NAD 214 in protection mode

[ear4life]

Hello!

I hope I can receive some help in here. I have an old nad 214 with several problems. Well some of them are gone by now I think.. I bought it as not working ( stayed in protection). After reading about some of the usual reasons why the protection circuit could be on, I think I need some "new thinking" -fresh eyes on it

What I have done so far is:

-Replacing the output caps (C202-C205 + C901-C902)
-changed the output relay
-adjusted the dials on the left and right channels to 20mV (after 30 minutes)
-redone all solder joints
checked:
-C226
-D212
-R233
-C224

Okay... When I now turn the amp on it goes out of protection, and clicks the relay ON, and lights green in the powerlamp. BUT when I connect the speakers to the binding posts it clicks back in protection... funny... disconnecting the speakers again and it comes back on. I found out that when I switch to bridge mode, it goes in to protection as well I think that's where I need some help. Something in the output is wrong or out of specs.

I have heard that the IC 201 could fail as well, but I´m having a harder time finding a replacement to check with.

I hope some of you know something about the old NAD amps

Best regards
Martin

 

[Mooly]

First test has to be to measure the DC voltage from ground to the junction of the four 0.22 ohm emitter resistors. It should be essentially zero volts DC. Do that for both channels and report back with the result.

 

[mcd99]

Warning: Be very very gentle with the PCBs especially the PSU section. Sometimes the only thing holding them together is the silk screen.

 

[ear4life]

Hello

Thanks for replying! Well I´m reading about 40-50 mV on the emitter resistors

 

[Mooly]

And does it go into protection mode if either the left or right channel has a speaker connected ? That is to say, are both channels behaving the same ? Or does it only trip when both are connected ?

 

[ear4life]

Hello
I did something stupid! I was doing some measuring, and accidently dropped one pin of the multimeter out of my hand and down in the amp, where it killed a 1SS143 diode by shorting it to another component

I will get back if I can find a substitute.. maybe a 1N5614 would do it... Thanks so far.

Regards
Martin

 

[Mooly]

Oh dear, thats something we've all done at some point. Which diode was it... could be other damage. If it was D301 or 303 (and whatever the other channel is) then the amp will work without them. They are used to limit the drive at clipping point by clamping to the rails.

 

[ear4life]

It was the D309 that died.. It is the exact same diode as the D301 an D303. (left channel) I could swap maybe... just to begin with?
Still not proud of that little stunt

 

[Mooly]

A mere minor mishap

Leave it out of circuit if you haven't got anything to replace it with and power the amp up with a bulb tester to confirm no other damage is done. They only come into play under extremely high signal levels when they clamp the signal to the rails. All other times they do nothing so can be omitted for testing.

 

[ear4life]

Okay, I´m back on track

I took the diodes out of both channels, and when i turn on the amp it clicks green... Nice!

It does look like the amp has a couple problems. When the speaker are connected to the left channel it stays in protection. When speaker are connected to the right channel MAJOR 50 Hz hum comes out! I will check all solder areas again.

 

[Mooly]

This is where a scope is really needed. Any AC on the output (particularly HF oscillation) won't show as a DC offset. Is it 50 Hz hum (50 ?? pure deep tone) or a harsher buzz ? Could be anything... you'll just have to check it all carefully.

 

[ear4life]

Yes you are right! I have to check the value of all components. I would say it was a harsh bass tone.... more like a square tone than a sinus tone. I don´t have a scope, only my little multimeter.. But thanks for your time and help! I will get back when I find out more.

Regards
Martin

 

[Mooly]

A harsh tone could be PSU ripple (that's at 100Hz and harsh sounding) caused by a break in the print somewhere or a missing/low value reservoir cap... many possibilities really.

 

[ear4life]

Well I was testing with a little JBL control 1... I might have bin fooled to think it was 50 Hz.. The speaker does not have much sinus response down there.... I don´t know.. it was only a second, but the signal was not clean. I will disconnect all the cabs and check there value, and then for shortcuts..