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Old 2nd July 2014, 01:24 PM   #1
HandH is offline HandH  England
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Default Help with NAD L40

Hi, I have a NAD L40 CD receiver which has stopped working on the right channel regardless of the input (tuner, CD, etc)

So I swapped the speakers around but no change so it isn't the speakers.

I opened it up and swaped the fuses around, no change either & no blown fuses.

I can't see any obvious burn marks or bad capacitors etc and both channels work fine through my headphones.

The only thing I did notice was that a transistor for the working left channel on the main board ( Q225) got fairly hot to touch but the right ( non working) side remained cold (Q226) so I presume it is faulty or something feeding it is faulty. I have attached a picture of the main board, Q226 is in a black heatsink on the bottom right.
If anyone has had a similar problem or got any ideas what is wrong I'd be much obliged for your help.
This NAD has a dedicated pre-amp out rather than jumpers so would it be OK just to use it as a tuner / CD pre-amp connected to another power amp even though it is still wired to its own internal power amp? This might be the cheaper option for me rather than an expensive repair.

Thanks for your help
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File Type: jpg L40 main board 1.jpg (658.0 KB, 136 views)
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Old 3rd July 2014, 03:23 AM   #2
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About 95% of amplifier problems are due to output stage failures, including the very circuits designed to protect them. With one channel cold, the first thing to check is fuses M204, 205 but if they are OK as you say, the power has to be traced right to the amplifier and the relevant points in it. Here's a link to the service manual. Searching and posting one is a good way to make it easier for others to look at your problem: NAD L40 Manual - Compact Disc Receiver - HiFi Engine

This is one of NAD's weird asymmetric design output stages with Mosfet + Transistor output devices. The amplifier is AC coupled (4,700uF capacitor) so the overload protection relay only cuts the power to both channels. What you need to do is check DC voltages in the circuit to find where it went wrong. Some key DC potentials are already marked on the schematic (main board L+R ) Others can be compared with the good channel, so there is no lack of information regarding what is likely OK and what's not.

The question is, do you have the basic tools for soldering electronic circuits, small hand tools like screwdrivers, pliers etc, a DMM for accurate measurements and some grasp on the fundamentals electronics? Obviously, if parts are needed to repair the amplifier then it will cost a little money, perhaps more if trying to source obsolete parts like that BUK55 Mosfet, should it have failed. Otherwise, components are not inherently expensive, depending on where you buy and whether you wind up with fake parts which is what happens when you buy too cheap from unauthorized suppliers.

If you aren't confident or don't want to repair it, it's certainly possible to run your amp. as a preamplifier without the power amplifiers. Just remove fuses M204, 205 which removes the single +55V rail voltage supplies to each power amplifier and you still have the pre-out, as you say.
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Last edited by Ian Finch; 3rd July 2014 at 03:38 AM.
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Old 3rd July 2014, 08:01 AM   #3
HandH is offline HandH  England
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Thanks Ian,

I do have the tools & a meter & have down loaded the schematic so I'll have a crack at identifying the voltages as you suggested & get back to you.

John
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Old 3rd July 2014, 05:26 PM   #4
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Let's take it one step at a time. First you'll need to get yourself a form of current limiting, such as bulb limiter, using an incandescent lamp in series with the mains to your amplifier. Bulb Limiter for Testing

A safer to implement but perhaps expensive way, if you are not confident wiring 230V, is to solder a 220 ohm 1/2 watt resistor across the fuse holder and let that take any short currents rather than the amplifier components. It will heat quickly and burn in a fault condition if you are slow to switch off but they are cheaper than other parts. When either limiter is in place, then think about measuring.

Then make sure: No inputs or signals are selected, turn volume down to 0. Remove speakers or any other connections in or out, apart from power.

Establish whether all the transistors (apart from Mosfet) in the power amplifier are basically functional by measuring the Vbe (voltage between base & emitter) All should be ~0.65V.

Finally, watch where you place test probes and since many voltage measurements are referenced to ground, a clip for the negative probe to a ground point is safest. Then use just one hand with the positive probe for safety. There is mains power exposed in there , after all. Pressing hard with blunt probes also has a way of making them slip off the point and short out expensive bits. Take care.
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Old 5th July 2014, 06:26 AM   #5
HandH is offline HandH  England
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OK, built a bulb limiter. With a 100w bulb briefly bright to a dull glow and then off as far as I can see. With a 60w bulb dull glow then off. I have counted 30 odd transistors on the main board so it is going to take me a while to do the measuring! I'll get back with the info in the next day or so. There is an inspection plate on the bottom of the chassis to get at the underside of the PCB so hopefully it will make the task easier.

Thanks for your help, even if it doesn't get fixed at least I have learnt something.
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Old 5th July 2014, 07:48 AM   #6
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At this stage, you really only need to look at the power amplifier sections shown on pages 24,25 of the manual. There are 10 bipolar transistors in each channel which should be simple to test. They will be grouped near the heatsink and not hard to identify following the schematic and odd/even numbers for left/right channels. Q211, 212 will not respond like the Bipolar transistors as they are mosfets. Simply measure and record DC voltages at their gate, drain & source pins. I'd print out the pages and write on them.

If voltages appear very close from one channel to the other there may not be a problem in that section. You generally only need to look at differences like 5% or more, and if all voltages are consistently different by a small voltage or a few %, that could well be component tolerances or possibly a single fault affecting all circuit voltages.

Good work constructing a bulb limiter. That may also come in handy if you DIY build or do more repairs. It seems you don't have any immediate problems but that can equally mean that the problem has simply blown out that section of the circuit and it no longer works nor loads the power supply. The bright glow initially is the brief surge to the transformer followed by charging the electrolytic capacitors. That's quite normal and reassuring. When it stays bright, you do have a worry.
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Old 5th July 2014, 12:57 PM   #7
HandH is offline HandH  England
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Hi Ian,

Attached are the transistor readings but disaster has struck. On my very last reading I shorted out a pin on the right hand mosfet, bit of a spark & now the amp is is protect mode. I assume the mosfet is fried and needs a new one. I gather they aren't available so do you know an equivalent? ( I found IRLZ34 or IRLZ34N through Google) Should I change both channels?

I have taken out the fuses & the unit works through the pre amp & the headphones.

Anyway before my slip up I got three odd readings:-

Q203 (LH) and Q204 ( RH) voltages started at about 0.78v but rose slowly to well over 0.8v but never stabilizing at a steady figure As they are both doing the same this can I assume this is normal?

Q220 (RH) read either infinity or -0.43 when I swapped the probes around whereas Q219 (LH) read 0.86 This got my hopes up a bit thinking I had found the culprit, then the probe slipped on Q212.

Again thanks for the help
John
Attached Files
File Type: pdf NAD L40 transistor readings.pdf (16.5 KB, 17 views)
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Old 5th July 2014, 05:41 PM   #8
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Oh dear, and we were off to a good start there.

Search at the page header for BUK555 under 'view posts' and get a feel for what and why most "equivalents" are not. This transistor has an unusually low (logic level) threshold voltage and without that feature, wannabe equivalents just won't be. There is another feature with a high Rds(on) that is considered best for audio applications and again, this just doesn't match.

The direct part is available as BUK555 60B and I would normally advise not to buy from Ebay but this guy has bailed me out of very difficult situations a few times already. The risk of cash is not much but it is still yours if there are problems: 5pcs BUK555 60B Manu PHI Encapsulation TO 220 Powermos Transistor Logic Level | eBay
Alternatives (only a few) are posted in the thread search results.

Having given you the go-to guy, we don't actually know if you have toasted anything yet. With the bulb limiter in place, only milliamps can flow so a shorted or blown device is unlikely but I guess shorting the gate would cause other problems too. The diodes (measure the voltages across these) should also be checked because they are low current devices and blow easily.

The idea was also to compare voltages between channels so that you know what to look for but this is your only reference, don't slip, get some new probes or spin them against a fine grinding wheel to grind a nice, sharp point, around 60 degrees included angle on them! (I've been known to use the knife sharpener on an electric can opener for this!) Now you see the true DIY approach
As a further precaution, heatshrink tubing on the probes with just the tips showing is good or just slip on some PVC sleeving from power flex or thin, craft plastic tubing if possible, maybe adhered with a smidgen of glue.

Let's compare voltages at the BUK555 pins and see what has happened, rechecking previous measurements at other transistors and compare the measurements with the other channel. I'm a bit concerned that you read different voltages with reversed probes though. Is the battery in your meter OK, resistance reading near zero? (about 0.3 ohms usually due to lead resistance). Grab a small battery and give that a voltage test both ways, to be certain.
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Old 6th July 2014, 04:23 AM   #9
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I ran out of edit time but meant to first ask whether powering off then back on reset the protection switch. This gives you an indication of any damage caused. The protection circuit (actually Q216,218,220 + left channel circuit) monitors total current to each amp. by reading an excessive voltage developed across R248. It then turns off relay power and once triggered, I think it would hold that state until all power is removed so it would just turn amplifier power off again when switched on, if the overload wasn't cleared.

There is no indicator light or description of this in the owner manual either, and no connection to the microcontroller, so overload protection for the power amps is an independent circuit. The intriguing "XTALK" connections between the power amps have me stumped though - dunno how they treat cross-talk, as they appear to cross-link the amplifiers at high frequencies

It's likely that the bulb limiter, which drops mains supply voltage sharply with a load, actually alters the protection action by greatly reducing its supply voltage too. Still, we have to keep looking for what happened and clear any remaining faults.

Thanks for posting voltages you did manage to read. Most compare fine and the output devices showed no sign of failure. There is just the doubt with the odd readings that look to be failure and you realized it too. There is some damage that will have to be sorted. Unfortunately, the small transistors (2SC2240/2SA970) are now obsolete too so replacing them could be expensive due to scalping and postage . There are replacements from Fairchild (KSC1845/KSC992) but not so readily available. Still, most are in non-critical locations and substitutes won't be problem.
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Old 6th July 2014, 08:37 AM   #10
HandH is offline HandH  England
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Hi Ian,

I took your advice & changed the battery in my meter & also opened up the L40 case a bit more to provide better access because those mosfets are really awkward to get at. I then took the readings again (and again!) and attach them to this post. Probably best to ignore the first lot. Now they are all pretty close to each other and near your original estimate. The Mosfet I sparked is still reading too and virtually the same as the working left channel.

The only anomaly is Q220 which now reads 16.5v. I checked & rechecked this so I am confident of the reading. If Q220 is part of the protection circuit & is bad maybe changing it will sort things out?

I had a look on Ebay and took a punt on two 2SC2240 for 1.68 ( 3 dollars?)
So they should be here Monday/Tuesday I hope. I know there are counterfeit components out there ( I cant get my head around that, is there a big market for obsolete semi-conductors?) but it is cheap enough for me to take a chance.

Turning it off/on with/without the bulb limiter makes no difference, the front display says 'Protect' either way. I can hear the relay click though.
If I pull both fuses it comes out of protection & works as a cd/tuner pre-amp so I am no worse off.

Thanks for your help thus far
John
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File Type: pdf NAD L4 Transisitor readings 2.pdf (14.1 KB, 7 views)
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