioned incandescent light dimmer. Anatech is the resident expert on these.
I believe that growling is a sign that the main filter capacitors need to be replaced.
I believe that growling is a sign that the main filter capacitors need to be replaced.
Thanks Jaycee. I am hoping Anatech step in too. I fear I have bothered him a bit much lately. He is very helpful.
I went ahead and replaced three smaller caps that were obviously rounded - C103, C104 and C202 with equivalent values and it cleaned up the sound a bit. More detail and separation. C103 and 104 are in the input circuit.
I adjusted the bias down to 3.5mV as I believe that is where it needed to be because of there are extra resistors on top of the square ceramic resistors at the test points. It reduced that level of the buzz but did not eliminate it. It is actually kind of at a tolerable level.
I have PS caps left over from another project that would fit - they are 10,000uf vs. the 6800uf though.
Thanks Jaycee.
I went ahead and replaced three smaller caps that were obviously rounded - C103, C104 and C202 with equivalent values and it cleaned up the sound a bit. More detail and separation. C103 and 104 are in the input circuit.
I adjusted the bias down to 3.5mV as I believe that is where it needed to be because of there are extra resistors on top of the square ceramic resistors at the test points. It reduced that level of the buzz but did not eliminate it. It is actually kind of at a tolerable level.
I have PS caps left over from another project that would fit - they are 10,000uf vs. the 6800uf though.
Thanks Jaycee.
Hi,
Speaking of transformers I checked the top two bolts that go through to a nut on the other side. I am able to turn the bolt fairly easily with a phillips screwdriver and the nut on the other side turns too. There are nylon washers between the bolt and nut and transformer. I do not know if that it normal? If not, I see that I can get the transformer out and likely tighten those down.
Jaycee, there are six bigger caps on the PS board, 2 x 80v x 6800, 2 x 50v x 6800 uf and 2 x 80v x 2200uf. Which ones of the smaller ones do you mean in your post above?
Thanks folks!
Speaking of transformers I checked the top two bolts that go through to a nut on the other side. I am able to turn the bolt fairly easily with a phillips screwdriver and the nut on the other side turns too. There are nylon washers between the bolt and nut and transformer. I do not know if that it normal? If not, I see that I can get the transformer out and likely tighten those down.
Jaycee, there are six bigger caps on the PS board, 2 x 80v x 6800, 2 x 50v x 6800 uf and 2 x 80v x 2200uf. Which ones of the smaller ones do you mean in your post above?
Thanks folks!
The BOLTS through the transformer should have soda straws and insulated washers before the nut. This stops a SHORTED TURN in the transformer. Tighten up the bolts as this may lower the mechanical noise (HUM). Most of the noise/HUM is where some thing is loose and vibrates. A good solution is to fill the transformer with some fluid in a vacuum chamber that will cure/harden and stop any noise.
Duke
Duke
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