Crescendo Millennium offset problem

[Elbert]

Hi!

Just connected and powered up my Crescendo Millenium amp (Elektor 2001).

According to the original article, the first thing to do, is to check the DC voltage on the output, and if this was higher than 0,1 v, then "something is wrong".

Well, I measured as much as -7v, so something was obviously wrong.

Quickly switched off and checked for any obvious visual, olfactory or thermal signs of catastrophic failure, but everything looked OK.

I then switched on power again, and the offset would drop to -1,2 v approximately after some 5-10 seconds, where it seemed to stop.

Switched off again, and re-checked for hot components.
Two 5w resistors on the protection card were warm, but in the article it said that "these resistors may become warm during certain failure modes and are therefore of 5W rating", so I assume that was "normal"

I then swapped in another amplifier card; Same result.

I then swapped in another protection/bias card; same result.

So unless something instantly fries on power-up, I assume that the problem is systematic rather than isolated to one mistake.

apart from that, I'm a bit clueless regarding what the problem could be..

Does anybody have some general advise based on my description of the problem?

Has anybody had similar issues with this amplifier in particular??

Any input would be greatly appreciated!

Best regards,

[Rikard Nilsson]

The schematics would help a lot to come up with ideas....



/Rikard

[Elbert]

A good point Rikard!

I guess I assumed someone could have an answer based on past experience.

Anyway, found some images of the schematics for both the amplifier and the protection/ bias circuit!

Hopefully, someone better versed in Amplifier and Analogue circuitry than me can see what to look for..

[nigelwright7557]

Perhaps your simply not allowing the amp to settle before measuring the DC offset ?

How is it after 1 minute ?

[tiefbassuebertr]

To determine the reason for the offset, you must investigate the voltage amplifier and the push pull buffer separately.

Probably an error in the voltage amplifier is the reason. For checking this stage allone, remove tthe MOSFET T12 and T13. Make a short connection between drain and source of T11 (T11 = Vbe multiplier).
Disconnect the right side of NFB resistor 12 K (R23? - hard to read exactly) and connect it with the short connection between drain and source of T11.

If the offset is still present, the output power stage isn't definitely the reason.

Now you can perform DC measurement both in pos. and neg. half and compare.

What type of ref LED is D1 and D2 (either 1V6/red or 2V1/yellow) ??
1V6/2,1-0V65 = 0V95 : 270R = 3,5/5,3 mA through diff amp (voltage across R8/9/13/14 each 1,75/2,65V)

Idle current through predriver (VAS):
3V9 - 0V65 = 3V25 : 33/39R = 98/83 mA

If I can see your results, I can say more.

[wahab]

You should check wether it s the amp itself or the DC servo that
are the source of the misbehaving.

First is to test the amp alone as follow :
-- remove R4 to disconnect the DC servo.
-- replace R2 by a 12K.
-- connect a 220uF cap in serial with R22.

Power on and measure the output dc voltage.
It should be at very low value, since the amp
has now a DC gain of 1 in respect to the input
offset.
If it works this way, then it s the DC servo that
is misbehaving.
If not, then perhaps the amp is oscillating,
in wich case it can be corrected by adding
a compensation that is ABSENT in the original
schematic...

[Elbert]

Oh, relief!

I was starting to fear that I'd be stuck and clueless with my decade old obscure amplifier project, but advice and ideas suddenly rush in!

First of all, I'm not sure I'd like to keep the amp on for as long as one minute in case something overloads etc.. and even on the odd chance that it should eventually settle down to a normal offset voltage, the initial conditions tels me that there is something wring nevertheless.

I've used red LED's so that shouldn't be a problem. red in another thread about a guy that had used green ones, but it seems that only resulted in a slightly high bias current, not an offset problem like I have..

The approach of identifying if the problem is with the amp or the DC servo makes very good sense, and here i get a description on how to do it too, so I'll start there!

And thanks for all replies so far!!

[Elbert]

Oh, one more thing..

Wahab, the bias servo return is connected to the card with a contact, is it sufficient to disconnect this, or should I still unsolder R4 for some other reason?

[wahab]

Originally Posted by Elbert

Oh, one more thing..

Wahab, the bias servo return is connected to the card with a contact, is it sufficient to disconnect this, or should I still unsolder R4 for some other reason?

Right, so let R4 in place and just disconnect the contact...
And don t forget to ground the input before output DC measurements..

[Elbert]

Wahab, thank you!!

I followed your instructions, and when I switched on the power, I measured 0,17 V!

Admittedly, not the "below 0,1v" quoted as normal in the magazine article, but I think this proves that the culprit is somewhere in the DC-servo circuit on the protection card....

Now I need to find out what I've done wrong with the protection card.. seems like I've made a systematic error as I had exactly the same result with both cards I tested..

The only deviation I know of, is that I used an OP177G opamp in stead of the originally specified OP77.

To the best of my understanding the OP177 should be a direct replacement for the OP77.. could this be where I went wrong??