Replacement transistors, NAIM NAP90

[wahab]

Are the red circled resistors values the actual ones on your amp ?.

[gilwe]

The reading is correct for all resistors in the input stage. No shorts are seen on the circuit.

[wahab]

Ok , replace Q3 emitter resistors (56R) by a 470R as a first step.

Assuming all the transistors are good this should reduce output DC offset
to a few dozen mV.

The 27K resistor should be reduced to 1K for faster setting of the circuit
at power on and better caracteristics as well.

BC239 are 30V devices , so the max power supply should be +-30V ,
BC550 are a better choice if ever you have some.

[Ian Finch]

Re #38.
The voltages you measured tell us the 752 is turned on and the 652 is off. On the good channel both are partly on, so there is a DC balance between them, as with all the voltages measured there. Since you've replaced all the semis and the problem remains on the bad channel, there is a fault in the remaining resistors or PCB tracks or even a connection or part remaining in circuit that should not be there. The feedback path to Q2 is still connected to the output stage resistors and the limiter caps. Don't rule out soldering shorts or open connections as a cause either.

Make sure it's not the signal input as there is no input capacitor shown which would prevent DC at the input - this could have a profound effect. Like Wahab, I'm suspicious of the schematic as I think there are some parts not shown. I wouldn't mess with the intentional imbalance of the emitter resistors in the input pair transistors, though. That is what Naim sound is partly about - "fix" it and you don't have a Naim any more. The other channel is fine with them as is.

[gilwe]

So basically, based on the information I provided, you are convinced there is a problem with the observed behavior, which I should sort out before placing the output stage back,
and would recommend NOT to place the output stage back as long as I don't get the readings similar to the ones on the good channel ?

Is it possible that the readings are because there is no output stage currently ,and it might become correct once it is back ?

[wahab]

Originally Posted by Ian Finch

Re #38.
The voltages you measured tell us the 752 is turned on and the 652 is off.
.

If the said resistors values are genuine then the input stage current
is as high as 10mA.

The differential will settle the ZTX752 base to 0.65V or so , hence
the current through Q1 will be only 0.65mA , all the rest , 9.35mA ,
will try to flow through Q2 via an inadequate 27K resistor .
This is likely the cause of the circuit disfunctionality offset wise.

[gilwe]

The "good" channel is out of the circuit btw, meaning I have its positive and negative main supplies disconnected from the power supply (FWIW)

[gilwe]

I deleted every trace that is not relevant on the schematics, with the output stage transistors off...
attached here. does it gives a better picture ?

Another finding which may give us some hint (?)

If I disconnect the resistor, which goes from the negative rail to the 10uf cap,
the reading between the E and C of the BC639 (ZTX652) now become 0v,
while the reading between E and C of the BC650 (ZTX752) become 55v ...

Just the opposite then it is when the resistor is not disconnected (when E-C of 752 is 0v and E-C of 652 is 55v).

Does that give some direction ?

I couldn't see and short or bad connection. I checked it a few times. The resistor values are correct. The diodes are new and are placed in the correct orientation :/

Thanks again!

[Ian Finch]

Your conclusions, #45 are about right. Earlier, Wahab raised the issue of the 56R resistor (circled) in the emitter of Q3. It looks wrong and I believe 560R is more likely. Wahab's 470R may be better by calculation from the schematic. These could be 4-band colour coded which would be green-blue-black-black or yellow-violet-black-black respectively. If 3 band instead, green-blue-brown etc. In any case, measure the voltage across the resistor and at the base (from the negative rail) of Q3. That will confirm whether the total current regulation is within reasonable bounds.

'Good idea to delete the unused parts. Now, are you referring to the 1k resistor at the base of Q2? This sets the feedback ratio. Without it, the amplifier is not an amplifier. I think the arrow pointing down is wrong though. Normally, there would be capacitor of 10-100uF from that point to ground. Is that what you also refer to? Perhaps you mean the 1.8k resistors towards the output side. These should both be removed as a measure to eliminate possibly faulty parts like those caps from the circuit. (a limiter transistor and diode there have failed already)

You will need the other channel in operation again to compare some more voltages when we get the operation more normal, with voltage levels of the signal path closer to 0V.
Exasperating, ain't it?

[gilwe]

Hi Ian,

Yes, I wa referring to the 1.8K on the schematics. When the bottom one is out, the voltages flips so the reading between the E and C of the BC639 (ZTX652) is 0v while the reading between E and C of the BC650 (ZTX752) become 55v.

While tracing the circuit I found that the schematics here doesn't 100% matches the actual circuit. Values of resistors can be a little different and there is a missing part or two on the schematics.

Are you sure I don't get these odd voltages only because the output stage is taken off ?

You will need the other channel in operation again to compare some more voltages when we get the operation more normal, with voltage levels of the signal path closer to 0V.

What exactly do you mean and how do I check this ?