Updates:
1. Driver Board:
All the carbon films in the driver board section have been replaced with Metal films from MultiComp(Thanks Hyperion) rated at 0.5 watts each, and a tolerance rating of 0.1%, better temp co-efficient.
Some things i noted and wanted to share, the MultiComp Metal Films resistors are a tad bit bigger than the 0.33 watt Carbon Film resistors, you might want to adjust the spacing of the resistors, so you atleast get around 0.5mm to 1mm spacing between resistors placed beneath each other or the sides .
I did check each and every resistor before replacing with the multimeter to check for their rated values before soldering them in.
Also verified for trace continuity after the solder work was done.
What is left are couple of Carbon Films in the protector section, which i had missed to place an order for, and the potentiometer from Bourns, the Bourns potentiometers that i have, have the second wiper pin closer to the pins 1 and 2 , i can adjust the pin so they fit in the holes properly, but i am going to check if i can find a potentiometer that has a wider spacing for the middle wiper pin.
Inspired by work done by Hyperion, i have sourced the Fish spine beads to be used for non-flammable resistors, so i wait for them to arrive.
Ronito6 is a knowledge bank, and am very very thankful to him for the support rendered.
Driver Board: Front
Driver Board Rear:
Only Carbon Films in driver board replaced, and couple of other carbon films in protector section which i had already.
The small transistors mounted in heat sinks have been re-greased and soldered back.
I noticed that one of the A968 transistors, had been leaky and had very low Hfe gain compared to the other A968 transistor, i have replaced them with the below transistors, from Fujitsu i suppose, these measured well and equal to the Toshiba A968 transistor, so i replaced them both with the ones from Fujitsu.
I would want to know, if you guys have any inputs with this replacement used, in case i am wrong, i will reconsider my choices.
Heat Sink(Transistor Mount Assembly):
I have replaced the emitter cement resistors, with those from Vishay Dale LVR series if remember correctly, and the base resistors with non-flammable ones from Vishay, i have to admit , i am not happy done with the work done on Base Resistors, the key is to bend them properly, in such that you can remove the transistor mount screws easily, i had only 8 pieces of these resistors, and was not able to do this job properly.
I might add some more solder to the emitter resistors, the base resistors are just mounted and not soldered for now, as the base wires feed in there, and i will solder each and every one of them during this work.
Side Note: The transistors mounts were removed prior to this work being done, the emitter and base contacts of the TO-3 transistors were cleaned, with a dose of Deoxit, followed by IPA wash completely, as i found some heat sink paste inside the holes where the emitter and base leads feed in.
If one removes the driver board completely isolating it from the heat sink, its very easy to get this job done.
Lesson Learnt (Mistakes Were Made But Never Again) !:
After desoldering the components from the driver board, i rubbed it with a dose of IPA, to clean the contacts before soldering, little did i notice that one of the solder pad, used to mount transistor A968B(collector) pin, had come off, and the another solder pad(Preamp Positive), was becoming a bit loose.
You can see the removed pad in above picture, i have temporarily soldered the transistor lead to the solder trace in PCB, i might use a small bit of wire, and then connect the two collectors together, to be on the safer side.
The Pad on extreme left has become a bit loose, i believe half of it still clings to the adhesive on the PCB, can you guys recommend a Epoxy , that i can use to stick this pad back.
Any inputs will be appreciated.
I immediately stopped the work, and inspected the boards for further damage.
The rubber adhesive on this board, in certain places, are quite weak, and i should have dosed, it with IPA after the solder work and not before, IPA also loosens up the solder pads easily if too much heat was applied, earlier, it is safe to use a lesser or very very minimal dose of IPA on areas on the PCB, where heat marks are present.
Eyelets, are a good way to go forward, but i do not have the tools required for using the eyelets, i will in near future when i have necessary funds, will do a proper work on the pads removed(count:1) /loosened(count:1) in above picture. !
Can you guys recommend/suggest me a way to fix the other pad that is a bit loose i do not want to go ahead with soldering on the loosened pad, as this might just make things worse.
Awaiting inputs from the Guru's of the forum.
Thanks in advance for all help and support rendered. !