I'm sorry if I offend you with my honesty, but state this with good intentions. Your comment of "I'm fairly new in electronics" concerns me - this repair is not a beginner job. This is a high voltage circuit with voltages than can kill you. Or at least zap the living s*** out of you. If you don't feel 100% confident doing this work, PLEASE don't do it. In addition, this is a very rare, expensive piece of gear to learn on. If you choose to send it out, I can suggest someone who has the knowledge to do the work properly. Check your PM's here.
That said, here is a list of what will be involved:
- knowledge of what the completed unit should be, performance wise, once the repairs are complete. There are very few techs with that knowledge. The ML-1 was a custom product, only a few hundred ever made. Most techs have never seen one in person, much less worked on one. There are units out there that such techs have tried to repair/modify with less than stellar results. There is no documentation publicly available, to my knowledge. And I looked real hard.
- confirm B+ transformer was not damaged. When disconnected from the line amp PCB, transformer secondary should measure 230VAC or so. Insulate conductors so they don't accidentally short against the chassis. Also disconnect 12V heater voltage and confirm that's OK. Insulate conductors. Disconnect input from front panel and output to rear panel XLR's, so you can work on the PCB. Remove PCB. Document how everything came apart via pictures and notes.
- if B+ transformer is OK, remove everything on the line amp PCB that comes after the B+. Rectifier, film caps, electrolytic caps, power resistor (the thing that got hot enough to delaminate the PCB), the burned and not burned resistors, and the zener diodes. Zener failure is a likely culprit for this meltdown. Known failure situation. I use higher current rated zeners, and parallel two for even more current capability. Many original ML-1's are likely due for replacement of the zeners and electrolyics by now, most were made in the 90's and the units gets pretty hot inside during operation, especially if not properly ventilated.
- Clean and repair PCB. I would probably try the detergent method Andy described, but from that pic would likely end up using a Dremel with wire brush (while wearing a respirator mask in order to not breathe the epoxy dust) to get down to clean material, then recoat with epoxy to strengthen the PCB and insulate the fiberglass core.
- Repair any damaged traces on the PCB. From the look of that pic, wouldn't surprise me to find trace damage on the other side of the PCB.
- Replace all removed parts with new parts.
- Reconnect line amp board with tubes removed.
- Power up with variac and check all voltages.
- Power down, put tubes in, power up with variac, check all voltages.
- ML-1 runs the cathode follower very hot, so it's likely time for a new tube. The original is 7308. Do not let techs or tube sellers convince you that 6DJ8 or 6922 is exactly the same as 7308, it is not. A common misconception/oversight. Check heater current on spec sheets. This is a series heater string, that is important. If you decide to use 6922, modifications will have to be made to accommodate it. Might be time for a new 5751 too. Measurement will tell that story. And use NOS tubes. I have not found modern tubes that compare.
- test to confirm unit meets original performance specs for frequency response, THD, EIN, etc.
Enjoy the finest tube mic pre ever made, IMHO.
