Hi ! This is my first post, so please forgive me if I violate any rules..
I'm about to repair an Electrocompaniet EC4.7 preamp. I have the theoretical and practical skills to do the job. But I have no service manual !! The ALPS motorized volume pot i scratchy. The replacement of the pot seems to be a simple soldering job. But the original pot is a non standard, hard to get, (low value), 2x5k version 🙁. Do any of you wise people know if this is a standard volume circuit (input at the "top" pin, output at the wiper and the "bottom" terminal is GND) ? or is it a more exotic circuit were the ohmic value is critical ? The amp is DC coupled, so I wonder if the low ohmic value was chosen to limit the impact of bias currernts in the following amplifier stage...
Best regards
SorenP
I'm about to repair an Electrocompaniet EC4.7 preamp. I have the theoretical and practical skills to do the job. But I have no service manual !! The ALPS motorized volume pot i scratchy. The replacement of the pot seems to be a simple soldering job. But the original pot is a non standard, hard to get, (low value), 2x5k version 🙁. Do any of you wise people know if this is a standard volume circuit (input at the "top" pin, output at the wiper and the "bottom" terminal is GND) ? or is it a more exotic circuit were the ohmic value is critical ? The amp is DC coupled, so I wonder if the low ohmic value was chosen to limit the impact of bias currernts in the following amplifier stage...
Best regards
SorenP
Hard to give absolute advice without seeing so I would suggest the following.
First measure to see if there is any tiny DC voltage across the pot... just for curiosity.
If the pot is wired conventionally it should be easy to see. Or it may be part of an "active" attenuator in which case the value could be more critical.
Look and see what devices the pot acts on. I suspect they will be FET's as the thing is DC coupled. If so there is no bias current issue. Could they be opamps (FET) ? Still no issue though.
5K sounds a low value so suggests a custom part maybe.
Maybe post some detailed close up pictures...
First measure to see if there is any tiny DC voltage across the pot... just for curiosity.
If the pot is wired conventionally it should be easy to see. Or it may be part of an "active" attenuator in which case the value could be more critical.
Look and see what devices the pot acts on. I suspect they will be FET's as the thing is DC coupled. If so there is no bias current issue. Could they be opamps (FET) ? Still no issue though.
5K sounds a low value so suggests a custom part maybe.
Maybe post some detailed close up pictures...
Hi!
The thread is old but I believe I can help others. I fixed one recently and copied the circuit diagram from the board.
The adjustment is described in the drawing for the Right channel. The procedure is the same for the left channel at P1 and P2.
The thread is old but I believe I can help others. I fixed one recently and copied the circuit diagram from the board.
The adjustment is described in the drawing for the Right channel. The procedure is the same for the left channel at P1 and P2.
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Hi,
Yet again, the thread is old, but I am just looking at an EC4.7 preamp and cannot figure out the DC adjustment. I can get 16V from P3/P4 or P1/P2 to GND, but for some reason there are 1ohm resistors soldered in between P3 and P4, and between P1 and P2 on the other channel. Adjustment on the DC pots does nothing, but that makes sense I guess. Should I remove the resistors? They look factory mounted, perhaps from an upgrade.
Regards,
Kristian
Yet again, the thread is old, but I am just looking at an EC4.7 preamp and cannot figure out the DC adjustment. I can get 16V from P3/P4 or P1/P2 to GND, but for some reason there are 1ohm resistors soldered in between P3 and P4, and between P1 and P2 on the other channel. Adjustment on the DC pots does nothing, but that makes sense I guess. Should I remove the resistors? They look factory mounted, perhaps from an upgrade.
Regards,
Kristian
Hi,
The DC adjustment on the volume pot must be done with the volume pot at maximum. Really, if the volume pot is at minimum, it will not be possible to adjust it because P3 and P4 will be in short circuit and the voltage between them will always measure 0V.
The DC adjustment on the volume pot must be done with the volume pot at maximum. Really, if the volume pot is at minimum, it will not be possible to adjust it because P3 and P4 will be in short circuit and the voltage between them will always measure 0V.
Ah, of course! Thanks a lot, I see that now. The reason for having this open in the first place is because of some scratchiness in the volume pot when adjusting at low/medium volumes. I understand that this is a common issue and should be caused by DC on the pot, but have not fully understood whether the scratchiness actually means that the pot has been damaged or if it can simply be improved by adjusting the DC offset. Not so easy to find Alps RK27 blue pots in 5k value. Seems to be only Chinese copies available?
Yes, it's hard to find. With EC4.7 off, turn the pot manually a few times. That's what I did. With the DC adjustment and this procedure the scratching stopped. Good luck.