neutrik tt402a , reading 20db low

[NorfolkDaveUK]

Hey all hope you are all well, I have an issue with a neutrik tt402a I recently bought , I'm a reel to reel tech for the most part so , this will come in very handy if I can make it work well. I'm using a ferrograph RTS 2 and APU combo at the moment . So here's the story .

I bought it on eBay and it was filthy , and the case was battered but inside is was pretty much mint. , I have recapped the power supply , and a few other board , i.e. the meter driver board , the gain chain etc , and here's where I'm at . So when I bought it it would read freq perfectly , but wouldn't read amplitude at all , I traced that to a problem on the "spare filter" board and on removal of that , it brought back the ability to read amplitude , however , its reading it 20db low . So if I feed 0dbm (775) from my rts into it , it reads it as -20dbm , likewise if I send a signal from the tt402a to the rts , it will read 0dbm on the rts and -20db on the tt402a .

Now here is the problem , this thing is very rare , I can't find a schematic for it anywhere , believe me I have hunted and hunted , and there is nothing , I contacted neutrik , no reply  . So , I have no clue really other than my reasonable knowledge of electronics and audio gear / logic . If anyone has any idea what's going on here or where I might gets some hints as tomhow to put this right I would be very very grateful .

Thanks in advance guys and happy sparking !!