HP3312 Restoration

2019 5/10

HP3312 Final Amplifier

It's could deliver 0.1 Vpp while the output actually on 10 Vpp when i received it.   It pretty simple to located the problem stage,  the final amplifier. The Service manual is quite awful. The following picture  marked the transistor number, it's help.

Q501, Q502, Q503 responsible amplify the wind band signal, need transistor Ft exceed 1 Ghz. All of them is metal CAN transistor, no possibility to get same type transistor. I try my best to keep it style avoid use plastic transistor.

Find faulty transistor

The power transistor, Q509 is pretty dead,  EC is dead shorted.  I replace it with a Motorola 2N5160. 2N5160 is better transistor but the quiescent current might change a little bit.

Q509 is not the only transistor blew up, there is other transistor faulty,  output is obviously abnormal. It's hard to trace what's happen there, the DC voltage on joint of 2 emitter seem right, near to zero.  

Check signal level follow the 501->502/503 -> 504/506. It suggest the Q506 is faulty, because Q504 collector had much high signal level than  Q506 did. But when i cut off the Q506, it's good offline.  

Q506 must be pulled down.  Even a transistor's BE seems both conducted from either way(but not dead shorted), that's not a strong signal to prove the transistor dead! I made mistake cut several good transistor off the board, than I had to replace it with 2N5109 or 2N5160.

I finally identify the last falty transistor,  Q507, even though the Q507's emitter is seems ok, around 0 volt, but the DMM reading is wiggling, not a solid stable value. It turns out the Q507 BE is totally open! 

Conclusion

I broken a very important transistor, a PNP 1.4 Ghz, replace it with a 2N5160,  the character is difference. Now the 10Vpp amplitude sustained under 11 Mhz, high frequency the amplitude go low to 8 Vpp. Not big deal, square wave go round  above 8Mhz.  One friend tell me even all transistor is original,  the 13Mhz square wave is not possible to be perfect.