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Old 5th November 2010, 06:42 AM   #561
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Quote:
Originally Posted by Ruwe View Post
Hi ciariello,
First check the current through the 68 ohms resistor in the emitter of ZTX 653. If it is about 8.5-10 mA you're good.

If your bias current is not adjustable most likely the Vbe multiplier doesn't work (the transistor between the ZTX653 and ZTX753). Check the pin orientation and the resistor values around. The voltage for the right bias current is about 2.35-2.38V between the collector of ZTX653 and ZTX753.

One of your channels has DC offset problem. Probably something at the inputs resistors or the feedback resistors. Check the values of the resistors.

... And get rid of the fakes, but I don't believe they are causing the problems.
i replaced the MJ15003 with genuine ONSemi.
i replaced the MJE15030/31 with genuine ONSemi.
i replaced the BC550/2N5551 (Q1-Q2-Q5) with genuine Toshiba 2SC2240
i replaced the 220-330-470pF capacitor with film capacitor Wima (i can't find 39pF)
the DC offsets are changed.
On channel 1 the dc offset value starts from 20mV and increases in proportion to the temperature of the ZTX until 35-37 mV.
On the other channel the dc offset starts from -120mV and increase until 0mV and over in proportion to the temperature of the ZTX.
i think (LIKE YOU )the fakes transistors wasn't the overheat cause..
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Old 5th November 2010, 07:03 AM   #562
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i readed the current value on both 68 resistors ...we are around 10-11 mA but the voltage between the ztx collectors is 1.995 on channel 1 and 2.018 on hte other.
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Old 5th November 2010, 07:04 AM   #563
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It is JUST another strait "blameless" , just another flavor.....what is the big deal. This is the "monday" amp , sym for tuesday ,blame es for thursday, leech for friday. Unless there is something REALLY wrong , an offset of under 10 Mv will be typical ... at least go through 10 differential pairs to find 2 similar ones !! I run this circuit at 6mA , never hot ... just "luke warm" ...
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Last edited by ostripper; 5th November 2010 at 07:07 AM.
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Old 5th November 2010, 07:24 AM   #564
Ruwe is offline Ruwe  Europe
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Quote:
Originally Posted by ciariello View Post
i readed the current value on both 68 resistors ...we are around 10-11 mA but the voltage between the ztx collectors is 1.995 on channel 1 and 2.018 on hte other.
Hi,

10-11mA is a little bit on the high side, but is acceptable. Both ZTX will be hot, 60C is well expected.

When you turn the bias setting pot, does the voltage between the collectors change? This means that your current in power transistors is also changing. Both changes must be related.

Your measured voltage looks slightly low to me, but I'm using different transistors. Is your current in the power transistors normal (around 25 mA) with this voltage?

The DC offset is not much related to the hot transistors. It's fixed by the differential pair at the input. If it is shifting you should look there for faults.
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Old 5th November 2010, 07:38 AM   #565
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hi and thanks for your reply.when i try to set (on both channel ) the bias they don't change and became ALWAYS 25mA.Now i powered ON the ampli and I connected 2 JBL CONTROL 5 (4 ohm) with a NAD 502 CD player to check if everything works well ... and I must say that even with these speakers (not great but better to sacrifice them!) sounds really good and " appears "that the ZTX become less warm.... I'll keep everything a little less in practice and then I will tell you the changes. Thank you all for your help
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Old 5th November 2010, 06:21 PM   #566
Ruwe is offline Ruwe  Europe
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Quote:
Originally Posted by ciariello View Post
hi and thanks for your reply.when i try to set (on both channel ) the bias they don't change and became ALWAYS 25mA.Now i powered ON the ampli and I connected 2 JBL CONTROL 5 (4 ohm) with a NAD 502 CD player to check if everything works well ... and I must say that even with these speakers (not great but better to sacrifice them!) sounds really good and " appears "that the ZTX become less warm.... I'll keep everything a little less in practice and then I will tell you the changes. Thank you all for your help

Hi,
Well, then the Vbe multiplier doesn't work properly.
Check if the transistor is connected correctly, according to schematic, and that the board labels correspond to the actual pin-out of the transistor. Sometimes there are many errors on these boards.

Also check that the trimmer works, by measuring changing resistance between its pins when you turn it. The voltage between the ZTX collectors must change when you turn the trimmer, as well the bias current.
Start bias current adjustment always with the trimmer turned for max resistance, usually counter-clockwise on most boards. Also have in mind these trimmers are multi-turn, so to see a change you have to turn it more than, let's say, few turns. Then it suddenly gets quite sensitive and current change is significant.

The DC offset should be very stable. It doesn't change at all, and is usually somewhere in 5-30 mV range. It's not affected by the load/no load output.

Do you have an oscilloscope? It would be nice to check if that DC offset is actually a DC. It could be some mild high frequency oscillation that your multimeter sees as DC.
Regards
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Old 8th November 2010, 11:13 AM   #567
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I did several tests / readings with an oscilloscope and there are no self-oscillations on the outputs of the amplifier but it is DC.
Finally I checked all the connections and pinout of the transistors and they are all correct (datasheets and schematic diagram in hand).
multi-turn potentiometers works well by changing the value.
Conclusion ... the amp sounds great shorting the inputs is VERY VERY VERY quiet, the ZTX heat but the temperature is still around 60 C.... "BUT " the bias is still not able to be changed and remains on 25mA.
I think that they should be to leave everything as is and enjoy weeks of work ...
Thanks to all.
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Old 9th November 2010, 08:31 PM   #568
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Hello,

I am currently putting together two monoblocks with the naimclone boards from ebay aswell. I trashed the components that came with the kit and replaced them with the parts mentioned in the NC220 schematic posted somewhere in this thread aswell.
Output trannies are NTE-58 replacements from ON Semi

On the NC220 schematic both Q7 and Q8 and C3 and C4 are not mentioned, with what can they be replaced since Q1 - Q6 are different now aswell?
(Q1-Q2-Q3 are BC546, Q4 is ZTX753, Q5 is 2n5551, Q6 is ZTX653, Q10 is MJE15030 and Q11 is the MJE15031)

Could someone push me in the right direction?
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Old 10th November 2010, 04:03 AM   #569
Ruwe is offline Ruwe  Europe
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Quote:
Originally Posted by Dilirium View Post
Q1-Q2-Q3 are BC546, Q4 is ZTX753, Q5 is 2n5551, Q6 is ZTX653, Q10 is MJE15030 and Q11 is the MJE15031
Hi,
Looks very good to me. I don't have experience with these drivers and power transistors though.
Just an idea: Buy BC546B or C for the input stage. High gain transistors are beneficial in this position. Or you can pick and match them by hand. hfe of >300 is best, IMO.

Quote:
Originally Posted by Dilirium View Post
On the NC220 schematic both Q7 and Q8 and C3 and C4 are not mentioned, with what can they be replaced since Q1 - Q6 are different now aswell?
These are part of the protection circuit that is not used in most kits. If you want it, use C3,C4 10uF/50V and Q7,Q8 BC546/556 or 2n5551/2N5401. The transistor choice is not critical, this thing is supposed to stay normally off.

Last edited by Ruwe; 10th November 2010 at 04:14 AM.
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Old 10th November 2010, 07:01 PM   #570
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OK, thanks for the info! Got enough of all those laying around so looks like I am good to go again.
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