Help With 60Hz Hum PLEASE
I think my post about XLR cables not being plugged in and there being noise was an error. That makes more sense why it doesn’t matter if the Marantz is powered on or powered off with the power cord disconnected from the power outlet. |
jo3533, Did you buy the Adcom amp new or used? I believe you said in a previous post the amp always had the hum since you have owned it. Is that correct? Did you click on the two links I provided for the sound of 60Hz and 120hz? Does the hum/buzz sound you hear sound like either one of them? Is it actually a hum or a buzzing sound?
I looked at the schematic wiring diagram you provide in your last post. Do you have a diagram that shows the power transformer and DC power supply? You said in an earlier post you had the amp worked on because the center channel had went out. You said:
Did the service tech say anything about if anything in the rectifier circuitry had shorted to the metal chassis or anything like that? Was any fuses blown? The reason I ask if you look at the schematic wiring drawing, look at the XRL connector pinouts. Look at pin #1 ground and follow the line down the page and you come to R49 a 1/4 watt resistor which is in series with the line. Follow the line on down the page where it ends at the earth ground symbol. This is the circuit that connects the signal ground to the metal chassis of the amp. By chance was this 1/4 watt resistor blown because of a B+ short to the chassis? If the connection of the signal ground to the metal chassis is open, not made, then the outer metal case/chassis could cause noise problems. If the signal ground is floating above the chassis I wonder what that does to the input sensitivity of the input circuit of each channel of the amp. Do you have a multimeter? If so I would be curious if there is continuity from pin 1 on any XLR input connector to the metal chassis of the amp. I would first check for DC voltage from Pin 1 to the chassis. Amp powered on. If the 1/4 resistor is good you should read zero DC volts. (Just guessing the value of the resistor is 10 ohms or less.) If that is the case turn off the amp. Unplug it from the AC power wall outlet. Then check for continuity from Pin 1 to the metal chassis of the amp. Al, If by chance you are reading this post I would appreciate your thoughts. Jim |
jo3533, Here is a quote from Charles Hansen.
http://www.audioasylum.com/audio/general/messages/52/525622.html |
jea48, I bought the ADCOM used. One channel was not working, and it would not turn on at all when first purchased. The tech was not very forthcoming on what exactly he replace. He did mention the bridge rectifier however he never confirmed if he replaced it. I have tried to contact him several times and have not received a response......Dunno whats up with that. There are no traditional fuses on this unit. It has had some mod completed by ADCOM that uses some other sort of safety. YES, I listened to the two links and it sounds just like the 60hz buzz! I mean its dead on with the recording. I do have a diagram that shows the DC power supply and I will post it in the shared dropbox folder. I want to point out that the transformer itself produces an audible 60 hz buzz that can be herd while standing next to the unit. I have read this is normal. Using my multi meter I checked for continuity between the chases and the #1 pin on the XLR jack and yes, there is continuity. There is also continuity between the chases and the outer shell of the RCA jack. I went doen to Stereotypes in Daytona Beach, FL and they were nice enough to allow me to borrow a set of TARA LABS XLR IC for a small deposit. I plugged in the Tara Labs cables and the noise was still there! Rats....it was worth a try. almarg, the 7800 series uses the same amp boards for the GFA-7805/7807. You will see the same markings for the power supply schematics. Link to service manual below. https://www.dropbox.com/sh/p7w1zmad9dwhx3o/AAC1ZY5kCxocLwKCn_nfTgBNa?dl=0 |
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