Sony receiver in protector mode. How to check transistors?

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Old 11-18-13, 04:37 PM
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Sony receiver in protector mode. How to check transistors?

Hey all!
I found an old response to the same problem with the same receiver written by ecosseman. 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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Old 11-18-13, 05:25 PM
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Welcome to the forums.

If the unit is in protect then it usually means a pair of output transistors has shorted.
I identified the three major output amp pairs.

Make your tests with the unit unplugged for now. You'll need to wait a short while for the two large power supply filter capacitors (big, round, black) to discharge before making tests.

The white parts in the blue square's are the dual emitter resistors. Two resistors that share a common center connection. Very low value.... usually around 1 ohm.

. Set your ohmmeter to Rx1 (unless it has a diode setting). Work with one transistor at a time in a pair. Check between each lead. The short should be pretty apparent as you'll find one pair with one or two transistors shorted and the other two pairs that don't show a short or possibly no resistance at all on the Rx1 scale.

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Old 11-18-13, 05:54 PM
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Hello PJmax,
Thanks for your reply!
With lead do you mean leads as transistors have 3(what I call pins)? My multimeter only have a OHMX1K setting which I suppose 999ohms to many? And with my multimeter would I put the red or black wire on the resistor connection and the other on one of the leads(pins), does it matter which lead?
I hope you have patience with a newbie like me
 
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Old 11-18-13, 07:31 PM
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The pins on a semiconductor are called leads. Just focus on the transistors. Check one at a time. Put the red probe on one lead and the black probe on another. If you see a low resistance... you've found a shorted part. Otherwise.... leave the red where it is and move the black to the third lead and read it. Now leave the black probe where it is and move the red probe to the other lead.

You want to check all possible combinations of the the three leads.

You should find a pattern. You'll probably find two pairs check very similar and one pair exhibits very low resistance.

Don't worry.... I have a lot of patience.
 
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Old 11-18-13, 09:31 PM
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Alright, so I did that and all the left ones have similar pattern and the right ones have the same pattern. Tomorrow I'm going to get another multimeter from work and try it again just to double check.
 
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Old 11-20-13, 05:50 AM
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Resistance measurements do not prove anything unless at least one of the leads to which you are touching the meter probes has been unhooked from the rest of the circuitry. (Or for probing screw terminals at least one of them has all the wires unhooked from it.)

In this case, if you did not unhook a transistor lead, you would not know whether a short was in the transistor or somewhere else nearby in the circuitry.

With transistors or diodes or other semiconductors, it will make a difference whether you put the red or black probe on a given test point. In this case you will need to do it both ways. Typical battery operated hand held multimeters use batteries (DC) to do resistance measurements.

Also be aware that the test current from a multimeter can fry small transistors simply by making a resistance test.

To bleed off filter capacitors, read off the voltage rating on the capacitor, multiply by 100, get a one watt resistor of that value and touch the leads of that resistor to the two leads of the filter capacitor for ten seconds. It is not necessary to unhook the capacitor from the circuit. Do not use a jumper wire to bleed the capacitors, the sudden discharge can damage a capacitor.
 

Last edited by AllanJ; 11-20-13 at 06:30 AM.
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Old 11-20-13, 09:59 PM
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You want to first establish if you have an output transistor(s) blown. You don't need to forward bias them at this point and there aren't any small transistors in the output stages of that amp that will short from the test meter.
 
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