fault finding BJT replacement in Sony STR-DA5300ES power amp

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tfboy

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Joined 2006
#1
My father's STR-DA5300ES home cinema amp keeps kicking into protection mode when turning on. Before that, it worked fine, but all of a sudden, turned off, kicking into protection mode. Apparently, there was that slight burnt electronics smell too (I wasn't there).

After unplugging all the speakers, it still does it, so I reckon some of the transistors on the power amp section have failed causing a short - which would be consistent with the smell.

I've now removed the power amp board / heatsink assembly and using my nose 🙂D) it seems quite obvious that something has blown! Although visibly everything looks OK, something is not quite right. I'm actually rather shocked to see the state of the soldering and flux.

3659792117_NM2F3df-O.png


From the looks and smell, I have a feeling it's the Sanken 2SB1647 and/or 2SD2560 from a channel that have failed.

I need to figure out how to test them and identify exactly what has failed.

For those who know this stuff (it's a long time since I did any power analog audio stuff), I suppose these are the best / most stable transistors to use and you wouldn't recommend changing them to anything else? A bit of searching has revealed the service manual of the amplifier, and this is the power amp board circuitry:
3659772984_cqFGQQN-O.png


Any other thoughts? Thanks for any ideas 🙂
 
#2
The soldering looks fine tbh.

You need to confirm by measurement if any devices have failed short circuit collector to emitter. Also check ALL those low value emitter power resistors and the smaller ones too.

A good chance there is damage further back in the driver stages if you find the transistors have failed. I've no suggestions on replacements I'm afraid, those are power darlingtons with inbuilt bias resistors.
 
#3
Mooly, I think I owe you a beer or two!

Upon closer inspection the small 100 Ohm R1567 has a bit of its outer casing missing! Checking the large R1566 shows open circuit on one of those dual resistors.

3659827775_p6jSvdG-O.jpg


Maybe the transistors are in fact fine, and it's just the resistors that have failed. I'll go through checking C-E shorts on them. But more interstingly, I recall the front right channel sounding a little muffled compared to the others when setting up speakers with pink noise. And, it's that channel that has the failed components...
 
#4
I wouldn't get your hopes up that quickly tbh. There is a 99.5% chance of finding at least one output transistor failed and more importantly, failure of the driver stages (which aren't shown on that diagram). Its a classic failure mode.
 
#5
Thanks Mooly.
The Q1558 2SB1647 is shorted C-E. Other components look OK, going back up that channel checking other stuff where I can.

Parts needed so far:

R1566 - dual 0.22R 5W resistor.
R1567 - 100 ohm resistor. Easy enough
Q1558 - Sanken 2SB1647. Available on ebay
 
#6
 
#9
Small update: replaced the blown 100R resistor, and tested the various components in that audio path.

I've replaced the dual 0.22R 5W with some beefier resistors and the 1647 Darlington.

I'm waiting to get back to my dad's to plug it back into the chassis. With any luck, it'll live again.
 
#12
A bulb tester is a mains filament bulb (typically a 60 or 100 watt light bulb) that is wired IN SERIES with the mains supply.

If there is a fault such as the output stage wanting to draw excess current, then the bulb lights and hopefully saves the transistors.
 
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