SX-838 won't come out of protect

[zebulon1]

MTF calls them a "Ticking time bomb" catastrophic damage when one lets go. Also found in the 1010 amp board circuits.

 

[LesE]

MTF calls them a "Ticking time bomb" catastrophic damage when one lets go. Also found in the 1010 amp board circuits.

This is especially true in a design like the SX-838 amp since when the 2SC1451 fails open, the bias current goes to max. I think the OP caught a break when his failed short.

Yes, the 2SC1451 was frequently used in Pioneer products during the mid 70s. The SA-9900 may be the worst in this regard with 18 2SC1451s along with 4 2SA809s which is it's PNP complement.

 

[nickhead]

I finished rebuilding the power amp board, and it looks like I screwed it up somewhere. And I screwed it up identically on each channel. Back to protect with the following voltages on the protection board:
1: 34v
2: -13v
3: 34v
4: 0
5: 34v
6: 34v
7: 34v
8: 34v
9: 35v
10: 35v
11: 35v
12: 7.5A

Which leads me back to my work on the power amp board. All of the inputs are good. Others:

09: -49v
10: 22v
12: -13v
14: 35v
17: 22v
18: 22v

Q5 and Q6 were replaced with KSC3503DSTU's. On both base is reading 22v, collector 33v and emitter 22v.
Q1, Q2, Q3, Q4 were replaced with KSA992FBU.

I double checked all of the pinouts before soldering them in. All tested good and all should be in correctly. I also replaced all of the electrolytic caps.

Any ideas where I messed up?

Nick

 

[nickhead]

Never mind. Apparently when I put the power amp board back in I missed soldering some of the pins. She's sounding good now!

 

[Watthour]

Well, there's always THAT as a solution. Good find. Reviewing our work is always a good practice. More than once I've found a tranny stuck in the wrong way around just before I was ready to throw the switch to try it out.