Death and Life of a Nait3

[nickcase]

Back in the bowels of time I acquired a NAP90 board and a CD3 trafo with the dubious intention of building a Nait3 with the remains of a 92R.

Roll forward 5 years and my break from the craft is over

Having never powered up the NAP board before I had no idea if it worked.

It didn't. Magic smoke poured from the left channel and tracks around the rectifier fried.

There were two obviously fried resistors. R10 from the soa circuit and the 100R from the emittor of TR10.

Turns out the nasty looking output trannies were both continuous across all legs and therefore shorted the 35 - 0 - 35 rails :eek

So, stripped them out along with the soa circuit on both channels. Cleaned everything up.

Fitted four new BD911's to replace the BD743A's.

Before trying again I built a light current current limiter to reduce damage if something is still amiss.

You can see it in background here.

Worked a treat, no bangs or smoke.

Rails measured at 35 - 0 - 35. Let's check the bias ...

Here's the right channel - 3.9mV across both 0.22R resistors.

But here is the troubled Left - 1.487 Volts!

Clearly something is not right on that channel.

Any ideas what to check first?

 

[Paul R]

The adjustment pot?

Paul

 

[nickcase]

Haha!

Full 'lock to lock' adjustment of the trimpots yielded:

L chan: 1.9 to 1.3 V

R chan: 1.7 to 3.9 mV

An order of magnitude difference between the two channels.

 

[df_genius]

That pot will probably be for bias so leave that as-is for now. Next step is to check the voltages on all the transistors (B-E should be 0.6v-0.7v I believe), have you replaced the tants? you could have a bad feedback cap.

 

[nickcase]

Tants will be replaced, Wimas for input and MMks for feedback.

Was going to leave mods until the circuit is working though, I suspect resistors, diodes or trannies before tants. Happy to be wrong though.

 

[nickcase]

Update: The 0R22 resistor on the collector of TR12 output tranny was open circuit.

Replaced and bias voltage across them is now looking sane.

Cheers

Nick

 

[neiljadman]

Did you also check the DC offset across the speaker outputs? Should be less than 50mV

 

[Simon Dawson]

Hey Nick, should I recognise that 92R?

Simon

 

[nickcase]

Hey Nick, should I recognise that 92R?

Yes! She's some mileage on the clock.

Oh the adventures she's been on ... from a harem of ALWSR's and bulbous film caps to tales of being a remote control for a starfish

Thought it would be a fitting retirement to 'return to the source' as a little integrated.

You still hanging around here then?

Nick

 

[nickcase]

Hoping for a bit of advice/sanity check regarding transformer arrangements.

I am using a CD3 transformer with a single centre-tapped secondary. This is powering the amplifier circuits.

The original Nait3 transformer had additional taps for the pre-amp power section which I do not have.

I have another 30VA transformer with 25 - 0V secondary which I planned to use here.

The question: is it safe to use two different transformers where the 0V on the secondary on one is common with the centre-tap 0V of the other?

Something like this:

***Trafo1*** ***Trafo2***

+35V ------ ---- 25V

0V --------------------------- 0V

-35V --------

I am concerned about phase differences between the transformers.

 

[Tony L]

I wasn't expecting the loom band craze to make it as far as mains cables.

 

[nickcase]

I wasn't expecting the loom band craze to make it as far as mains cables.

Next up ...

 

[juz400]

Ive just had a lookie at sum piccys on me hdd,
looking at your first pic,
the top two pins are for the naim logo led`s
the second two pins are for the preamp psu a single winding on the traffo I presume
I dont remember the layout from memory but I think the 0v (centre tap) is connected to the -ve of the 4700uF. I think it sould be ok as the pre psu is 0v (ish) here too

If Im wrong, please someone say so!!!

Hope I was of some use
Juz

 

[trancera]

I'm not sure you would link the transformers like that, couldn't the power amp then use the smaller 0V as a return?

Can they not just be run separately, as long as you have DC blocking as per any pre / power combo?

 

[Emile_A]

Should work fine, Hacker did this on the design of the HackerNAP, 2 transformers -> 2 HackerCAPs (full wave rectification) 0v tied together on the amp board.

 

[nickcase]

the top two pins are for the naim logo led`s

Interesting, thanks Juz.

couldn't the power amp then use the smaller 0V as a return?

I had wondered about that too.

Hacker did this on the design of the HackerNAP,

Ok cool, missed that action.

Giving it go. I'm linking the two pins that Juz mentioned, they are redundant in this arrangement and can provide a return to the common 0V for the 2nd trafo.

Cheers

 

[nickcase]

Quite a few issues needed to be sorted out but we now have both channels playing

Two problems remain:

1. Trip and reset at modest volume. Both channels 'trip' when turning the wick up to 8 on the pot. By 'trip', my current limiter lightbulb briefly flares and the sound clicks off for 8 secs. After which it comes back until the next drum beat.

The soa circuits have been removed from the NAP 90/5 board, so I've not yet figured it out whether the pre-amp side or the NAP side is doing this.

2. Powering on the amp with left channel speaker connected draws high current. The lightbulb flares and stays bright. Without the L channel speaker connected the lightbulb briefly flares and music can be played (within the limits described above).

Any ideas to help the fault finding?

Not tried it without the current limiter yet ...

 

[nickcase]

wondering if this is amp instability caused by the speaker and cable.

a crappy cheap driver on 18inches of bell wire is certainly not the same as 3.5m of NACA5!

 

[trancera]

Wind an inductor to suit?

Unsure what will 'trip' for 8 seconds though except a relay.

 

[nickcase]

Wind an inductor to suit?

Was just looking into that I recall my NCC200's use a 15R inside 16 turns or so.

Just rigged it up with 5m of K20 - no difference, still 'trips'.

Unsure what will 'trip' for 8 seconds though except a relay.

Hmm, thanks, lots of those on the 92R, perhaps the mute relay is dodgy.