So.. R8 measured 1,171V across and 995 Ohms, R13 Measured 1,074V across and 995 Ohms...
A more noticeable diffrence in voltage here...
A more noticeable diffrence in voltage here...
So far, so good...
Last, but not least, can you measure the VBE of
T7 and T9 ?...
IT should be logically different since the voltage at their
respective bases is different as the amp seems to
compensate for the different VBE of these devices leading to the
slightly high dc offset..
Also, can you measure the rails voltage as well as the voltage
through R25 and R27.
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Sure thing!
T5 had a Veb of 0,724 and T9 had a Veb of 0,684
R25 measured 33,6Ohms and 0,443V across, R27 measured 33,6 Ohms and 0,338V across.
Rail voltage was 46,3 volts +/-
Again, some non-symetry...
T5 had a Veb of 0,724 and T9 had a Veb of 0,684
R25 measured 33,6Ohms and 0,443V across, R27 measured 33,6 Ohms and 0,338V across.
Rail voltage was 46,3 volts +/-
Again, some non-symetry...
I assume this is rather T7 ???.....
btw, what is the quiesent current? just measure the voltage
through R34 and R35....
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We are close to end the thing...
The offset is tracked, as we have those 0.17V as the sum
of the difference between T7/T9 respective VBE plus the
difference in voltage drops of R25/R27...
The offset is tracked, as we have those 0.17V as the sum
of the difference between T7/T9 respective VBE plus the
difference in voltage drops of R25/R27...
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If you ask me, I have a feeling that the thing has decided to end it self.. I thought this was going to be a relatively straightforward project, etch some cards and solder the components according to the description.. not so!😕
I can only be extremely gratefull that someone has the patience to help..
Well, we, or more correctly you Wahab, have tracked down the offset, but what is the root-cause???
I can only be extremely gratefull that someone has the patience to help..
Well, we, or more correctly you Wahab, have tracked down the offset, but what is the root-cause???
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Is the quiescent current correctly adjusted.???.
If not, the amp will compensate but one output device
will force an offset since it has lower threshold conduction
than his symetrical...
Adjust P1 to have about 22mV through R34 and R35...
Be extremely cautious for this setting as you should first
adjust the trimmer to have no current at all (P1 at maximum resistance)
and then trim cautiously while watching the voltmeter...
If not, the amp will compensate but one output device
will force an offset since it has lower threshold conduction
than his symetrical...
Adjust P1 to have about 22mV through R34 and R35...
Be extremely cautious for this setting as you should first
adjust the trimmer to have no current at all (P1 at maximum resistance)
and then trim cautiously while watching the voltmeter...
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Hmmm.. sounds like we are getting closer to something here...
I have not adjusted the quiescent current yet.. the pot is set at minimum . The building instructions in the elektor magazine stated that the pot should be turned all the way left, i.e. to the lowest current setting to reduce the risk of ecessive current on power-up.
The next thing to do was to check the output offset on tp3. It then said that if anything more than 0,1V was measured, there was a problem somewhere that should be investigated... and that is as far as I got.
The recommendation is to adjust for a current of 200-250 mA, corresponding to 0,044-0,055V measured between tp3 and tp1.
I'll give this a try tomorrow. perhaps it takes out the offset? Perhaps the DC servo works afterwards??
Anyway, thank you very much for the help so far, I would have been seriously stuck without your kind assistance! 🙂
I have not adjusted the quiescent current yet.. the pot is set at minimum . The building instructions in the elektor magazine stated that the pot should be turned all the way left, i.e. to the lowest current setting to reduce the risk of ecessive current on power-up.
The next thing to do was to check the output offset on tp3. It then said that if anything more than 0,1V was measured, there was a problem somewhere that should be investigated... and that is as far as I got.
The recommendation is to adjust for a current of 200-250 mA, corresponding to 0,044-0,055V measured between tp3 and tp1.
I'll give this a try tomorrow. perhaps it takes out the offset? Perhaps the DC servo works afterwards??
Anyway, thank you very much for the help so far, I would have been seriously stuck without your kind assistance! 🙂
An unbalance somewhere in the amp..
The DC offset is a by product of the amp trying
to cancel it..
We assume that the critical components are correctly matched,
but if we can t do better with the things as they currently are,
you ll have to unsolder the said components and procede to
an accurate enough matching..
That s often THE problem with symetrical differentials, but as soon
as it is made, and providing it has been correctly designed, you ll
have an exceptionnal amp...
Well,
After a day of car-fixing, its time for amp-fixing..
I hooked the multimeter to tp1 and tp3 and switched the power on. The voltage was very low, so I turned the potmeter up untill I got 0,044V. actually had to turn the pot about 2/3 turn.
I measured the offset voltage, still -1,7V...
Right now I'm just letting the amp sit and warm up, but initially the findings are not too encouraging, some change in offset was hoped for..
After a day of car-fixing, its time for amp-fixing..
I hooked the multimeter to tp1 and tp3 and switched the power on. The voltage was very low, so I turned the potmeter up untill I got 0,044V. actually had to turn the pot about 2/3 turn.
I measured the offset voltage, still -1,7V...
Right now I'm just letting the amp sit and warm up, but initially the findings are not too encouraging, some change in offset was hoped for..
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is tp1 to tp3 across both emitter resistors?
44mVdc across the pair of resistors sounds OK.
Andrew,
The voltage is measured across the emitter resostor R34 for T12.
The building instructions states that one is to measure across this when setting the quiescent current and that the setting should be 44-55mV.
Just checked the voltage across the other emitter resistor, and it is excactly the same, i.e. 44 mV
The amp is now luke-warm but the voltage has not changed since I switched it on cold. The offset measured between 0 and tp3 is stil -0,17
The voltage is measured across the emitter resostor R34 for T12.
The building instructions states that one is to measure across this when setting the quiescent current and that the setting should be 44-55mV.
Just checked the voltage across the other emitter resistor, and it is excactly the same, i.e. 44 mV
The amp is now luke-warm but the voltage has not changed since I switched it on cold. The offset measured between 0 and tp3 is stil -0,17
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44mV across one emitter resistor is unusually high.
Is tp3 on the output line? and tp1 on the emitter end of the resistor?
Where is tp2?
Is tp3 on the output line? and tp1 on the emitter end of the resistor?
Where is tp2?
tp1 (test point) is between emitter and the emitter resistor. tp2 is situated correspondingly for the oher emitter resistor.
tp3 is between both emitter resistors, i.e. on the output before the filter consisting of a 1 ohm resistor and an inductor in parallel.
I have no basis for judging the voltage across the emitter resistor, I've only adjusted it to the value descrribed in the article. 😕
No smoke or burned smell after about 20mins of power-on, temperature on and arround the output transistors is approx. 36 deg C according to my indrared thermometer.
tp3 is between both emitter resistors, i.e. on the output before the filter consisting of a 1 ohm resistor and an inductor in parallel.
I have no basis for judging the voltage across the emitter resistor, I've only adjusted it to the value descrribed in the article. 😕
No smoke or burned smell after about 20mins of power-on, temperature on and arround the output transistors is approx. 36 deg C according to my indrared thermometer.
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As you requested Andrew:
tp1-tp2 = 0,87V
tp1-0V = -0,132V
tp2-0V = -0,22V
tp3-0V = -0,175V
tp2-tp3 = -0,043V
I can't see any surprises here and it all seems to add up...
tp1-tp2 = 0,87V
tp1-0V = -0,132V
tp2-0V = -0,22V
tp3-0V = -0,175V
tp2-tp3 = -0,043V
I can't see any surprises here and it all seems to add up...
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Hi,
two measurements show -170mV and -175mV. One shows -1700mV.
Can we assume the output offset is ~-170mV?
Are these with no DC servo? the output open circuit and with the input shorted.
What is the warm offset with the servo in place?
Sorry Andrew,
It was -0,17V NOT -1,7V..😱
for the last series of measurement, I used a different voltmeter (the other was monitoring emitter resistor voltage) and a higher resolution, so that's why I got 0,175V.
All the last measurements were done with one meter at the same setting.
There is no DC servo, I'm still using the set-up/ modification described by Wahab, input is shorted and output is open circuit.
I do not know what the warm offset with servo in place is, I didn't dare to let the thing stand and warm up when i measured 1,71V offset. Allso the 5w resistors on the protection card (where the DC servo is located) got quite warm after a while as the card was in protection mode due to the unnormal voltages it detected..
It was -0,17V NOT -1,7V..😱
for the last series of measurement, I used a different voltmeter (the other was monitoring emitter resistor voltage) and a higher resolution, so that's why I got 0,175V.
All the last measurements were done with one meter at the same setting.
There is no DC servo, I'm still using the set-up/ modification described by Wahab, input is shorted and output is open circuit.
I do not know what the warm offset with servo in place is, I didn't dare to let the thing stand and warm up when i measured 1,71V offset. Allso the 5w resistors on the protection card (where the DC servo is located) got quite warm after a while as the card was in protection mode due to the unnormal voltages it detected..
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