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Old 15th January 2013, 02:14 AM   #1051
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That's an unusual schematic - certainly has a lot of tweaks that aren't original or on other clones - not that they affect this problem.
A TIP41 for the Vbe multiplier? Now that's odd, since it doesn't have to be attached to anything like the heatsink.
If Q5 is reversed, C6 is faulty etc. there will be similar problems. It's worth checking Q5 pinouts on your meter,
since most now have a transistor Hfe test socket to verify transistor operation.

Have you bought/built 2 channels and compared relevant parts and operation?
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Old 15th January 2013, 04:49 AM   #1052
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Quote:
Originally Posted by Ian Finch View Post
That's an unusual schematic - certainly has a lot of tweaks that aren't original or on other clones - not that they affect this problem.
A TIP41 for the Vbe multiplier? Now that's odd, since it doesn't have to be attached to anything like the heatsink.
It's from beginning part of this thread, not mine.
Quote:
If Q5 is reversed, C6 is faulty etc. there will be similar problems. It's worth checking Q5 pinouts on your meter,
I read that the newer release has fixed the misprint on the board. I looked at the photo of one posted a year ago (NAP-140 Clone Amp Kit on Ebay) and if his board worked the way it's assembled, I figured mine would too. Anyway, I'll check those to see if they are correct. Thanks.

Quote:
Have you bought/built 2 channels and compared relevant parts and operation?
I only bought 1 as a test unit which will be for the center channel of my home theater. If it works out fine, then I would get a pair for my main speakers.
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Old 15th January 2013, 04:54 AM   #1053
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Originally Posted by kclusa@yahoo.co View Post
You can measure the voltage drop between Collecotr and Emitter of Q5.
If R17 is defective, Q5 Vce will not change.
I just checked and R17 is fine. So it's something else I guess. Thanks.
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Old 15th January 2013, 07:20 AM   #1054
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I checked Q5 (BC546) and it's bad. My local Radio Shack doesn't carry BC546. What are acceptable substitutions?
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Old 15th January 2013, 08:11 AM   #1055
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Default Vbe multiplier transistor types

This is a rare opportunity to try almost anything! Any small audio NPN with Vceo> 30V and Icmax >250 mA should be ok.
e.g. BC546,7,8,9,550 MPSA06,42 2N5550,1. You should have no trouble as long as you get the appropriate pinout right.
Choose one with the EBC sequence, up or down, so that it fits easily if the PCB holes are also in a straight line.
Odd that BC546 was bad...
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Last edited by Ian Finch; 15th January 2013 at 08:19 AM.
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Old 15th January 2013, 10:41 AM   #1056
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If the bias is changing it will show as a change in voltage across R29 or R30 . Rotel used to say 4.3 mV for the bias on their amps using the same resistor . It will be something in that region .Doubtless the instructions say something similar ? The way that this bias works makes it almost impossible that nothing is happening . When settled you might like to replace the bias pot with fixed resistors , it is ultra safe if no measuring errors occur . Some will say it sounds better that way . The argument is that the cermet used for adjustable resistors and the associated wiper is not ideal ( noise ) . I think if looking at the simplicity and the risks the fixed resistors version is better . Risk is getting it wrong , long term risk if the variable resistor fails ( and it will , espeailly if a tropical country ) . Set it and forget it if so .
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Old 15th January 2013, 04:02 PM   #1057
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Originally Posted by nigel pearson View Post
If the bias is changing it will show as a change in voltage across R29 or R30 .
I checked but it won't change no matter how many turns I make on trim pot. R29 stays at 2mV and R30 stays at 4mV.

Thanks for the tips on fixed resistor.
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Old 15th January 2013, 05:46 PM   #1058
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Just questions that have to asked to check measurements:
When powered,
First, is the meter set to low or autorange volts DC range and not some other range that appears to give readings?
1. Measure the voltage between the output connections. (have no other connections or input leads fitted)
There should be some DC voltage < 40mV typically.
2. Is Q5 replaced now and does the voltage across it (E-C) not vary when you adjust the pot?
3. Does the voltage across the pot (top to bottom on sch.) not vary when you adjust it? If voltages there
don't change, then remove the pot and test it out of circuit. Since the amplifier works, presumably
you must have tried audio satisfactorily with it, the output transistors must then be OK and correctly
installed so that leaves only Q5 and/or the pot. in doubt.

I'm following your posted schematic, which is different to your actual PCB and schematic so you'll need to cross-check.

4. Measure the voltage across R12 (68R) It should be ~0.4V for 6mA current through the VAS/bias generator circuit.
If the current is there, the previous checks must show voltages across Q5 and the pot. too.

At the output stage,
5. Measure between the emitter of Q11 and collector of Q12. This is the same as across both output "emitter" resistors.
It should be twice the required 5.5 mV for each resistor or 11 mV when the proper bias is set. Try adjusting again.

6. Measure Vbe (The voltage drop from base to emitter) for the output transistors and drivers (Q9-12).
It should be in the vicinity of 0.6V. Unless a solder joint has failed, the bias current must be there if the voltage
is being supplied by the bias circuit.

7. Is there a heatsink fitted and does it get even slightly warm after 10 mins or so?

Phew! Try these taking extra care not to slip with the probes, shorting the transistors and wrecking your good work!
Report back what you find plus any revelations or eureaks hopefully. Stay with it.
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Last edited by Ian Finch; 15th January 2013 at 06:02 PM.
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Old 16th January 2013, 06:15 AM   #1059
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Quote:
Originally Posted by Ian Finch View Post
4. Measure the voltage across R12 (68R) It should be ~0.4V for 6mA current through the VAS/bias generator circuit.
Did you mean R13 (680)? I don't see R12.

I'll go through those check points later today and report back. Thanks.
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Old 16th January 2013, 06:24 AM   #1060
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Hi
It's fuzzy but look at the emitter of Q6 - the resistor to the negative rail. It's just a convenient series resistor to monitor the current through the second stage (VAS) of the amp.
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