68 Magnavox AstroSonic Bias Help

Alex Marette

New Member
I have an 68 astrosnic console chassis number:
R220-02-BA

My amplifier has a unique silicon npn/germanium pnp output stage pictured below.
upload_2019-2-13_22-51-3.png

Unfortunately, my germanium is leaking on my left channel and subsequently, the output power is very low. The old transistors are bandix 111p1 and the 111n2. Some research shows that the proper replacement is the nke 219mcp. Both transistors in the replacement complimentary pair are silicon so I will need to rebias the pnp transistor.

This is where I will need help. I've built the circuit on spice to try to gain some intuition Im having trouble finding the best way to bias the pnp.
-What resistor(s) should I replace and with what rough value? My guess would be the 150k to -25.5V in the PC10 along with R72 possibly.
-I dont have a personal silly-scope so I can check for actual clipping so that just leaves me with checking the DC voltage at the output. Should I adjust values until the dc output voltage is the same 15VDC that is specified in the schematic?

I have engineering background but my focus is antenna design, digital communication, and embedded systems. They do not teach a whole lot in the field of analog circuits anymore...or maybe I just didnt pay attention my amplifier design class...whoops.

Any help would be greatly appreciated! Thanks,
Alex
 
Back
Top Bottom