A sorry tale of woe and stupidity: NAD C370

Jump to Latest
#142
OK, it had to be checked.

I'm assuming you haven't a scope so lets try a little trick instead. If you connect a resistor and a cap in series across the speaker output do you see the same offset voltage across the cap as across the terminals. Lets say use a 1k to 10k resistor and a small cap. A non polarised type would be best like a 0.1uF but even a small electrolytic would do.

The R and C form a filter that will remove any high frequency AC and leave just the average DC value.

Any big difference in the steady state offset result.

Don't do this yet... but maybe we should try full mains, and I think we just have to do that at some point to be sure we're not chasing a non existent issue... but of course there is always a risk doing this.

Have to go 🙂
 

Attachments

  • Untitled.jpg
    Untitled.jpg
    20.3 KB · Views: 89
#144
Mooly said:
OK, it had to be checked.

I'm assuming you haven't a scope so lets try a little trick instead. If you connect a resistor and a cap in series across the speaker output do you see the same offset voltage across the cap as across the terminals. Lets say use a 1k to 10k resistor and a small cap. A non polarised type would be best like a 0.1uF but even a small electrolytic would do.
Click to expand...

No scope i'm afraid.

I wired up a 10uF electrolytic in series with a 10k resistor.

The steady state offset is pretty much identical across the cap as across the terminals. 😉

Mooly said:
Don't do this yet... but maybe we should try full mains, and I think we just have to do that at some point to be sure we're not chasing a non existent issue... but of course there is always a risk doing this.
Click to expand...

Oops too late! I already gave this a go with the preset back to 500 ohms.

No smoke or bangs - the speaker still pops though.

Mooly said:
Have to go 🙂
Click to expand...

Take care and thanks again Mooly 🙂
 
#145
Good to know it survived full mains 😉

Its quite difficult keeping track of this mentally without actually having a physical item in front of me so if any of these questions seem a bit random...

You mentioned in post #43 that R337 had failed open circuit.

We could do with looking at the possible paths the current needed to zap that resistor might have flowed and that is most like via R338 and R339 and L302.

Those very low value resistors would be worth a second look. Some of these types can go high in value by just a few ohms and if that was intermittent/variable it could cause issues.

When isolated they should read pretty much exactly as marked plus the series resistance of your meter leads which is probably around 0.2 ohm.

Just thinking aloud now... so we are pretty sure the output isn't oscillating so that means the noise at power off seems to be caused by the offset changing rapidly in the few 10's of milliseconds between mains disappearing and the relay dropping out.

So is the offset changing with a change in supply voltage and if so why.

We should always measure the rails in any fault-finding exercise. In particular are the - and + 68 volt lines correct. These feed the front end section of the power amp.

We need to measure these and confirm they are OK.

Are the - and + 18 volt supplies OK? These are across the Zener diodes D301 and D302.

The 10k feed resistors R310 and R316 would be prime suspects as they are listed as 0.25 watt parts and they dissipate... 0.25 watts 😱
 
#146
Mooly said:
We could do with looking at the possible paths the current needed to zap that resistor might have flowed and that is most like via R338 and R339 and L302.

Those very low value resistors would be worth a second look. Some of these types can go high in value by just a few ohms and if that was intermittent/variable it could cause issues.

When isolated they should read pretty much exactly as marked plus the series resistance of your meter leads which is probably around 0.2 ohm.
Click to expand...

Will double check these again 😉

Mooly said:
We should always measure the rails in any fault-finding exercise. In particular are the - and + 68 volt lines correct. These feed the front end section of the power amp.

We need to measure these and confirm they are OK.
Click to expand...

Will do - what is the best ground source to use when checking the rails? TP4? One of the connector pins like pin 4 of CZ304?

Mooly said:
Are the - and + 18 volt supplies OK? These are across the Zener diodes D301 and D302.

The 10k feed resistors R310 and R316 would be prime suspects as they are listed as 0.25 watt parts and they dissipate... 0.25 watts 😱
Click to expand...

Will check these too 😉
 
#149
Those rails are OK but how about these? Are we on full mains now or back to the bulb?

TP4 is perfect but using test points makes me nervous as it's so easy to slip with a probe. The speaker negative terminals should be as good. You can always do a quick confirmation that no voltage difference is present between them.
 

Attachments

  • Annotation 2020-07-27 195920.jpg
    Annotation 2020-07-27 195920.jpg
    99.1 KB · Views: 94
#150
Mooly said:
TP4 is perfect but using test points makes me nervous as it's so easy to slip with a probe. .
Click to expand...

🙁 Just as you posted that I indeed managed to slip with a probe.

I tested D302 and got 17.4V.

I went to test D301 and slipped - a small flash, not really any bang just a small fzzt.

Anyway, I frazzled D301 and will need a new one. I tested all the resistors around it and they are OK...bah!


After testing with mains I have been back on the bulb since.
 
#155
Hmm... accidental shorts, it is so easily done. Do you know what you touched and to where...

If it was a rail to the Zener then the Zener is probably short. If it was 'something else' to the Zener then you have pulled current through other parts which may have caused a problem.

That replacement looks fine, it dissipates less than 100 milliwatts in circuit.
 
#156
Mooly said:
Hmm... accidental shorts, it is so easily done. Do you know what you touched and to where...

If it was a rail to the Zener then the Zener is probably short. If it was 'something else' to the Zener then you have pulled current through other parts which may have caused a problem.

That replacement looks fine, it dissipates less than 100 milliwatts in circuit.
Click to expand...

Pretty sure it was the little bar that runs directly underneath the zener - 3J24.

This appears to connect R309 with R310 along the +V68 track - fingers crossed it was only the zener that got hurt.

The parts should arrive on Thursday- along with the hook type probes. Hopefully they will help me avoid more probing slips.
 
#157
OK 🙂

So it looks like the +68v line got shorted to the Zener. If it was the striped end of the Zener then the Zener will be zapped. If it was the other end then that is ground and the Zener would be OK.

Either way the +68v line took a hit. Now that line is stabilised and so it's worth checking the 33 ohm fusible resistor and also that the transistor isn't short circuit. The resistor should protect the transistor but you never know.
 

Attachments

  • Annotation 2020-07-28 115838.jpg
    Annotation 2020-07-28 115838.jpg
    114.1 KB · Views: 136
#158
Mooly said:
OK 🙂

So it looks like the +68v line got shorted to the Zener. If it was the striped end of the Zener then the Zener will be zapped. If it was the other end then that is ground and the Zener would be OK.

Either way the +68v line took a hit. Now that line is stabilised and so it's worth checking the 33 ohm fusible resistor and also that the transistor isn't short circuit. The resistor should protect the transistor but you never know.
Click to expand...

Yes, that resistor looks like its gone open - the transistor looks OK.

Going to be a pain to swap out that resistor - i'll have to remove the main board which doesn't look easy.
 
You must log in or register to reply here.
Top Bottom