NAD 304 still has problems after replacing faulty resistors and relay

EstueraIV

Member
2018-09-22 1:44 am
This amplifier has been sitting and collecting dust for about a year, I gave up trying to repair this even after following the simple repairs that are supposed to fix this fading channels issue, which was replacing the R333 and R334 resistors and the relay. Despite replacing these, it still has the channel fading issues. Im gonna give this another try and see if someone can help me out.

So these are the components that i replaced:

1. The two R333 and R334 resistors (both 47k 1/2 watt) with two 47k metal film 5% 1 watt resistors. (Pictures)

2. The DEC DH2U 24VDC relay with a Relpol RM84 24VDC relay. (Pictures)

3. The R742 (6.8k) and R743 (3.9k flame proof) resistors with new ones, the previous owner used a combination of two 16k resistors in parallel in the R742 spot because it looks like the original blew up, i cant tell what happened but i got rid of that parallel configuration and replaced it with the original value. The R743 flame proof resistor was right next to it and it was possibly damaged so i replaced it with a new one as well. I also replaced a nearby diode just in case. Here are some pics of this mess. (Before) / (After replacement)


When i replaced these faulty components, i can turn on the amplifier, but the red-light lights up for about 3 seconds and then turns to green, and when i turn it off it goes back to red and it fades away after a few seconds. Heres a video demonstration of this.

Before i bought this off of craigslist, the owner did tell me that it had some audio problems, he didnt specify the exact issue but he said he fixed it. After a few months, i started getting issues. The owner disappeared so now im stuck.

I also think i potentionally made it worse because of my soldering skills were really bad, i lifted some of the traces but i soldered the pins to the next component in line, this could be a problem.

Any ideas where i should start or what to do?
 
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bullittstang

Member
Paid Member
2013-05-09 2:14 pm
Ft. Worth, TX
I would start with as many high resolution pictures as possible, and post them directly to the forum site not links.
Also have you tested all voltages especially when the “fade happens” is it the relay going into protection or something else?
How long does it take to fade out and does it happen every time or sporadic?
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
I've seen that reversal of the protection lamp sequence on power-down with similar NAD PE amps, not just that 304 model. The protection indication is a little complicated and monitors both voltage and current at the amplifier, before the relay. Still, it all rests on an old faithful type protection chip, IC701 (TA7317). These are pretty reliable but the cheapo small electrolytic caps around it are not. Leakage currents and failures are probably interfering with operation of the indicator LEDs and possibly other functions.

These Power Envelope models are all much the same and hellishly complex to follow on the PCB so I simply snip and replace the caps there around the chip, to avoid having to remove, test and replace every one carefully. ESR testing can be done in-circuit but not many testers are reliably capable of it.

Assuming you have the schematic, I would check and replace all the small elcaps there around the chip. Even if it makes no difference, that's still a good precaution and after all, the amplifier is working now - a good result so far. Before connecting speakers though, check DC offset there at the amplifier outputs and before the relay contacts. That might indicate the root cause of the failures if the higher rails now don't switch in to deliver the full specified power envelope. These are dual rail amplifiers, similar to class G and deeper problems may not be noticed until the higher supply rails boost kicks in as needed.
 

EstueraIV

Member
2018-09-22 1:44 am
I would start with as many high resolution pictures as possible, and post them directly to the forum site not links.
Also have you tested all voltages especially when the “fade happens” is it the relay going into protection or something else?
How long does it take to fade out and does it happen every time or sporadic?

The images i provided are actually in full resolution if you open them directly in another tab.

Other than that, I think its more of a muffled sound? The right channel sounds okay (I dont remember how it normally sounded before), but the left channel has significantly less bass than the right channel.
 

EstueraIV

Member
2018-09-22 1:44 am
I've seen that reversal of the protection lamp sequence on power-down with similar NAD PE amps, not just that 304 model. The protection indication is a little complicated and monitors both voltage and current at the amplifier, before the relay. Still, it all rests on an old faithful type protection chip, IC701 (TA7317). These are pretty reliable but the cheapo small electrolytic caps around it are not. Leakage currents and failures are probably interfering with operation of the indicator LEDs and possibly other functions.

These Power Envelope models are all much the same and hellishly complex to follow on the PCB so I simply snip and replace the caps there around the chip, to avoid having to remove, test and replace every one carefully. ESR testing can be done in-circuit but not many testers are reliably capable of it.

Assuming you have the schematic, I would check and replace all the small elcaps there around the chip. Even if it makes no difference, that's still a good precaution and after all, the amplifier is working now - a good result so far. Before connecting speakers though, check DC offset there at the amplifier outputs and before the relay contacts. That might indicate the root cause of the failures if the higher rails now don't switch in to deliver the full specified power envelope. These are dual rail amplifiers, similar to class G and deeper problems may not be noticed until the higher supply rails boost kicks in as needed.

Where exactly in the relay should i measure for offset? which pins? i cant tell where in the schematic diagram, but i did measured the DC offsets at the outputs and these are the results.


  • Right channel: min 0.5mv | max 1.9mv
  • Left channel: min 17.7mv | max 19mv

Also the the left channel has significantly less bass than the right channel. I will replace the caps around the chip tomorrow once i get the caps
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
The offset voltages are OK, though the amplifier is just idling with only the low voltage rails in operation. That should be the case until momentary peak program levels switch in higher voltage boost stages to permit higher peak power levels. That's what these "Power Envelope" models are about but it's also their Achilles' heel. We measure the amplifier output voltages before the relay to ensure that any relay contact or protection faults that may interfere, are avoided. If no problem, no need to.

We can only test in the low power mode with simple voltage checks. Strictly speaking, an oscilloscope and the skills to use it are necessary to fault-find the PE operation. As your offset measurements are OK, there's no longer a need to locate the appropriate connections to the relay. However, if you can't identify the connections from the PCB patterns in the service manual, note that the amplifier outputs each pass through a coil (L301 or 302) and parallel resistor (R422 or 424) before the relay. You should be able to identify and make contact there if necessary.
 

EstueraIV

Member
2018-09-22 1:44 am
The offset voltages are OK, though the amplifier is just idling with only the low voltage rails in operation. That should be the case until momentary peak program levels switch in higher voltage boost stages to permit higher peak power levels. That's what these "Power Envelope" models are about but it's also their Achilles' heel. We measure the amplifier output voltages before the relay to ensure that any relay contact or protection faults that may interfere, are avoided. If no problem, no need to.

We can only test in the low power mode with simple voltage checks. Strictly speaking, an oscilloscope and the skills to use it are necessary to fault-find the PE operation. As your offset measurements are OK, there's no longer a need to locate the appropriate connections to the relay. However, if you can't identify the connections from the PCB patterns in the service manual, note that the amplifier outputs each pass through a coil (L301 or 302) and parallel resistor (R422 or 424) before the relay. You should be able to identify and make contact there if necessary.

ahh I did something wrong and now the amp is going in protection mode with the red light. I was measuring the voltage across the 422 resistor and it read 0v, I thought my multimeter was bad, so I randomly chose to measure the voltage across the 306 resistor and there was a small spark on the left side of the resistor, could've been on the probe and then I heard the relay switch and the amp is now stuck in protection mode. The resistor itself seems fine as I can still measure it's resistance. Jesus christ I dug myself a deeper hole now, I think I should just get a professional to repair it, I'm afraid that I will make things worse just like I did now...
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
That's likely the wise choice. The resistor and coil form an RL output filter that should never have any measurable DC across it, since the resistor is shorted by the coil at DC. Unfortunately, it's very difficult to identify faults when you don't have some basic design theory or know what to look for in advance. I'm sorry to say that the repairs will not be cheap, particularly after the repairer sees the rough work, apparently done by the previous owner. Best check they are familiar with the design of such NAD PE models - these are not simple amplifiers.
 
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I also had repaired this model with the two 47k 1/2 resistor busted (open). after replacing this it turns on red then green. when off it turns red again but its normal. Had you tried playing music with it? when relay activates it means no dc out.


well before I messed it up even more yesterday, it played music, but the left channel had very little bass, I think the right channel sounded normal, I forgot how it normally sounds but it seemed fine. It's gone completely in protection mode now that I shorted something while trying to measure the DC offset
 

Junm

Member
2008-12-23 5:52 am
Dau
ohhh thats bad...when you fix the problem try playing music without the preamp, remove the jumper for pre out at main amp in. if still unbalanced the problem is on the amp. check the balance pot and input caps.

Check your output trannies if they still alive..
 

Ian Finch

Member
Paid Member
2010-04-11 4:22 am
Coffs Harbour, NSW
At this point, it would be wise to get professional help. Yes, anything can be fixed provided we know how the circuit should work, how to assess what is needed, including test instruments and genuine spec. parts, in order to replace any damaged ones and have good spares.
As mentioned earlier, this isn't an average amplifier. It operates with dual pairs of power rails where the higher rails are only switched in according to power demand. To understand and address its common problems requires a bit more than poking about, as in the simple designs we are familiar with here and in most older commercial products where there are no tricky power circuits that need an engineer to analyse.

If you are causing more damage as you test a circuit (yes, I've done that!) it's time to stop and consider what your knowledge and experience so far has taught you. We need to study more than just a schematic before diving in with probes, unaware of the dangers of slipping and shorting circuits and voltages that can destroy even the largest semis in a flash. Still, there are several aids to testing with meter probes that should be in use as appropriate. You may also need three hands and two pairs of eyes to use a meter whilst making adjustments if you are trying to hold standard probes in place at the same time. No wonder we have problems :D
1. Sharp, hard probe tips to penetrate and "grip" in solder joints.
2. Clip leads (alligator or crocodile clips on either end of insulated wire, about 30cm. (not great but cheap)
3. Hook probes - the normal probe leads are replaced by leads with a spring loaded hook at the tip, chosen to be small enough to reach into closely spaced wiring and make reliable contact whilst your attention is on the meter. (variable quality and sizes, many too big, expensive etc. and some very cheap but too flimsy to last long)
 
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EstueraIV

Member
2018-09-22 1:44 am
Just wanted to make a quick update. I managed to find another NAD 304 that was not working for $13. It had severe distortion problems, so I replaced the R333 and R334 resistors, and the relay and to my surprise it works! No more distortion and it plays normally. My old NAD 304 however is still toast, im probably going to recycle it or sell it for parts as i dont have an oscilloscope to do further troubleshooting.

As for the new NAD 304 i just repaired, is it advisable to recap the amplifier at this time?