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Old 30th July 2013, 02:04 PM   #311
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Terry, are there any troubleshooting jumpers/resistors installed for the latest round of measurements? Is there a load resistor?

If you are using 0R47 for R47 etc, and you are showing -1.42V DC offset, then Q20 is passing ~120 mA : (1.41-1.35)/.047. But that doesn't match the -1.38V at Q21's emitter. It's hard to imagine how Q21's emitter could be more positive than the output.

Are D7 and D8 back in the circuit? Either way, something is off that the Collectors of Q10 and Q11 are significantly closer to the rails than their bases. For example, if the diodes are in circuit and Q10 is conducting it's collector should be D7's Vf below D7's anode or .281V. There is no connection to pull it more positive. The Vbe shown for Q10 and Q11 are not enough to turn them on. Are you sure that 3.22 and -4.50 voltages are measured/marked at the correct location?

To further clarify, degeneration resistors are R7,8,9,10
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Old 30th July 2013, 02:11 PM   #312
bonfis is offline bonfis  United States
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Quote:
Originally Posted by still4given View Post
Well I thought I was getting there. I replaced Q14 and there was no change. Then I replaced Q16 and Q26. I brought it up on the variac and was able to get it up to 90Vdc rails without it swinging negative. I had only about -1.4V offset so I decided to take some measurements so we could get it the rest of the way. I had just finished filling out the schematic when I realized that I missed the base of Q16. So I went back out and powered it up again to get that reading. What do you know, it swung negative again. I did notice that the rails have gone up to 91.5V. So I just tried it, and if I bring it up slowly on the variac, I get to about 87V rails with about 1.4V offset. As soon as it hits 88VDC rails it swings to -33Vdc offset.

So, do you guys have any idea what could be causing it to do that?

I'm attaching the readings I got just before it swung negative.

Thanks, Terry
Hi Terry-
Have you measured the voltage on the collectors of Q18-21 while you bring up the rails?
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Old 30th July 2013, 02:41 PM   #313
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I believe R7-10 are 330R and the differential input current should be 2.5 mA per leg, so the voltage across them should be roughly .825V.

Do measure the voltage at the junction of R2, R3 and R5. This will help determine if Q1 and Q3 base currents are within normal limits. Current should be ~tens of microamps, so across 1K2 you should see a few tens of mV.

I guess that's enough measurement points for a while.
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Old 30th July 2013, 02:46 PM   #314
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Quote:
Originally Posted by BobEllis View Post
Terry, are there any troubleshooting jumpers/resistors installed for the latest round of measurements? Is there a load resistor?

No, I removed all jumpers and replaced diodes. Nothing on the output.

If you are using 0R47 for R47 etc, and you are showing -1.42V DC offset, then Q20 is passing ~120 mA : (1.41-1.35)/.047. But that doesn't match the -1.38V at Q21's emitter. It's hard to imagine how Q21's emitter could be more positive than the output.
I am using .33R resistors for R45-R48 if that matters.

Are D7 and D8 back in the circuit? Either way, something is off that the Collectors of Q10 and Q11 are significantly closer to the rails than their bases. For example, if the diodes are in circuit and Q10 is conducting it's collector should be D7's Vf below D7's anode or .281V. There is no connection to pull it more positive. The Vbe shown for Q10 and Q11 are not enough to turn them on. Are you sure that 3.22 and -4.50 voltages are measured/marked at the correct location?

Yes, D7 & D8 are back in.

To further clarify, degeneration resistors are R7,8,9,10
Note: The measurement on the above chart were taken before the "avalanche" set back in.

Here are some readings I got this morning with a quick test.
With the rails set at 85V
Q1E= -.594
Q2E= -1.318
Q3E= +.537
Q4E= -.203
R3-R5J= -22MV
R7-R8J= -1.426
R9-R10J= +.664

With rails set to 90V
Q1E= -6.62V
Q2E= -7.79V
Q3E= -14.28
Q4E= -17.48
R3-R5J= -6.00V
R7-R8J= -7.58
R9-R10J= -15.14

Quote:
Hi Terry-
Have you measured the voltage on the collectors of Q18-21 while you bring up the rails?
I will check those.

Thanks
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Old 30th July 2013, 03:44 PM   #315
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Sure smells like multiple front end differential issues. Step 1 replace all 4 devices. Measure again.
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Old 30th July 2013, 05:06 PM   #316
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Why didn't just replace them earlier?

I just replaced Q1-Q6. Now everything is beautiful. Offset is about 33mv. Everything checks side to side. I had no idea the front end had such a huge affect on everything. Thanks to all of you for sticking with me through this. I sure hope someone else can learn from my mistakes. Once I get everything put back together I'll give a report on how it sounds.

Are there any readings you guys would like to see while I still have it all out of the case and accessible?

Thanks again!
Blessings, Terry
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Old 30th July 2013, 05:13 PM   #317
AndrewT is offline AndrewT  Scotland
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Check the Vdrops of the 4 degenerations resistors. This will tell you the imbalance in the currents passing through the 4 input transistors.
Check the Vdrops of the Vas degen.
Check the Vdrops of the base stoppers.
Check the change in output bias current (Vdrop of the emitter resistors) as the amp warms up from cold to fully warmed up.
Artifically heat up the sink to raise the output temperature to mimic extreme weather heating. And check the output bias at this artifically high temperature.

While doing all this repeatedly recheck the output offset.

Connect your source and recheck your output offset. Switch off your connected source and check the output offset.

When you are happy that it passes all these tests, then connect a cheap disposable speaker and listen for a bit.

Finally build and fit a speaker delay circuit that also detects excessive output offset and triggers that output switch. Now you can fairly safely attach your expensive speakers.
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Old 30th July 2013, 05:40 PM   #318
wg_ski is offline wg_ski  United States
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The front end probably got severely overloaded while troubleshooting. When an amp heads for the rail, it can break down (zener) the input transistors. 85 volts at the inverting input, zero at the noninverting = not good. It doesn't always kill the input devices but it can. Op amps often use protection diodes between the inputs to keep that from happening. A pair of 1N4148's in inverse parallel (or four in series parallel) would do the trick.

For next time. Leave it alone now that you've got it working.
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Old 30th July 2013, 05:41 PM   #319
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Congratulations Terry. Andrew gives sound advice for ensuring long term reliability since many of your parts have been stressed.

If you need a speaker delay/protection circuit I can send you the one used in the Hafler DH-500. It fits the requirements Andrew sets out and is designed to run off of ~90V rails.
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Old 30th July 2013, 05:53 PM   #320
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Quote:
Originally Posted by AndrewT View Post
Check the Vdrops of the 4 degenerations resistors. This will tell you the imbalance in the currents passing through the 4 input transistors.
Check the Vdrops of the Vas degen.
Check the Vdrops of the base stoppers.
Check the change in output bias current (Vdrop of the emitter resistors) as the amp warms up from cold to fully warmed up.
Artifically heat up the sink to raise the output temperature to mimic extreme weather heating. And check the output bias at this artifically high temperature.

While doing all this repeatedly recheck the output offset.

Connect your source and recheck your output offset. Switch off your connected source and check the output offset.

When you are happy that it passes all these tests, then connect a cheap disposable speaker and listen for a bit.

Finally build and fit a speaker delay circuit that also detects excessive output offset and triggers that output switch. Now you can fairly safely attach your expensive speakers.
Hi Andrew,

I would love to check all of that. Would yo be so kind as to look at the circuit and give me the resistor numbers for the Vas degen, and base stoppers?

Thanks

Hi Bob,

I used to own a P500 Hafler. I have the manual in pdf somewhere. I would like to have the circuit for the speaker protects if you wouldn't mind. I guess I had better put one in this amp.

Thanks again.

Last edited by still4given; 30th July 2013 at 05:56 PM.
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