Hi everyone.
I am working on a heathkit AA-100, wich is a 7591 push-pull integrated. It has what sounds like a 120hz hum to me, on both channels (I compared the hum to 60hz and 120hz tones from a generetor app in my iphone). I suspected the filter caps in the power supply so I replaced those with new JJ multi section electrolytics (They fit the original holes perfectly, really satisfied with that). There was also a paper 20uf cap that I replaced with a new 20uf electrolytic, one resistor that was directly on the cap was replaced too. The 20uf cap has a selenium diode and a resistor in parallel with it so I replaced the diode with a 1n4007 and I changed the diode that’s in series with it for a 10k instead of the 8.2k that was there originally to compensate for the lower resistance/higher efficiency of the 1n4007. There was a paper/wax capacitor between the fuse and the ground so I replaced it with a film cap of the same value.
so from now that’s all I’ve done to it, but the hum is still there. It’s a bit better now but still not usable. When there is no input, the amp is practically silent and I can get the hum pots to lower the slight noise to a satisfactory level, but as soon as I connect my phone in the aux in and start a song, the hum is there. Also, when I did my tests after replacing the filter caps, the amp lost all its bass and I never came back since. I tried other inputs. All the tubes tested very good on my eico 667 tube tester. The power amp section has been recaped by someone else before but I can’t confirm that it ever worked well after that since the owner bought it as is.
what could cause a total lack of bass on both channels ?
since the hum is 120hz and present on both channels I suspect that it’s caused by the power supply, but nearly every cap in the supply is new. Do you have any idea as where I should start looking ?
thanks a lot !
I am working on a heathkit AA-100, wich is a 7591 push-pull integrated. It has what sounds like a 120hz hum to me, on both channels (I compared the hum to 60hz and 120hz tones from a generetor app in my iphone). I suspected the filter caps in the power supply so I replaced those with new JJ multi section electrolytics (They fit the original holes perfectly, really satisfied with that). There was also a paper 20uf cap that I replaced with a new 20uf electrolytic, one resistor that was directly on the cap was replaced too. The 20uf cap has a selenium diode and a resistor in parallel with it so I replaced the diode with a 1n4007 and I changed the diode that’s in series with it for a 10k instead of the 8.2k that was there originally to compensate for the lower resistance/higher efficiency of the 1n4007. There was a paper/wax capacitor between the fuse and the ground so I replaced it with a film cap of the same value.
so from now that’s all I’ve done to it, but the hum is still there. It’s a bit better now but still not usable. When there is no input, the amp is practically silent and I can get the hum pots to lower the slight noise to a satisfactory level, but as soon as I connect my phone in the aux in and start a song, the hum is there. Also, when I did my tests after replacing the filter caps, the amp lost all its bass and I never came back since. I tried other inputs. All the tubes tested very good on my eico 667 tube tester. The power amp section has been recaped by someone else before but I can’t confirm that it ever worked well after that since the owner bought it as is.
what could cause a total lack of bass on both channels ?
since the hum is 120hz and present on both channels I suspect that it’s caused by the power supply, but nearly every cap in the supply is new. Do you have any idea as where I should start looking ?
thanks a lot !
I went back to the amp after publishing this post and realized that the lack off bass was caused by the volume pot. I double checked everything that I did and found that I soldered the 20uf cap on the wrong tab of the strip. Soldered it at its place and the hum is gone. Problem solved !
Problem solved ! And it wad my fault. Lesson learned I guess !
But i’d like to understand the function of this part of the schematic since it was part of the problem. Could someone explain what it does ?
thanks !
But i’d like to understand the function of this part of the schematic since it was part of the problem. Could someone explain what it does ?
thanks !
Wouldn't back bias come off the center tap? Looks more like a 50VAC bias tap on the transformer.
You’re correct it’s off the CT. Look at the ground references, CT goes to ground, the bias is -xxV (I can’t tell if it’s a 30 or 50) to ground.
When you are ready too get into it a bit more here's a great thread from AudioKarma.
https://www.audiokarma.org/forums/i...or-the-heathkit-aa-100-integrated-amp.204753/
Cheers, S.
https://www.audiokarma.org/forums/i...or-the-heathkit-aa-100-integrated-amp.204753/
Cheers, S.
Well… I reworked the solder on all the pots, most of the sockets and started to recap the preamp with the parts I already had on hand since everything sounded right and it seemed clear that the volume pot caused the lack of bass because I could get the amp to sound good by wiggling it. Now that i’m shure that the pot is soldered properly, the bass is gone and I can’t find anything else that changes the sound when I move/touch/shake, etc. And I have a slight hum that wasn’t there earlier today.
read the beginning of the aa-100 post. I realize that I might have opened a can of worms… took the amp in for a quick recap for a friend, now its looking like a complete rebuild…
read the beginning of the aa-100 post. I realize that I might have opened a can of worms… took the amp in for a quick recap for a friend, now its looking like a complete rebuild…
Yes, you are correct, that is just a half wave rectified 50V tap, with resistor divider to set the bias voltage, and CT is at ground.