Exposure IV....fire in the hold!!

[Tonewheelkev]

I've recently lost at least one Isobarik mid driver. This EXPIV RH channel would appear be the culprit.

I thought I'd rule out failure of the O/P transistors...so removed/tested/refitted these, thinking it was a good idea to do this first!!

Next, powered the thing up, and was in the process of comparing voltages between the Good LHS and Bad RHS....when R30 (100 Ohm) burst into flames.
A bit of excitement there.....and a 1st for me.

Wondering if the 'driver' transistor SM2177A just above R30 has failed....centre pin goes to R30

 

[colasblue]

Actually it's more likely to be a fault with the output transistor.

They can test good sometimes at low voltage but in reality aren't good at their operating voltage.

The Amp John & Jake fixed recently actually had a similarly burnt R30 but the driver transistor was fine. It's actually massively overspec'd and quite unlikely to fail.

You can't get an exact replacement at any sensible price but I imagine pretty much any commonly used To220 PNP driver transistor would do at a pinch if it has gone. The much more modern MJE15033 would be a fairly safe bet.

If you're using tantalum feedback or bypass capacitors then they're likely to be toast too.

 

[Tonewheelkev]

Thanks for getting back.
I've replaced all Tants on this, and my other Exposure stuff after realising how many of 'em had failed in a VII preamp I have.

So it could still be the EXP O/P trannies......they appeared OK with my DMM, but that can't be relied on....right!!!

Can I rely on DMM to test the SM2177A.....or can that also give an unreliable result?

If the EXP O/P transistors have gone... is it likely that something 'upstream' has had a hand....or can they just give up the ghost over time?

 

[colasblue]

If it tests bad then it's definitely bad, so worth doing.

It's not usually something "upstream" that kills O/P transistors, more likely something downstream such as short circuit or low load causing overheating.

Expo amps are absolutely not suited to driving Bricks since their heatsinking is limited and getting O/P transistors hot will certainly shorten their life. Your desolder and resolder may actually have tipped it over.

I assume it's actually gone "phut" with no load attached so quite likely to be an O/P transistor breakdown. Even with major upstream faults the O/P transistors can easily survive an output voltage "hard over" when no load is connected if they're working OK.

You should actually be able to power it up and test voltages upstream with the power transistors out of circuit. I'd suggest test TR10, re-install if seems OK or replace if not, replace R30, power up without O/P transistors and see what happens.

The trouble with the dual Sziklai stage is that the driver transistor can short without dumping a load of current through the power transistor, and a power transistor b-e short could easily blow out that 100R without hurting the driver transistor.

If it holds without the power transistors then TR10 is OK, if not and the 100R fries again you've found your problem.

 

[Tonewheelkev]

If it tests bad then it's definitely bad, so worth doing.

It's not usually something "upstream" that kills O/P transistors, more likely something downstream such as short circuit or low load causing overheating.

Expo amps are absolutely not suited to driving Bricks since their heatsinking is limited and getting O/P transistors hot will certainly shorten their life. Your desolder and resolder may actually have tipped it over.

I assume it's actually gone "phut" with no load attached so quite likely to be an O/P transistor breakdown. Even with major upstream faults the O/P transistors can easily survive an output voltage "hard over" when no load is connected if they're working OK.

You should actually be able to power it up and test voltages upstream with the power transistors out of circuit. I'd suggest test TR10, re-install if seems OK or replace if not, replace R30, power up without O/P transistors and see what happens.

The trouble with the dual Sziklai stage is that the driver transistor can short without dumping a load of current through the power transistor, and a power transistor b-e short could easily blow out that 100R without hurting the driver transistor.

If it holds without the power transistors then TR10 is OK, if not and the 100R fries again you've found your problem.

A fair bit to be going on with there....thank-you.
This EXP IV is only driving the mids (in Aktiv system).....I had hoped that'd be OK???

 

[colasblue]

I dunno. My VIII is uncomfortable driving Kans and two B110's in parallel must be a "heavier" load in principle, but eliminating the extreme LF ought to make it easier.

If it wasn't running hot then it's a a bit more of a mystery.

 

[Tonewheelkev]

O/P transistors....
EXPN (15003 position, next to burnout)....pins open circuit in both directions.
EXPP (15004 position) ....E/C Short

The SM2177a shows sub 0.6v drops between pins1/2....and 1/3 so I think this is behaving correctly.

Getting somewhere