Hi,
My Dared MC-7P started making a strange hum. I tried testing and replacing all the tubes, but that didn't work. I opened it up and found two blown resistors, R35 and R36. Now that they are burnt I can no longer read the color coding. Does anyone know what the value of these two resistors is based on a schematic or by looking at your own unit? I can't find a schematic anywhere online and I can't be sure of the values based on any of the photos others have shared of the inside of the unit.
Also, if anyone has thoughts on why these two resistors blew, I'm open to ideas. Without the schematic, I'm not sure of where they sit in the signal path.
You can clearly see the blown resistors in the photo.
Thanks for any and all help.
Jeremy
My Dared MC-7P started making a strange hum. I tried testing and replacing all the tubes, but that didn't work. I opened it up and found two blown resistors, R35 and R36. Now that they are burnt I can no longer read the color coding. Does anyone know what the value of these two resistors is based on a schematic or by looking at your own unit? I can't find a schematic anywhere online and I can't be sure of the values based on any of the photos others have shared of the inside of the unit.
Also, if anyone has thoughts on why these two resistors blew, I'm open to ideas. Without the schematic, I'm not sure of where they sit in the signal path.
You can clearly see the blown resistors in the photo.
Thanks for any and all help.
Jeremy
I found what I think is the schematic for your preamp here: https://www.diyaudio.com/community/threads/6v6-regulator-issue.299216/
Scroll down about halfway and you'll see the hand drawn schematic sideways. R35 & R36 are 47k. The one on the right looks like it's going to pin #6 (plate), so that makes sense.
jeff
Scroll down about halfway and you'll see the hand drawn schematic sideways. R35 & R36 are 47k. The one on the right looks like it's going to pin #6 (plate), so that makes sense.
jeff
Thanks so much to each of you. I could not find that schematic. Clearly I need to do some more work locating information in the forum.
I'll measure and possibly then replace the resistors.
I'll measure and possibly then replace the resistors.
Just use google. When you replace the resistors, be sure to leave a bit of space between the pcb and the resistor body, so they will run cooler.
jeff
If you replace them, get the highest wattage that will fit the board, and stand them
1/4" to 1/2" up off the board.
1/4" to 1/2" up off the board.
They read 49.7 and 49.1. My understanding is that those are still within tolerance.
However, maybe I should still replace them. Is there any reason that resistors in this shape would produce a hum? I haven't done any other troubleshooting other than swapping out tubes. I only noticed that these two looked burnt.
However, maybe I should still replace them. Is there any reason that resistors in this shape would produce a hum? I haven't done any other troubleshooting other than swapping out tubes. I only noticed that these two looked burnt.
Will do. And I believe I found a good 5w version at Parts Connexion in Burlington ON. I see you are on the other side of Canada. I'm in Toronto so will have to decide if I want to drive to the burbs or have it mailed. Thanks again.
Replace them, but new ones will not fix the hum.
Does the hum reduce with a lower volume control setting?
Is the hum in both channels equally?
Does the hum reduce with a lower volume control setting?
Is the hum in both channels equally?
Yes, the hum is in both channels.
The hum gets louder when the volume is turned up. It also starks to crackle as the volume is turned up.
It's incredibly loud, especially just as the amplifier turns on.
The hum gets louder when the volume is turned up. It also starks to crackle as the volume is turned up.
It's incredibly loud, especially just as the amplifier turns on.
The electrolytic caps in picture seems to be chinese very cheap crap ... the life is very short for those
The rectifier tube I'm using is an older JAN tube and I've tested it and swapped in a few other 5U4 tubes that I know are working in other amps.
It seems like I may need to pull the board out and start testing capacitors. I've built a tube amplifier from a kit and done a bit of soldering and recapping, but this will definitely be the most elaborate fix I've tried.
Suspect the capacitor just after the rectifier, since that one works the hardest.
Of course, others could also be bad.
Desoldering board mounted capacitors can be difficult, without damaging the board.
Never try pulling the lead out while heating it. Heat it, and let it fall out by itself.
Of course, others could also be bad.
Desoldering board mounted capacitors can be difficult, without damaging the board.
Never try pulling the lead out while heating it. Heat it, and let it fall out by itself.