Stack
Active Member
Hi,
I've recently recapped my KA-801 and wanted to fine tune things. I've had specific problems with the bias and DC offset, and was wondering if anybody could lend ke some friendly advice.
For the preamp DC offset (which I'm now thinking I should've left alone in the first place) the spec wants you to descend the lower rear panel, hook a DC meter to the test points, and turn the tiny screw until you get 0V.
My first problem is that when I hook the meter up and power on the amp (from a power strip without touching anything of course) the voltage reading fluctuates wildly, by as much as .75 volts! For about ten minutes it varies up and down until it settles in a fluctuation pattern of 30ish millivolts. Is this normal behavior? It makes it hard to get an accurate reading.
Second, the pots seem to be incredibly sensitive. Putting any force into a turn will move you by half a volt! I was able to get a sort of fine tune by shimmying the (insulated) eyeglass screwdriver with the tiniest inclination in the direction that I wanted, but is there a better way? Does it make sense to replace these pots with something a little less finnicky?
Finally, this might be just my imagination, but adjusting one channel seemed to mess up the other. After I thought I'd gotten everything square, I put the amp back together and the right channel (the first I adjusted) was off to the point of clipping! Back to the bench it went. In your experience, does it make sense to get a second meter so that both can be measured at once, or was this maybe user error?
For the bias, my problem is much more straightforward: the pots don't adjust down far enough to reach the 18mv reading that the maintenance manual calls for. These are 500 ohm pots. In this case, does it make sense to replace these with higher resistance (perhaps 1000 ohm) pots so they can be adjusted down? I'm not in a position to diagnose and replace all the resistors, transistors, and diodes in this amp right now (I just finished a full recap and don't have the patience at the moment).
The left channel can be brought down to 33ish mv and the right channel 22ish mv. What long term effects would happen with the bias this high? Does it make sense to "match" the channels up to 33mv or just keep them set to minimum?
Any help would be greatly appreciated! Thank you for your time.
I've recently recapped my KA-801 and wanted to fine tune things. I've had specific problems with the bias and DC offset, and was wondering if anybody could lend ke some friendly advice.
For the preamp DC offset (which I'm now thinking I should've left alone in the first place) the spec wants you to descend the lower rear panel, hook a DC meter to the test points, and turn the tiny screw until you get 0V.
My first problem is that when I hook the meter up and power on the amp (from a power strip without touching anything of course) the voltage reading fluctuates wildly, by as much as .75 volts! For about ten minutes it varies up and down until it settles in a fluctuation pattern of 30ish millivolts. Is this normal behavior? It makes it hard to get an accurate reading.
Second, the pots seem to be incredibly sensitive. Putting any force into a turn will move you by half a volt! I was able to get a sort of fine tune by shimmying the (insulated) eyeglass screwdriver with the tiniest inclination in the direction that I wanted, but is there a better way? Does it make sense to replace these pots with something a little less finnicky?
Finally, this might be just my imagination, but adjusting one channel seemed to mess up the other. After I thought I'd gotten everything square, I put the amp back together and the right channel (the first I adjusted) was off to the point of clipping! Back to the bench it went. In your experience, does it make sense to get a second meter so that both can be measured at once, or was this maybe user error?
For the bias, my problem is much more straightforward: the pots don't adjust down far enough to reach the 18mv reading that the maintenance manual calls for. These are 500 ohm pots. In this case, does it make sense to replace these with higher resistance (perhaps 1000 ohm) pots so they can be adjusted down? I'm not in a position to diagnose and replace all the resistors, transistors, and diodes in this amp right now (I just finished a full recap and don't have the patience at the moment).
The left channel can be brought down to 33ish mv and the right channel 22ish mv. What long term effects would happen with the bias this high? Does it make sense to "match" the channels up to 33mv or just keep them set to minimum?
Any help would be greatly appreciated! Thank you for your time.