Sony receiver in protector mode. How to check transistors?

[ll979]

Hey all!
I picked up a Sony receiver on the street and when started "protector" keeps flashing and nothing works. I found an old response to the same problem with the same receiver written by ecosseman in a different forum. Questions and picture are following :


Hi Gavin
Hope you haven't paid someone to repair the 915 by now.
Have repaired the d915 a few times.
1) Short output transistors or drivers (all on ali heatsink).
Easy to check in situ with power off. Usually only one pair fail at a time.
2) IC701* internal failure, giving Vcc and/or Vdd to output transistors.
With power on .....compare volts on o/p transistors (they won't all be
short).
Each pair of transistors should have Vcc and Vdd respectively,
on the centre pin.
3) If any of above have failed, check both halves of resistors R715, R672,
R765.
4) Check all fusible resistors R759, 760, 629, 655, 654, 668, 664,710, 709,
717.

*Note Check data sheet of IC701 (STK3102) on DataSheetArchive.com.
Can be interesting to research fault before fitting new components.
Regards
Tom


Now, know very little about electronics and could use some help in translating Toms words.
1)I have an old analog volt meter, with that how do I check if any of the transistors are shorted?
2)Does he mean to check only the center pin on each pair, AC or DC? All Left transistors in each pair reads 0V on center pin and all Right reads 50VDC and 110VAC on the center pin. But, the voltage on the R pins and L pins vary.
3 and 4) How do I check them? If they show any voltage on both sides of the resistor they're good?

Thanks for any help or advice!

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[KrisBlueNZ]

I suggest you buy a digital multimeter. You don't need to spend much - $20 will get you something reasonable. Analogue multimeters are cool but measuring resistance is different from digital meters and it's probably best to avoid confusion.

In general when a direct-coupled output stage fails, all or most of the semiconductors fail at the same time, and one or both of the rail fuses blows a short time later. This is likely to include the driver IC, which may have failed first.

In your case, it's possible that some of the transistors have survived. Check the fuses but don't replace any of them if they're blown until you've replaced the damaged components.

Get the part numbers for the driver IC (STK3102 is mentioned in your post) and output transistors, and see whether you can get replacements. (Use Google.)

When testing for shorted output transistors, do it with the power OFF. The important measurement is between the middle lead and the right hand lead (when they're oriented as shown in your photo), but if the device has failed, you'll usually measure a short (or nearly) on all combinations of leads. Measure resistance each way round. If the meter reads more than around 1000 ohms (1 kilohm) both ways, the device is probably OK. If in doubt, remove it and re-measure; it should measure open circuit in both directions between those pins. (The meter will indicate overload, as it does when the probes aren't connected to anything.)

When everything has been replaced, take the positive and negative rail fuses out and replace them with resistors of around 47 ohms, rated at 1 watt or higher. If there's still a fault, the resistors will prevent the components from being damaged. If there's a fault, the resistors will get extremely hot very quickly though, so don't get too close to them when you turn it on!

When you're measuring voltages with the power ON, slide some insulation onto the meter probes, so that if the probe slips, it can't accidentally short a transistor lead onto the heatsink. That's a sure way to break something!

I've had some experience repairing a similar Sony product - a STR-DE915. Almost the same part number as yours. I replaced a Sanken driver IC and some other parts and it was working fine. When I gave it back to the owner, after a while it failed again. He told me he spoke to a Sony representative who implied that problems are common with these units if they're manufactured in a particular Asian country - I don't remember which country it was, sorry. But the upshot is that you may be fighting an uphill battle with this one.