I have a 2105 that I completely recapped about 2 years ago. I posted about the restoration when I did it:
The amp has been sounding great but lately I hear some snap crackle pop on the left channel that I have not diagnosed yet. I am going to pull the amp and try some cleaning and switching of the boards to find the problem but while it is out of the system I want to do a few things that I did not do during the recap.
I want to go ahead and finish the job by updating the output driver boards according to the service bulletin/kit instructions that I have found posts about on AK. There are some suggestions that I don't quite understand that I would like some clarity about. I will refer to the left channel part numbers in the questions for brevity. Most of these questions are about the transistors since I have limited experience picking new/changing transistors.
1.) It is suggested that all the Bias Resistors on the board be replaced as they are very critical. Which ones would these be?
2.) The instructions indicate the replacement of Q111 and Q113. Should I purchase as matched pairs, either as the npn/pnp pair, or as matched for the left and right boards, or matched both ways?Is it even possible to purchase as matched pairs or do I need to learn to match them myself and buy a certain number of them to go through?
3.) Q107(NPN) and Q109 (PNP) are referred to as a Darlington Pair and/or Sentry Monitors. When purchasing replacements do I need to get matched pairs of these? ( this is not part of the update and will only be done if necessary)
4.) I did purchase the replacement diodes D103 for the thermal tracking from the factory when I did the previous work to the unit. I plan to leave the original diodes installed since they seem to be working fine and keep the new as backup. Is this what you guys would suggest?
5.) Q101 and Q103 - Would you guys suggest replacing?
6.) Q105 - Replace or leave?
7.) I also plan to replace the inductive resistors on the heat sinks and re-seat the output transistors on the heat sinks with new thermal paste. Do you guys think this is prudent? Do I need to get the resistors from the factory or will any wirewound of the same type that are not sold as non-inductive do the job? I think all the OTs are ok but I guess I will try to test them with a multimeter while I have them out.
Below is a scan of my board with some notes from two years ago. The resistors are noted in red what they were marked as and smaller in blue what they tested as in circuit. All the 5% resistors are highlighted, the rest were 10% tolerance.
Also there is a pic of the schematic of the board and an attachment of the update instructions from the factory.
MC2105 restore to former glory / parts list
MC2105 restore to former glory / parts list I finally decided to restore my beloved MC2105. I am a hobbyist, not an engineer so my thoughts on circuit theory are to be taken with a grain of salt. That being said: I do have some insights that may help people that don’t do this work every day...
audiokarma.org
The amp has been sounding great but lately I hear some snap crackle pop on the left channel that I have not diagnosed yet. I am going to pull the amp and try some cleaning and switching of the boards to find the problem but while it is out of the system I want to do a few things that I did not do during the recap.
I want to go ahead and finish the job by updating the output driver boards according to the service bulletin/kit instructions that I have found posts about on AK. There are some suggestions that I don't quite understand that I would like some clarity about. I will refer to the left channel part numbers in the questions for brevity. Most of these questions are about the transistors since I have limited experience picking new/changing transistors.
1.) It is suggested that all the Bias Resistors on the board be replaced as they are very critical. Which ones would these be?
2.) The instructions indicate the replacement of Q111 and Q113. Should I purchase as matched pairs, either as the npn/pnp pair, or as matched for the left and right boards, or matched both ways?Is it even possible to purchase as matched pairs or do I need to learn to match them myself and buy a certain number of them to go through?
3.) Q107(NPN) and Q109 (PNP) are referred to as a Darlington Pair and/or Sentry Monitors. When purchasing replacements do I need to get matched pairs of these? ( this is not part of the update and will only be done if necessary)
4.) I did purchase the replacement diodes D103 for the thermal tracking from the factory when I did the previous work to the unit. I plan to leave the original diodes installed since they seem to be working fine and keep the new as backup. Is this what you guys would suggest?
5.) Q101 and Q103 - Would you guys suggest replacing?
6.) Q105 - Replace or leave?
7.) I also plan to replace the inductive resistors on the heat sinks and re-seat the output transistors on the heat sinks with new thermal paste. Do you guys think this is prudent? Do I need to get the resistors from the factory or will any wirewound of the same type that are not sold as non-inductive do the job? I think all the OTs are ok but I guess I will try to test them with a multimeter while I have them out.
Below is a scan of my board with some notes from two years ago. The resistors are noted in red what they were marked as and smaller in blue what they tested as in circuit. All the 5% resistors are highlighted, the rest were 10% tolerance.
Also there is a pic of the schematic of the board and an attachment of the update instructions from the factory.
