Sony TA-F3000ES - doing my head in

[keland_uk]

Sony TA-F3000ES

Ok hopefully someone will spend the time to point me in the right direction.  This has all the hallmarks of a short somewhere, but I can't find it.

Maybe someone could also do a "quick" teardown of what the "protection circuits" are doing, The differential amplifier voltages all look ok.. Q506 collector voltage was at zero, and once removed the B+ that was on the bases of Q510 & Q511 became -ve

 

Blown fuses, Dim bulb tester lights up on applying power

1) Removed MOSFETS Q512, Q513, Q612, Q613. Q512, Q513 were bad.
2) Checked voltages on both channels, all appeared to be good (B+ 34.5V, B- 35V)
3) Replaced all MOSFETS, dim Bulb Tester lit up :-(
4) Removed Q512, Q513 - Amplifier powers up, but Voltage at 1 was 33V, @2 was 7.3V (Base of Q510 & Q511)
5) Removed Q510, Q511 both tested ok, same voltages at same points (In Hindsight don't know why I removed them,
its the voltage going into them that is stressing them out.
6) Removed Q509, tested ok, voltages the same.

so now into the SMD area.

Voltages in RED before removing Q506 - it was showing zero on its collector

Then blow me all the voltages went to B-,

So I'm pretty stuck now, I have an esr tester, and thought C505 might be a problem, since its ESR is 0.12,
 but tests ok when removed, albeit still low ESR and -ve voltages still present at Collector Q506 and at points 1 & 2

It could be Q508, but not shorted.

[YetAnotherTechie]

For now i can only suggest that you check whether B+ and B- can sustain a few amps each at the same time, if the main transistors blew, something upstream of B+ or B- might be damaged too.

[timenutgoblin]

Check C510 (330µF 25V). Maybe it's leaky? Probably not, though.

[gaztech]

To help us with this, a few questions:

What was the original fault? 

Was it playing up and then stopped? - Or did it just stop dead?

Both channels have the same fault? If so, if you disconnect output devices from each channel alternatively can you get one side to come up?

It's odd if both channels failed at the same time - if that is the fault...

Do you have any burned components on the board - especially from the original fault?

[keland_uk]

removed C10, its fine.

Resistors R519,520 and 521 are 440, 58 & 58 ohm

on other channel they are

R619,620 and 621 are 440, 58 & 94 Ohms


so R521 & 621 are different, the R621 channel seems to work, well voltages are within spec.

R520 & R521 measure 58 ohms out of cct. (20% out of spec)

[keland_uk]

What was the original fault?
Came with both B+ & B- fuses blown, removed MOSFETS, Q510 & Q511 Failed test (Peak DCA75), Q610/Q611 differnt type, passed ok. I've brought some new MOSFETS from a reputible source
Was it playing up and then stopped? - Or did it just stop dead?
Unknown
Both channels have the same fault? If so, if you disconnect output devices from each channel alternatively can you get one side to come up?
other channel voltages seem ok, once Q510/Q511 MOSFETS removed (Using Dim Lamp Tester DLT). New MOSFETS were swapped around and same problem
It's odd if both channels failed at the same time - if that is the fault...

Do you have any burned components on the board - especially from the original fault?
not that i can see, some heat damage to main board above the underside SMD devices, suspect thats normal

[strawberry]

BIAS fault, or driver open (floating gate)

[keland_uk]

Hurrah, or not really. D505 is around the wrong way!
It’s a SEL1501C. Lesson to be learned here, what's been changed if anything. I ASSUMED that the red and green LEDS D505 and D605 were different colours to help distinguish between the channels, silly me. Both are SEL1501C.I think they are non-tinted green with a Vf of 2v.
Red tinted one (D605) was 1.96v and around the right way, clear one was suspect at 7.6V!. Anyway I' replaced both with a Square green LED at 1.96V Vf. and lo and behold both channel voltages behave the same without MOSFETS.

Short Emitter of Q510 & Q610 to Bias connector between both 0.22ohm resistors. Both channels behave roughly the same. Put MOSFETS back into "6" channel and voltages back to where they should be!.

Mostfets back into channel "5". It doesn't light up the DBT . Q512/Q513 get rather hot compared to Q612/Q613.

onwards and upwards.

No1 lesson: identify what’s been changed, don't assume anything..... (D605 looked like it had been changed, D505 didn't but must have been....)