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Old 18th June 2014, 12:15 AM   #11
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That's good but I think you need a better meter - they are so cheap now, and you can check specifications before buying. Usually, you won't need to replace WW resistors immediately, even if the coating has burned. When you have replaced the small, burned MF resistors, you could try it like that, as the small WW resistors may be hard to obtain.

As always, use a bulb limiter when you power up a repaired amplifier.
I actually use a 100R/OW5 resistor in each power rail lead of my own DIY amplifiers and this also works well, with the benefit that 100R is convenient for measuring voltage drop and calculating current. This is very good for spotting trouble before it destroys your work.

Simply remove power rail fuses (if fitted) and solder the resistors across the fuseholder or use any method to insert them securely in the supply wiring during testing. As Naim use square pin connectors for the power, it should be easy to fit resistors temporarily by wrapping one lead around the pin and folding the other to insert in the connector.
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Old 18th June 2014, 05:15 AM   #12
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Quote:
Originally Posted by Ian Finch View Post
That's good but I think you need a better meter - they are so cheap now, and you can check specifications before buying.
May you suggest a decent one?
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Old 18th June 2014, 08:00 AM   #13
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This circuit may be of help. I found it on the web some time back, therefore I don't know how accurate it is.
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File Type: gif nap140_interesting amp.gif (55.6 KB, 170 views)
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Old 18th June 2014, 12:58 PM   #14
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Quote:
Originally Posted by ingenieus View Post
This circuit may be of help. I found it on the web some time back, therefore I don't know how accurate it is.
By the added artwork to the standard Naim circuit, I guess that was one of Les Wolstenholme's (Avondale Audio) Naim mod circuits and is close to his NCC200 design. (see below). The original is said to follow the early NAP250 (1975) design but any variations across many models up to the late 1980s were minor. The differences to the NCC200 (still offered for sale) are removing the VI limiter circuit, adding "anti-rail sticking" diode/resistors, output coil and damping resitor, substituted transistors and I imagine, his own degree of parts matching.

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Hi Calamaro,
I hesitate to recommend a DMM for you as your local suppliers may have different brands even though they may be the same as mine and come from the same Chinese factory! Fluke now have budget meters that are basic but reliable Chinese products and you can buy them from many suppliers. I think Fluke 15 is a good basic meter.
Multimeter UNI-T UT71E
Here is a cheaper model and still quite good - check the variations like A,B,D,E for features and different accuracy too. Take a close look at resistance accuracy and range of measurement possible - this can be a problem for low cost instruments, as you found. Multimeter UNI-T UT61B.
Buying direct from China can be cheaper than these examples too - shop around, read reviews by professional people, even buy a used one if the meter is a really good model too.
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Last edited by Ian Finch; 18th June 2014 at 01:27 PM.
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Old 22nd June 2014, 05:27 PM   #15
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Quote:
Originally Posted by Ian Finch View Post
When you have replaced the small, burned MF resistors, you could try it like that, as the small WW resistors may be hard to obtain.
Replaced the two burned resistors but one of them burned again when I've switched on
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Old 22nd June 2014, 11:56 PM   #16
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You haven't identified what these resistors are but guessing from the fuzzy PCB traces showing in your pic, the burned resistors were 100R from each OR22 resistor (not the output resistor) to the bases of TR7,8. i.e. the current sense voltage for the VI limiter circuits. If these are the burnt resistors (please check) it would be unusual if TR7,8 were not also shorted or the 47R resistors or capacitors also damaged. Test Vbe on TR7,8 and also test quiescent current by measuring voltage across either OR22 resistor. You should measure around 6mV or, if the reading is not clear, 12mV across the 2 in series. The output offset voltage (from the output to speaker ground) should be less than 40 mV.

Otherwise, the other 100R resistors from the same point are the driver emitter resistors and that points to problems with the MJE243/253 drivers (on small heatsinks). Test them for shorted C-E and Vbe. ) Voltage between base and emitter is always approx. +/- 0.65V for all bipolar transitors. Always compare voltages with the good channel so others understand what is likely wrong and you gain some clues for troubleshooting.

Note: The part numbers refer to Naim's schematic, similar to the last schematic posted above.
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Last edited by Ian Finch; 23rd June 2014 at 12:21 AM.
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Old 24th June 2014, 04:01 PM   #17
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Default Bad news

Checked again with my unpredictable DMM (a Fluke one btw) - while the two replaced resistors were in the flames - and:

-one output transistor is shorted
-its driver transistor (MJE243) has open C-E, Vbe is 11,58 volts
-the other driver transistor (MJE543) has open C-E, Vbe is 0,65v
-1n004 diodes between + and - rail are open
-DC offset from faulty channel: 29 volts
-maybe you remember that one ww 0R22 resistor was also burnt

and I noticed that plastic connectors from pcb to red speaker terminals are a bit melted

also I wonder why my bulb tester isn't glowing
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Old 24th June 2014, 05:28 PM   #18
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PS. diodes between + and - rail aren't 1N4004 as in schematics but they are BYF406
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Old 25th June 2014, 04:22 AM   #19
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Well, it seems you have trouble everywhere. I guess these faults in the amplifier were there all the time and that's why it was for sale (hopefully very cheap in that condition).

OK, I think from your description that just one side, TR9, TR11(MJE243) are blown. That would make sense and of course, the output offset voltage would have told you that if it was so high. Perhaps you misunderstood what was meant in post #4 or your meter has an intermittent fault/ flat battery.

I think all those parts TR9,10,11,12 should be replaced but this channel will then not match the other. MJE243/253 are not difficult to obtain and you only need 1 each.
The output transistors, TR11,12 may be marked BDY58 as the schematic shows or they could have been replaced earlier with Naim "house code" parts NA001 - neither will be easily or cheaply available. The best substitutes for quality sound will be MJ15003 but BUV20 is now also rated highly. Modifying Naim Audio power amplifiers

You already know what to do about burnt resistors - the smaller types are 500 or 600 mW MF resistors and the diodes are not critical- any 1A rated general purpose rectifier diode with a voltage rating of 200V or greater will be fine, so IN4004 is fine too. Before repairing the damaged channel, you should test the VAS transistors, ZTX653,753. these will cause the output stage to blow if they are damaged too, so take care to measure all transistors, noting that in some places it is not simple to test with certainty unless you remove them. I have found the cheap and basic Ebay transistor testers to work well for checking out small transitors and drivers. They actually identify, show connections and test almost any component you can find - this is just the cheapest example I saw @ ~14 Euros. There are many other versions with better features, connectors etc: LED Backlight Transistor Tester Capacitor ESR Inductance Resistor NPN PNP Mosfet | eBay

I suggest you repair the damaged channel first, listen and think about the sound quality and then consider changing the output transistors in the other channel.

First, open up the meter and check there are no loose sockets, connectors etc. It seems like there could be a poor connection somewhere but be careful not to move any adjustable pots. which set factory calibrations.
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Last edited by Ian Finch; 25th June 2014 at 04:42 AM.
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Old 25th June 2014, 11:09 PM   #20
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Ian, your help is very precious: thank you.
Well I'm going to order the parts I need, probably I'll try a couple of MJ15003 as they are cheap but if the amp will be repaired I would like to use original power transistors, aren't BDY56 in the old NAP 110? Found this picture on the web. They also seem a lot cheaper than BUV20. There are a few sellers on that auction site but I feel a bit anxious about fakes: do someone knows a reliable supplier?

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