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Stage Line 500W amp repair project

KrisBlueNZ

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Nov 28, 2011
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Fit the resistor fuses first and repeat the voltage measurements from post #295. Actually, just the base-emitter voltages on the positions for the four output transistors is enough, and the DC voltage at the output. If the base-emitter voltages are all less than 0.6V and the DC output voltage is less than ±50 mV then fit the quick-blow low-current fuses and measure all the voltages marked in post #295.
 

bikemech

Oct 23, 2014
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Fit the resistor fuses first and repeat the voltage measurements from post #295. Actually, just the base-emitter voltages on the positions for the four output transistors is enough, and the DC voltage at the output. If the base-emitter voltages are all less than 0.6V and the DC output voltage is less than ±50 mV then fit the quick-blow low-current fuses and measure all the voltages marked in post #295.

When you say and the DC voltage at the output, would that be from T4 to the collector on each T03?
 

KrisBlueNZ

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No, it's the point marked "~0V" in post #295, yes measured relative to T4. And the equivalent point on the other channel.
 

bikemech

Oct 23, 2014
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the new drivers came this morning, shal i fit them befor i take the mesurements?
 

bikemech

Oct 23, 2014
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AAAHHH damn it Q7 was dead to, back when i have been over the board AGAIN!! and have Q7 bc549 replaced, this is NPN correct?
 

KrisBlueNZ

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Yes. At least a dead Q7 wouldn't have caused any other problems.
 

bikemech

Oct 23, 2014
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Ok kris Q7 replaced,
output voltage on the left chanel is -089m
Q18=24.3m
Q20=24.3m

output voltage on the right chanelis 31.9m
Q28=00.0m
Q36=00.0m
 

bikemech

Oct 23, 2014
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Sorry for the late post, been really busy, will get the other fuses installed and take the rest of the measurements asap, am not going to give up in this, it can't be to far away now
 

bikemech

Oct 23, 2014
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Is anyone else willing to help out on finishing my amp, I have spent months on it with kris and it would be a shame not to finish it now
 

Arouse1973

Adam
Dec 18, 2013
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Hello I am a bit concerned about Kris also. I have messaged him but he has not responded, I am trying to find out if he is ok.
Adam
 

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Ah, this I'd the thread you were talking about. I'll try to read through it all and come up to speed before I reply.

Please send a message to me directly if I take more than a few days -- I am pretty busy right now.

As said in other places, we're all concerned when people suddenly stop posting :-(
 

(*steve*)

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Hi Pete, I've read through the thread once to get an overall feel for what is going on. I'm going to have to get a schematic and read through it again to try to gain more familiarity with the amplifier.

You've been doing some great work and I will do my very best to help.

You've probably noticed that different people have a different approach to troubleshooting. I have seen that you've been a little affected by this.

It's probably best that you concentrate on what just one person is saying to you, unless someone says *STOP!!! YOU'LL KILL IT*.

For the moment that person can be me. If other people make suggestions then I'll keep them in mind. If they are requests for simple measurements, there's no problem in making them if they're easy to do. The problem comes in more where two people ask you to remove different components etc.

The first thing to do is to measure the resistance of the replaced resistors (in circuit) and compare these with those of the other channel even before you apply power. Any difference of 5% or more should be noted, and don't apply power if the difference is > 10%.

The next thing that you can do yourself is to have all the various resistor fuses in place and to measure voltages at like points in both channels relative to the ground point you've been using. Make these measurements first in the area of the components which were recently replaced. Make sure that you don't have components removed from one channel that are present in the other channel. What you're looking for is "significant differences".

It will be hard for you to know what is significant, in some places a fraction of a volt difference is huge, and in other places several volts difference can be acceptable. Start by looking for large differences, say of more than a volt. If you find any of these, note where they are, then after you have a few, turn the power off and measure the resistance across nearby components looking for any difference (5% or more).

This will give you a quick "reasonableness check" on the replaced components. and may also spot any differences that we need to check out.

Note that if either protection LED illuminates then there is not much point in doing any of the voltage tests. You did a great job of jumpering the transistor do defeat the protection circuit last time. If either protection LED is on, I would jumper the protection transistor in *both* channels before doing the comparative voltage checks. This will eliminate any differences caused by the difference between a turned on transistor and a jumpered transistor.

If I've given you too much to do, relax, I'll be back again as soon as I read the thread again (which may be tomorrow).
 

bikemech

Oct 23, 2014
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Fantastic , thank you so much for helping out, I will wait for you to get back to me,
 

(*steve*)

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Here are some notes for myself (also for Pete to check) as I read again through the thread, this time having an overview and a schematic.

  1. The fault LEDs on the board indicate a power supply fault in one of the rails used by the output stage. These are the 4 rails provided by the 4 fuses which now have the "blown" indicator and sacrificial resistors. .
  2. The output of each channel consists of a single complementary pair of MJE15003/MJE15004, not the two pairs as shown on the circuit diagram.
  3. In the left channel, the MJE15003 was dead and replaced.
  4. You accidentally connected the cables to T19 and T24 around the wrong way (the un-fused +/- 45V power supply connections) resulting in both onboard fault indicators being lit.
  5. (4) resulted in the mains fuses blowing and Kris suggesting the replacement of D4-D7, D9, D10, Q1, Q10, Q11, and Q22. Q12 and Q13 were also subsequently replaced.
  6. After some testing of resistors, R70, R40, C11 and C21 were replaced. (R70 had gone high, both had been hot)
  7. Testing was carried out with the output transistors and the driver transistors removed.
  8. After a while it was realized that the protection circuit was disabling the amplifier, so the protection circuit was disabled with a link across Q2's collector and emitter. This disables the effect of the protection circuit, but the LED remains under the control of the protection circuit.
  9. Q7 was found not to be in intimate contact with the heatsink (an omission???).
  10. Q46 was removed as part of an effort to diagnose *why* the protection circuit was activating.
  11. A stray piece of copper was found floating around on the board. Removing this fixed a fault with the fan. (Which may have been the ultimate cause for the amplifier's failure...)
  12. The power supply rails read high (62V rather than 45V) but there is no load on them and they are unregulated so this is not an immediate cause for concern.
  13. After a lot of effort, R117 was found to have some sort of intermittent failure (which is pretty unusual, I certainly wasn't expecting it). After replacement the fault light was extinguished and subsequently the jumper across Q2 was removed.
  14. At some point all the removed/replaced transistors were fitted. I missed exactly where on my second read through, but they were back in place by here.
  15. Some voltage readings suggested insufficient bias on the left channel which Kris thought may be due to the "resistor fuses"
  16. An error caused several components to require replacement... I have a list of R30, R32, R89, R23, and R21 along with "the driver transistors" (which I assume to be Q12 and Q13). Later Q7 was found to have failed (R23 was the giveaway for this).
  17. Kris has left the building :-(
Pete, if you can check that I have listed everything that was replaced, and especially check point 16. I've not located all those dead components, but I want a complete list from your perspective so I can mark up the circuit diagram.
 

(*steve*)

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So the important things are:
  1. Comparative resistance measurements of all replaced resistors between the two channels (power off)
  2. Comparative voltage measurements between the two channels (power on, using "resistor fuses", but only if all resistances within about 5%)
  3. Check point 16 in the previous post for correctness. (do regardless)
Another thing that you've probably discovered is that freight can be the largest component of the cost of getting electronic components in small quantities. Given that there's always a risk that you'll damage the components again during repair, I always try to have more on hand than I will require. If you are left with unused parts, that's probably better than having the additional time and expense of purchasing more.
 

bikemech

Oct 23, 2014
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Thanks Steve,
It's going to be a little while before I get chance to get this done , I have a lot going on at the min, but I will be back!!
 

Arouse1973

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Dec 18, 2013
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So the important things are:
  1. Comparative resistance measurements of all replaced resistors between the two channels (power off)
  2. Comparative voltage measurements between the two channels (power on, using "resistor fuses", but only if all resistances within about 5%)
  3. Check point 16 in the previous post for correctness. (do regardless)
Another thing that you've probably discovered is that freight can be the largest component of the cost of getting electronic components in small quantities. Given that there's always a risk that you'll damage the components again during repair, I always try to have more on hand than I will require. If you are left with unused parts, that's probably better than having the additional time and expense of purchasing more.

Pete / Steve, just let me know what parts you need as you go along I'll see what I can do.
Adam
 
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