Pioneer qx-8000a broiled resistors

[Goldie99]

The problem with eBay transistors is you simply don't know - the only way you'll know, is to either check them, which is basically impossible without specialist equipment & knowledge, or to fit them & see if they fail when you push them...... but that can lead to the same sort of damage as you already have, or worse.

I prefer to opt for either new equivalent transistors, or eventually new old stock, but in all cases only directly from certain reputable suppliers. I don't have the equipment or knowledge to test them either, so I just go for the least risk option. I'm sure others will have their own preferences, but at least you're aware of the risks now.

If you search the Audiokarma forums, you'll find (readily available) equivalents for all of the transistors on the amp board, including the TO-3 outputs. I'll have a look in the morning as well, when I have time.

 

[ivo]

Ok, i removed Q3 and Q5 from the board and they are bad. Are the 2SA497 and 2SC497 same transistors? On the top view schem (page 53) q3 is 2sa497 but the same transistor on page 52 is 2SC497. Q5 on both pages is 2SC497. I already purchased two 2SA497 so now I need one 2SC497 OR MAYBE TWO OF THEM IF THEY ARE DIFFEREN KIND.. Im not sure if there is replacement with new one component- There is not data on the service book.
So probably ill take another risk with e-bay sellers

 

[ivo]

So far - two ouputs bad, three drivers, two emitter resistors and three resistors. Hm,m, not bad! obviously, the barbecue there had been fine...

 

[Goldie99]

Not good news - looks like every semiconductor in that amp circuit is gone. Hopefully it stopped at C3....if you're (at all) lucky Q1 will still be ok.

Anyway, 2SA497 is not the same as 2SC497.

# 2SA497 is a 'pnp' type transistor, correctly shown & drawn on the schematic on page 52, and the layout on page 53 (Q7 & Q8)
# 2SC497 is a 'npn' type transistor, correctly shown & drawn on the schematic on page 52 (Q3, Q4, Q5, & Q6)

Q3 is wrongly shown and drawn on the layout on page 53, and is an example of the sort of mistake that can turn up in service manuals, all too often. Q3 on page 53 can be identified as the mistake, as the symbol shown there is for an 'npn' transistor, but it's labelled 2SA497 - all '2SA.....' designated transistors are 'pnp' type, so it must be a mistake. That's another reason to keep track of exactly what's actually installed where - the part that's physically on the pcb was (presumably) working previously, so should be correct.

The service manual would have been written at a time when the original transistors were still in production, so no alternatives were needed or included. That's no longer the case, but a quick 5 minute search of the forum gives the following alternatives (all courtesy of 'markthefixer'), which I think are probably suitable here.

2SA497 - 'KSA1220', e.g., http://www.mouser.co.uk/ProductDeta...=/ha2pyFadugRA4BHq0Xt4UBKCc5BImdQsauZAXS4wH8=
2SC497 - 'KSC2690', e.g., http://www.mouser.co.uk/ProductDeta...=sGAEpiMZZMshyDBzk1/Wi8oN7VHZ91OkPdp4Tc8ksXs=
2SC793 - 'MJ21194G', e.g., http://www.mouser.co.uk/ProductDeta...%2bAR7wx%2bmvwh4wsKHB%2b1mdhDUX1oT/PHwheTdA==
http://www.mouser.co.uk/ProductDeta...%2bAR7wx%2bmvwh4wsKHB%2b1mdhDUX1oT/PHwheTdA==
The transistors can be relatively easily replaced. At this point however, I'd be more concerned with D1, the SV-4A diode - there are possibilities for replacing it, but there is no direct equivalent available. I think all you can do is leave it alone, and cross your fingers... unless anyone else can suggest a good way to check it (within minimal risk) ??

 

[ivo]

Goldie99, thank you for the information! I have never use the forum search, so I did it today and Oh! there is a lot of information from the forum guys!! You guys are doing great job here!! There is not a lot of forums as Audiokarma.Not a lot!!! I ordered the parts from Mouser electronic 4 pcs of each in case i burn out some of the transistors in the time of checking

 

[ivo]

Yes q3 is originally installed as 2sc497 what is npn on the schematic

 

[ivo]

Hi goldie99, im just watching the c
schematic right now and im wondering how R39 got burn out - the only way (my opinion) is C13 to get short and then r39 fire up, doesn't it? And if it is corect then R33 got fire too because it is just 0.5 ohm and the whole voltage hit the output 2cs793 ???

 

[ivo]

Hi GUYS! This is a question: What is going to happen if one driver is installed wrong - the base plug into emitter, collector in base and emitter in collector place? Is this connection going to harm the output transistor?? Thank you for any answer!
P.S. And the driver is the next to the output

 

[cademan]

A shorted transistor will cause most of the damage you are inquiring about here. The 0.5Ω resistor is the emitter resistor for that specific transistor so if it got fried, this would indicate that that power transistor is shorted.

 

[Goldie99]

.. im wondering how R39 got burn out - the only way (my opinion) is C13 to get short and then r39 fire up, doesn't it? And if it is corect then R33 got fire too because it is just 0.5 ohm and the whole voltage hit the output 2cs793 ???

The series combination of R39 & C13, in parallel to the speaker load, is what's called a Zobel network - it's used to stabilise the amp against potentially harmful oscillation. The usual reason for R39 failing would be the amp oscillating. I think it's very unlikely that C13 is at fault.

Overall though, the amp has continued to be used after the original fault(s) occurred, possibly with new fuses having being fitted (and a wrongly installed driver ?), so the damage has likely progressed 'deeper' into the amp circuit, to the point that the original fault is now difficult to identify.

As far as I can see, all you can do is systematically check all components of that amp channel, and replace/rebuild it accordingly.

... Thank you for any answer!....

Not sure where you're located, but I'm in UK, so work on UK time.... replies may seem slow.

 

[ivo]

Im at US. I found Q7 pnp transistor installed wrong. the base leg on the emitter hole, emitter on collector and collector on the base hole. And it is an original driver transistor, compare it with the other board. I marк off one leg of any transistor desoldered. Ill upload pictures later. Still waiting for the driver transistors, all other components are installed.

 

[ivo]

Whats mean "amp oscillating "? Is it like "unstable" because some of the components there do not function in the way they should?

 

[Goldie99]

Whats mean "amp oscillating "? Is it like "unstable" because some of the components there do not function in the way they should?

Yes, and no - yes, it is essentially 'unstable', but no, in my understanding the zobel is normally to protect against oscillation coming from / caused by interaction of the load (speakers) with the amp, rather than a faulty component in the amp itself. If you google 'zobel network' it'll probably explain it better than I ever can.

Interested to see the photos - find it hard to believe Q7 would be wrongly installed from new ? Did you get datasheets for the 2SA497 & 2SC497, to identify which lead is which ?

 

[ivo]

No i dont have datasheet for 2sa497 and sc497 i figured out which lead is what just visually.And i compared the position of the transistors with the second board which is absolutely same (this is quad amp). Now when i back in time i cant say 100% that i didn't mess the mark lead it was first transistor which i took out from the board, so may be im wrong - i cant either believe it is installed wrong from the Pioneers tech

 

[ivo]

The black dot on the board indicated where thr mark lead was but what i said it is may my mistake.

 

[Goldie99]

Short datasheets are both here - this is the 2SA version, just change to 2SC497 in the search box for the other (they're the same orientation):
http://www.semicon-data.com/transistor/tc/2sa/2SA497.html
http://www.semicon-data.com/transistor/tc/2sa/2SA497.html
I also edited your photo to add labels for ECB, where they should be, just by looking at the trace positions marked on the pcb

 

[ivo]

Sorry in my previous post i said "i can say 100% that i didnt make mistake" this is wrong actually i want to say "i can't"

 

[ivo]

Ok! Two outputs, three drivers transistors, two emitter resistors and three resistors are installed.All circuit board connections restored. DBT is build and ready for a test. Im seating in front of my baby and gain momentum to press the button. It's not easy...5 minutes...10... No, i can't do that! I need to call Jaky and ask her to press the damn button!If the bulb gets bright that is going to be her fault, not mine! Ok she came and press the dbt switch as easy as she turn on her TV!! And they saying the girls are very sensitive creatures!!!They are NOT! They are hard as stone. The dbt bulb remains dark. The receiver lights beautiful. All channels are working! Amaising sound!! I did it!!!
Goldie99 who lead me step by step and all the guys who helped me with good advice and diagnostics: THANK YOU, THANK YOU!!!

 

[Goldie99]

Excellent news & congratulations ! Before you finally button it up though - you should just carefully check the centre voltage, and idle current.

I couldn't see any instructions in the manual, but for the centre voltage you're looking for ca. 26.2V at the output end of R33 (it should be adjustable with VR1).

For the idle current, just measure the voltage drop across R35 - it should be ca. 25 mV if R35 is still 0.5R (it's marked on the schematic). It should be adjustable with VR3, VERY gently, it'll only need very slight movements, and takes time to 'settle' thermally. I prefer to use mini-grabber leads for measuring bias - it's far too easy to slip with normal probes, and short something out...

For both settings, you need no input, all controls centred, and volume set to zero - without the DBT obviously.

 

[ivo]

Thank you goldie99! What is mean ca.