NAP-140 Clone Amp Kit on Ebay - Page 92 - diyAudio
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Old 19th June 2012, 09:29 AM   #911
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No easy answer . The wattage should be about 0.4 Watts and the resistors look to be 0.6 Watts . The Vishay resistors below are sold by many and are small . They should hold together for further testing as they are 1 watt types . Try to get hold of an oscilloscope . There should be no large waveforms across the resistors when no music playing . Also test the output . Do check that you only have 40 V ?

I have also shown how 4 capacitors will generally improve the design . The exact place to connect to earth ( 0V ) is difficult to say . To the power supply earth seems favoutite . However most amps of the day took them to the incoming earth without too many problems . Douglas Self has good advice on this . The idea of the 220uF is to reduce the effects of inductance in the power supply cables . It also allows a cheap high grade local reservoir of power ( low Z cap like Rubycon ZL or Panasonic FC ) . I have often taken the power supply directly to the power transistors with 2.5 mm solid house wining cable ( removing the coloured wires to use ) . The smaller parts of the circuit are happier this way also . If so remove the original cables . Take the cables in a direct route .

Distortion In Power Amplifiers ( Section 5.5 )

4k7 Pr01 5% Metal Film Resistor Pack of 10
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Old 19th June 2012, 09:46 AM   #912
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Hi Clariello
The resistors you marked, with R8,9 set bias for the VI limiters (TR7,8) circuit and should not carry high current. Unless you have shorted or wrongly fitted transistors there. Still, you should be able to remove the limiter transistors and run the amp without them as many DIYs, in fact, do.

Though designs have changed since 2010, you might tell us who supplied tbe board because the parts supplied may not have matched the overlay with older kits, so check the pinout of the actual part number supplied against the PCB overlay and tracks to be certain. Many people had problems with some substitute transistors that may have worked, if the transistor was rotated to suit. Of course, once blown, you can't fix anything by changing position.
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Last edited by Ian Finch; 19th June 2012 at 09:50 AM.
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Old 19th June 2012, 10:08 AM   #913
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Quote:
Originally Posted by Ian Finch View Post
Hi Clariello
The resistors you marked, with R8,9 set bias for the VI limiters (TR7,8) circuit and should not carry high current. Unless you have shorted or wrongly fitted transistors there. Still, you should be able to remove the limiter transistors and run the amp without them as many DIYs, in fact, do.

Though designs have changed since 2010, you might tell us who supplied tbe board because the parts supplied may not have matched the overlay with older kits, so check the pinout of the actual part number supplied against the PCB overlay and tracks to be certain. Many people had problems with some substitute transistors that may have worked, if the transistor was rotated to suit. Of course, once blown, you can't fix anything by changing position.
hi and thank you very much for your fast reply.
I bought only the PCB on Ebay (Tubehunter) and i bought all the components on RS-Components,Distrelect,ecc....
I used original schematic diagram transistor except for BC550 (i used Toshiba 2SC2240L) and output transistor (i used MJ15003 ON-Semi)
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Old 19th June 2012, 10:12 AM   #914
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Hi Clariello , hi Ian ,

Good advice as it kills two birds with one stone to remove the transistor . it's odd that this should happen . If careful protection circuits are not required ( for careful read only use very loud rather than totally distorted , if a sub-woofer retain them ) .

At random I picked a freeware oscilloscope to download . Sound cards are cheap enough if an accident happens . If a laptop best not to .

Universal Software Oscilloscope Library freeware download - Real-time universal oscilloscope GUI ( Real-time plot ) DLL library. - Best Freeware Download

Last edited by nigel pearson; 19th June 2012 at 10:23 AM.
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Old 19th June 2012, 10:22 AM   #915
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hi nigel ...to remove the protection needs remove ONLY the transisotrs? or other components?
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Old 19th June 2012, 10:27 AM   #916
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Hi again Clariello .

Just the transistor would be fine . I notice you say about different transistors . The pin out is different on 2SC ( base , collector , emitter . I am sure you would have checked that ? )

Forgive me Ian I feel it was your suggestion . I am off for a few hours now .

Nige
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Old 19th June 2012, 10:34 AM   #917
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i know Ian . i downloaded the datasheet and I corrected the pins by turning the transistors in the right position.
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Last edited by ciariello; 19th June 2012 at 10:41 AM.
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Old 19th June 2012, 12:10 PM   #918
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Ok, It's good you are aware of the problems with Ebay kit PCBs. As nigel agrees, it would be OK to simply remove the TR7,8 to eliminate the limiter circuit from the problems and replace the resistors only if you wish, then test the amp operation, set a low output stage quiescent current for initial setup and measure any DC offset at the output. Stop now if there is a problem.

I would first pull the fuses and place 100R resistors across the holder to limit fault current a bit. Set the output stage bias to 30mA (measure by mV drop across Emitter 0R22 resistors. Then verify the Bias current shared through the LTP and VAS transistors by measuring voltage drop across a series resistor. You should have about 3mA and 7mA in each path respectively. BTW, what are your rail voltages?
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Last edited by Ian Finch; 19th June 2012 at 12:12 PM.
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Old 19th June 2012, 01:52 PM   #919
brianuk is offline brianuk  United Kingdom
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Removing the protection is a known mod on the original Naims. However, they do of course know that their amplifier is working on spec - it normally takes a very long time for things to go brown when worked hard, so it suggests something is a little off here.

38mA at the power supply end after 20 mins powered is the easy way to test the bias. Connect your meter in line before powering up and don't mess while powered up!

Then check no more than 50mV DC offset.

The output end capacitors are a good mod, minimum 50V, 63 as per Nigels markup. 470uf is ok. Take them directly back to the PSU 0V so axials could work best, especially if they reach in one go.
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Old 19th June 2012, 01:56 PM   #920
brianuk is offline brianuk  United Kingdom
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Nigel, not sure if I recall correctly. Is it not the easy way to remove protection by cutting the two diodes just on one leg, which can be done while board in situ?

(and touched with solder if needed again)
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