Beginners Road To Repair Part 2

[don@cascadefellowship.org]

Testing IC 2 in circuit.  TL072CN

Pin 3 connected to ground.  No pins shorted together.

Pin 1   Out 1   .748 volts (steady)

Pin 2   In 1-   .088 volts (steady)

Pin 3   In 1+   .002 volts (steady)

Pin 4   Vcc-    .753 volts (steady)

Pin 5   In 2+   .745 volts (steady)

Pin 6   In 2-   .207 volts (steady)

Pin 7   Out 2   .641-.646 volts (fluctuates)

Pin 8   Vcc+    15 volts (steady)

[tpowell1830]

Testing IC 2 in circuit.  TL072CN

Pin 3 connected to ground.  No pins shorted together.

Pin 1   Out 1   .748 volts (steady)

Pin 2   In 1-   .088 volts (steady)

Pin 3   In 1+   .002 volts (steady)

Pin 4   Vcc-    .753 volts (steady)

Pin 5   In 2+   .745 volts (steady)

Pin 6   In 2-   .207 volts (steady)

Pin 7   Out 2   .641-.646 volts (fluctuates)

Pin 8   Vcc+    15 volts (steady)

I am unfamiliar with this setup and have not done much troubleshooting on such a complex board, just to qualify what I am saying.

To be perfectly honest with you, as some of the more accomplished board troubleshooters will tell you, these numbers mean very little unless you describe the setup that you are using for your test, such as what meter you are using, what is the settings that you are using, what is the input signal? Are these opamps there to accept the audio input? Do you have the schematic to show where and how these are used? Are you measuring AC or DC? Are you injecting a test signal on the input?

I am sure there will be 40 more questions from more learned people.

[don@cascadefellowship.org]

Testing IC 2 in circuit.  TL072CN

Pin 3 connected to ground.  No pins shorted together.

Pin 1   Out 1   .748 volts (steady)

Pin 2   In 1-   .088 volts (steady)

Pin 3   In 1+   .002 volts (steady)

Pin 4   Vcc-    .753 volts (steady)

Pin 5   In 2+   .745 volts (steady)

Pin 6   In 2-   .207 volts (steady)

Pin 7   Out 2   .641-.646 volts (fluctuates)

Pin 8   Vcc+    15 volts (steady)

I am unfamiliar with this setup and have not done much troubleshooting on such a complex board, just to qualify what I am saying.

To be perfectly honest with you, as some of the more accomplished board troubleshooters will tell you, these numbers mean very little unless you describe the setup that you are using for your test, such as what meter you are using, what is the settings that you are using, what is the input signal? Are these opamps there to accept the audio input? Do you have the schematic to show where and how these are used? Are you measuring AC or DC? Are you injecting a test signal on the input?

I am sure there will be 40 more questions from more learned people.

Yes, of course.  You make excellent points!  I'm using my multi-meter (Fluke 117) to measure voltages at each pin with the red lead while the black lead is on ground.  I'm measuring DC voltages. The unit is powered on with no input signal.  The Gain and Drive controls are all the way down.  I'm using this as an opportunity to learn what each pin represents on the op amp and I'm documenting the voltages to see if there are any obvious signs of a malfunction.  I should preface that by saying, I'm learning these things myself and would likely not recognize an obvious malfunction if it fell out the sky, landed on my face and started to wiggle.

I have not net been able to find a schematic so I'm open to learning how to draw one up from scratch while doing this repair.

That said, I'm open to any and all advice.  The more newbie oriented the better.  I have no formal training in electronics repair but I'm getting better at learning to identify components and research online.

[tpowell1830]

Yes, of course.  You make excellent points!  I'm using my multi-meter (Fluke 117) to measure voltages at each pin with the red lead while the black lead is on ground.  I'm measuring DC voltages. The unit is powered on with no input signal.  The Gain and Drive controls are all the way down.  I'm using this as an opportunity to learn what each pin represents on the op amp and I'm documenting the voltages to see if there are any obvious signs of a malfunction.  I should preface that by saying, I'm learning these things myself and would likely not recognize an obvious malfunction if it fell out the sky, landed on my face and started to wiggle.

I have not net been able to find a schematic so I'm open to learning how to draw one up from scratch while doing this repair.

That said, I'm open to any and all advice.  The more newbie oriented the better.  I have no formal training in electronics repair but I'm getting better at learning to identify components and research online.

LOL, that should be a classic, nearly fell out my seat on that one...   

[don@cascadefellowship.org]

Yes, of course.  You make excellent points!  I'm using my multi-meter (Fluke 117) to measure voltages at each pin with the red lead while the black lead is on ground.  I'm measuring DC voltages. The unit is powered on with no input signal.  The Gain and Drive controls are all the way down.  I'm using this as an opportunity to learn what each pin represents on the op amp and I'm documenting the voltages to see if there are any obvious signs of a malfunction.  I should preface that by saying, I'm learning these things myself and would likely not recognize an obvious malfunction if it fell out the sky, landed on my face and started to wiggle.

I have not net been able to find a schematic so I'm open to learning how to draw one up from scratch while doing this repair.

That said, I'm open to any and all advice.  The more newbie oriented the better.  I have no formal training in electronics repair but I'm getting better at learning to identify components and research online.

LOL, that should be a classic, nearly fell out my seat on that one...   

All credit to Dan Aykroyd.

[Shock]

I can see Q19 which is a 7815C (+15V) and a TIP31C, TIP32C. What is Q22?

[don@cascadefellowship.org]

I can see Q19 which is a 7815C (+15V) and a TIP31C, TIP32C. What is Q22?

That's an L7915CV.

The output measures .755 volts to ground (steady).  And the input measures -2.365-2.372 volts to ground (fluctuates).

[Shock]

I can see Q19 which is a 7815C (+15V) and a TIP31C, TIP32C. What is Q22?

That's an L7915CV.

The output measures .755 volts to ground (steady).  And the input measures -2.365-2.372 volts to ground (fluctuates).

Well that may explain where the -15V went. What could cause a low input voltage? What do you think is the next step?

I already know the answers but it helps if you think it through.

[don@cascadefellowship.org]

I can see Q19 which is a 7815C (+15V) and a TIP31C, TIP32C. What is Q22?

That's an L7915CV.

The output measures .755 volts to ground (steady).  And the input measures -2.365-2.372 volts to ground (fluctuates).

Well that may explain where the -15V went. What could cause a low input voltage? What do you think is the next step?

I already know the answers but it helps if you think it through.

Based on my limited experience and my last repair, a short along the signal path before the input could cause a low input voltage.

Q19 Voltage Regulator L7815CV

Input 26.98-27.47 volts to ground (fluctuates, ramps up slowly)
Output 15.00 volts to ground (steady)
Ground 0

Q20 Transistor TIP31C

Base 28.23 volts (steady)
Collector 46.32-46.43 volts (fluctuates)
Emitter 27.62-27.64 volts (fluctuates)

Q21 Transistor TIP32C

Base -2.92-2.97 volts (fluctuates)
Collector -46.39-46.48 volts (fluctuates)
Emitter -2.43-2.49 volts (fluctuates)

Q22 Voltage Regulator L7915CV

Input -2.365-2.372 volts to ground (fluctuates)
Output .755 volts to ground (steady)
Ground 0

[Shock]

Ok sounds like you might be stuck for ideas.

You're close. A short on the regulators output also may cause the voltage to drop or an over current situation may cause the regulator to fail or be damaged.

So remove the -15V regulator and then see if the input voltage comes back up by measuring input to ground. Not all components are safe to just remove (there are exceptions) but this is a power rail and we can see shes pretty much dead anyway.

With the power removed and circuit discharged measure resistance (ohms) across the output pin to ground and if you got no input voltage measurement before in that previous check measure the input to ground as well. Check you have a low resistance path to other grounds. I can see on the PCB that the grounds appear common between the +15V and -15V their pinouts are not the same. Look at datasheets!

Note: to confirm if a circuit is discharged we can measure voltage across capacitors or the voltage across the place you want to take your measurement. Some devices like CRTs microwaves and large power supplies can hold charge for quite some time. Another way to make it easier to discharge circuits sometimes is to flip the power off while the device is on and then wait a little and unplug it. If you don't test for voltage before you check with ohms or diode test with certain meters you will damage them. You will wonder why you are getting bad measurements and it's another good reminder that if there is voltage present you need to then double check the device is off unplugged and isolated from the mains.

When I measure to see if a circuit is shorted I normally take positive to negative, negative to positive and then I do the same with a diode test. So you get resistance but also can "characterize" the circuit for a potential short and see if there is a charging capacitor on that circuit or a semiconductor or some other voltage drop. If you did this with the regulator still in circuit it could be misleading if two faults are present i.e. short on output causes the regulator to fail.

If you have a power supply that is high enough to supply the -15V input of the removed regulator (you go look up the datasheet) you can test it out of circuit. It may need a heatsink and/or get hot quick. If it produces an output voltage of -15V it may be inconclusive test still without a load (it's faulty only under load), but if gives you an idea and you can also measure it for a short.

If you have a regulated power supply that has a floating output and adjustable current limiting you can also power the amp directly from the supply by connecting it to ground and the output pin of where the -15V regulator was. By current limiting you can monitor the voltage across the op amp (where -15V should have been present) and slowly bring up the current. You would expect to see the voltage rise. If it doesn't you have a high current path to ground somewhere.

In case it's not obvious, if your bench test power supply is floating (no reference to ground/earth or to the circuit under test) by reversing the positive and negative over it becomes a "negative supply".

There are a lot of concepts covered here.