this MOD works in TTSH Version rev.1, rev.2, rev.3 (pcb version 7- 8.x)
The filter input is B.A.D. (Broken As Designed)
It has DC coupled inputs for the VCO outputs and the pulse, square and saw waves have a 5V DC offset.
There are no coupling caps inside the filter.
So it is *designed* to give a massive *THUMP* when the VCA opens.
If you trim the Filter offsets for minimal thump with these offsets still built in, the best setting depends on the patch...
Have a look a the PCB, the TTSH has one single mod pre-thought: adding caps to the filter input.
Values can be in range of 470nF to 1uF bilpolar - I used polyester or polypropylen capacitors
(Speaking of "making the thing usable": this is one of the changes I would consider mandatory if you want to use an ARP2600 for more than fx sounds. )