Roger Mayer RM56

I have found a bad transistor in my stereo solid state limiter but it has only a yellow dot on it and no other markings. Any ideas as to how I can come up with an adequate replacement part?
It was built in the early 70's to the best of my knowledge.

Can't find a schematic or even a picture of the thing.

Guitar Effects Pedals by Roger Mayer - Jimi Hendrix FX Guru - established 1964. (Handbuilt Guitar Effects Pedals, Guitar Effects, Vision Wah pedal, Mission Controller and more.) )



Fuzz Central -- Schematics and PCBs


**broken link removed**

Rare Stereo Limiter

Yes Roger only made a little bit of outboard. I did once see his console at a studio near grand central station.
This is a sweet limiter.
I got brave today and stuck a 512-BC547 NPN 45V in place of the bad transistor. We had just ordered a few for our BA183 cards.The new transisitor works pretty well It sounds great but the channel with the new transistor drifts up and down 2.0 db every 5 minutes or so I have to tweak the output level.

https://www.electro-tech-online.com/custompdfs/2009/05/BC547.pdf
What would cause drift?