Fender Acoustasonic sfx shorted? Ugh!

[Indyryder02]

Ok. If this is right, from the data sheet.
the 2Sa1016 and 2sc2362 both read BCE from left to right. 123

so I checked em again..
Q13 16.25, -23.90, 24.05
Q14 24.44, -22.73, 23.88
Q9. .61, 23.94, 1.27
Q10 .-69, 23.88, -.06

Q13 is indeed very suspect.. I tried a diode test on it and 14.
The only difference I found. Was with positive lead on C, neg on B. 14 conducts and rises to just over 2v
this same test on 13 is OL
Not sure if that is also conclusive.
I’m learning a great deal of these transistors work by all this though! And I’m very grateful to you for helping me understand! No one else will! Thank you!!

[g1]

Ok. If this is right, from the data sheet.
the 2Sa1016 and 2sc2362 both read BCE from left to right. 123

Which datasheet are you using? See attached.

[g1]

Looking again at your readings, and reversing E & B, I have to wonder if fusible resistor R241 is ok?

[Indyryder02]

I had to look at a couple as it didn’t look very clear.. like it was backwards.. so these are actually ECB?

[g1]

Yes, they are ECB. And see post #63 if you missed it.

[Indyryder02]

Looks like 241 is bad.. as well as 239..
smh! I’d bumped something previously while testing. And these had smoked a bit. I must have just tested them for an open or short with my fancy auto meter. Which fooled me majorly! I used my ancient fluke and immediately saw they were open... I also never considered this, as it was done after the initial problems. So I assumed it was something else from previous.. (which of course there still may be.. but also wondering if the mismatch previously or trannies was causing me to burn up Q35 and 31.

I’ll have to pull the board and make the replacements to the resistors. Then retest and see where I’m at. Hopefully be headed in the right bias direction then!

I want to figure up my measurements from earlier and figure out how you were able to detect 241 being a culprit..
allowing too much positive voltage to 13?
Im sure you can explain it better. But I really love being able to diagnose the circuit in this manner. This has been most informative!! Thank you very much!

[g1]

Q13 emitter was at approx. 16V. It should have been near the rail voltage of around 24V, like Q14 emitter. After getting the E&B straightened out with regard to the pinout, the low voltage at Q13 emitter pointed to R241 not allowing the voltage through.

[Indyryder02]

Ok. I get it. That makes perfect sense. I hadn’t had a chance to go back and look at the values yet. After looking at the measurements again corrected for EBC. I see it. very cool!
Thank you so much! I’ll get those corrected and test again.
I can’t imagine I’m fully out of the woods yet on this. (Haven’t been that lucky yet on this demon!) But so much closer! Solid state is very similar in many ways to tube, but it definitely has its own characteristics to be aware of. Much appreciated!

[Indyryder02]

So. Replaced resistors. Brought it up slowly. Correct voltages at all the test points now... brought it up to full voltage.
it idled steadily enough for me to test points again.. still all ok..
really thinking it might be ok... then blamo!! It took out Q35 and 31 again. . Both shorted from E-C again.
When I checked all the outputs to make sure they were similar.. if anything.. I think the Emitter voltages were slightly off .. everything else looked good. Bias voltages were in spec. Bummer for sure!

[Indyryder02]

Checked all surrounding resistors and diodes.. everything checks out.. ugh! Love to find the cause of this! Can’t be far off... same two. And bias voltage right before it was good...
u less something upstream went to thermal runaway and knocked the rest out before I could detect it..
so aggravating as it all checked out good... and then even idled fine for a couple mins! Then just out of nowhere the needle on the amp meter pegged a couple times and I knew that was it again. Sucks!!

[Jazz P Bass]

"then blamo!! It took out Q35 and 31 again. . Both shorted from E-C again"
Are we testing this, at this point, without a load attached?

[Indyryder02]

No. I had a load attached.

[Jazz P Bass]

Sounds like genuine 'fake' transistors.
Where did you purchase them?

[Indyryder02]

Same two.... all have been replaced from same batch of new.
only these two burn up. Third time now.

[g1]

Output devices were fully secured to heatsink?
The output transistor's emitter voltages are critical within tens of millivolts. That is how you measure the idle current (mV across emitter resistors). So 'a little off' may have been more off than you might think.
I trust you powered it up without load first to verify no DC on output. That is an important step that must not be bypassed. Running it without a load for a few minutes may have revealed something running away but the output devices might not have blown with no load connected.