This is how it looks like before it was dismantled. It is sealed in some sort of rosin and requires heating before the transformer can be taken out.
After the extraction, the core lamination (obsolete now) are then cleaned up before it can be reused. The winding and bobbin are discarded as they no longer can be used. Due to the non-standard size of the core lamination, a customer bobbin has to be made to get a perfect fit. Standard size bobbin will not fit properly and there shall be unwanted design deviation that will not get it back to the original condition and performance.
The original bobbin / winding are slowly and carefully unwinded to record down the original turns and taps location for rewinding later. Luckily the damage was not that bad that prevents us from doing so. If not, another unit might be needed to complete the big picture, or smart guesstimate based on physical unit + theoretical data will be required if no references are available.
Another challenge was presented to us when it is time to rewind it - the wires used are of diameter of 0.05 with 3000 rounds of primary turns with a center-tap!!! It is quite difficult to rewind and had to be manually done slowly. If it is broken, what's done will have to be thrown away and new one redone.
After a 3 day affair, finally it is done! It will be shipped back for testing before another 3 remaining units are repaired with the same methodology & processes.
Refurbishing or repairing input transformers will be worth doing so if it is those vintage exotic ones such as those Western Electric types where a pair could be U$1K - U$2K nowadays, or for those looking to restore expensive vintage equipment to original condition that can fetch pretty good price in the market.