Wombaton
Active Member
Almost done!
Last week, I finally managed to revisit the Main Amp PCB, plucking out two sets of old trimmers, replacing them with new Bourns trimmers and doing DC offset / bias adjustments per @eizner23 instructions (post #42 above). Here is the image of the top-down view of the Main Amp PCB with the new trimmers installed. The ones that I've chosen (from a horde of various options in Bourns catalogue) seem to fit PCB openings fairly well and could be accessed for adjustment from above with relative ease:
This makes the offset adjustments a breeze, while for bias adjustments the amp has to go on a side with belly exposed.
Checking out the old trimmers revealed that their actual values are above spec resistivities by more than 10%. Could be age-related drift or maybe they were not that accurate to start with.
Both adjustments were done successfully with the new trimmers; DC offset was reduced to ~0.1 mV, bias voltage (28.2 mV) was matched within +-0.2 mV.
After this, the amp was fully assembled and moved to my main setup for some more critical listening (see below).
The only maintenance procedure that still remains to be done is refreshing thermal grease on the outputs. It is a mundane procedure, however, I am wondering what people use to clean off the old grease? I have a backup KA-5002 heat sink, from which I already removed the transistors and harvested their mica sheets:
As can be seen from the photo, there is plenty of ugly residue left behind, which could be pretty difficult to remove mechanically. Plus, the heat sink can be scratched in the process. Are there any other methods for removing it without sanding or scraping it off?
THX,
Wmbtn
Last week, I finally managed to revisit the Main Amp PCB, plucking out two sets of old trimmers, replacing them with new Bourns trimmers and doing DC offset / bias adjustments per @eizner23 instructions (post #42 above). Here is the image of the top-down view of the Main Amp PCB with the new trimmers installed. The ones that I've chosen (from a horde of various options in Bourns catalogue) seem to fit PCB openings fairly well and could be accessed for adjustment from above with relative ease:
This makes the offset adjustments a breeze, while for bias adjustments the amp has to go on a side with belly exposed.
Checking out the old trimmers revealed that their actual values are above spec resistivities by more than 10%. Could be age-related drift or maybe they were not that accurate to start with.
Both adjustments were done successfully with the new trimmers; DC offset was reduced to ~0.1 mV, bias voltage (28.2 mV) was matched within +-0.2 mV.
After this, the amp was fully assembled and moved to my main setup for some more critical listening (see below).
The only maintenance procedure that still remains to be done is refreshing thermal grease on the outputs. It is a mundane procedure, however, I am wondering what people use to clean off the old grease? I have a backup KA-5002 heat sink, from which I already removed the transistors and harvested their mica sheets:
As can be seen from the photo, there is plenty of ugly residue left behind, which could be pretty difficult to remove mechanically. Plus, the heat sink can be scratched in the process. Are there any other methods for removing it without sanding or scraping it off?
THX,
Wmbtn
