Crescendo Millennium offset problem

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Wahab,
does that circuit measure Vbe sat?
I see ~0.34mA of base current and ~1.1mA of collector current.
This is Ic/Ib ~3.5 which is a saturated ratio.

Change the collector resistor from 10k to 1k0.
Change the base resistor to a 1M0 pot and a 10k series resistor.
Adjust the pot until the voltage across the 1k0 indicates the required test Ic value.
Read off the voltage across the 10k to indicate the base current.

Now you have Vbe @ test current and hFE @ test current.
 
Wahab,
does that circuit measure Vbe sat?
I see ~0.34mA of base current and ~1.1mA of collector current.
This is Ic/Ib ~3.5 which is a saturated ratio.

Change the collector resistor from 10k to 1k0.
Change the base resistor to a 1M0 pot and a 10k series resistor.
Adjust the pot until the voltage across the 1k0 indicates the required test Ic value.
Read off the voltage across the 10k to indicate the base current.

Now you have Vbe @ test current and hFE @ test current.

Right Andrew, the transistor is< saturated, but this is still valid
in respect of the VBE.
^This circuit allow to take account of eventual difference
of the internal Re and Rb and thus allow better matching..
 
Puzzle in deed, wether or not I can fit the pieces together remains to be seen! 🙂

Anyway, I'll do some measurements on T1-4 on the amplifier card, both with and without DC servo and see if I can identify anything strange..
 
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Well, here are the first results measured with the servo disconnected and modifications as per Wahab's intstructions with 220uF in series with R22 and 12k R2:

T1 = Vbe 0,63 Vce 45,6
T2= Vbe 0,63 Vce 45,2
T3= Vbe 0,6 Vce 43,7
T4= Vbe 0,2 Vce 43,5

It was a bit difficult to get the measurement pins (1k resistor legs) in there, so I was not able to hold the measurements for long, hence they may not be 100% accurate..

But it seems obvious that T3-4 has a problem with Vbe... what do I make of this??? 😕
 
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As I'm still fumbling in the dark, I figured I'd just check some other voltages as well, just to see if I could find anything "unusual"...

So I decided to look at the T5/D1 and T6/D2 current sources..

Vbe was a rock steady 0,66V for both T5 and T6, so that looks normal.
I then measured voltage across R17 and R20, the resistor between the supply rails and Collector; 1,1V for both.

I then measured Vec.. for T5, this was 10,5V, for T6 it was 11V! is this right?? and if not, what could be causing it??
 
Wahab,

As is obvious by now, I am not in much of a position to question your advice,

But for the sake of satisfying my curiosity and for the slim chance that it would educate me; How do you deduce that T4 is the likely culprit??

And were the voltages across T4-5 ec in any way suspect or related to this??

I am only asking from sheer ignorance, not from doubt! 🙂
 
But for the sake of satisfying my curiosity and for the slim chance that it would educate me; How do you deduce that T4 is the likely culprit??

And were the voltages across T4-5 ec in any way suspect or related to this??

If T4 has a 0.2V Vbe, then it seems faulty as it should be
about 0.6V whatever the output DC offset.
It can t be lower by physical principles..
 
Oh, c..p!

This is a bit embarrasing, but nevertheless.. I decided to make one more measurement, just to make absolutely sure..

I must have fumbled a bit, again, hard to get those measuring pins in the right place, but on my second attempt I measured 0,55-0,56 on both T3 and T4...:ashamed:

Out the window went the obvious answer... my apologies

Seems the Veb on the T3-4 pair is slightly lower than for T1-2.. could this have some significance together with the 0,5v difference I measured in the current sources??
 
It s good...
Check the value of the resistors R8 and R13...
The vce voltage are not in the expected range.
So measure the voltage through these resistors, then
power off the amp , wait for the supply caps to
completely discharge and check them with the ohm meter...
Unless it s like the diode tester, not accurate, it will be
of great help..
 
Rikard,

I see where you are comming from here, but do you think that a difference of 10 mV and the corresponding difference in current of 4.0926 mA vs 4.0555 mA indicates a fault or gross transistor mismatch?

I'm no expert, but I suspect that one would have to do some fairly critical component matching to match current at this level of difference??

Following on from that, if it was that critical, I guess this would either have been highlighted in the building description or more commonly known??

Based on gut-feeling, I would tend to agree with Wahab that less than 1% difference "seems close enough"..

Well, the Ohm and voltage functions on my meter is pretty good, so I can trust it for that will measure R8 and R13! 🙂
 
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