Death and Life of a Nait3

[MJS]

The pre-amp has a delay that mutes the output when you turn it on. I suspect your limited supply is causing the rails to sag and then this circuit fires.

 

[trancera]

Add a diode then cap to hold the relay if so?

 

[nickcase]

Thanks for the lead, that sounds like a strong possibility, the naim logo leds blink off at trip time.

I'll observe the rails behaviour prior to working out how to splint the relay.

 

[nickcase]

I'd replaced the LM317 in the pre-amp power supply with a LT1086CT, which checking the datasheet states a max input voltage of 25V. The BHC cap was giving 38VDC after rectification so ...

I bypassed the monolithic reg and lashed up a replacement power supply in form of an ALWSR set for 24V, a new bridge rectifier and replaced the BHC cap with a brand new 6800u Kendeil.

The amp still trips.

The DVM shows steady 23.7 on the rails; at trip it dips to 17V and bounces back up.

What do you think? Problem must be on the pre-amp section. Looks like the 92R's mute relay is the single relay sat on the output of the gain stages.

Wondering how to test it, think I might just bypass it and connect speakers a few seconds after power up.

 

[nickcase]

... as I can't figure this out.

Taking the current limiter out of the equation there are no trips, however above quarter volume the amp boils the 100R resistors between the base / emitter of the output trannies.

With the current limiter back in and using an external power supply all looked fine and dandy.

I want a single box solution so I split the ground returns between the power amp section and the pre-amp power on the NAP board. Unfortunately, with the power amp and pre ground separate I measured 18V DC output at the speaker terminals

Perhaps the amp sections are still screwed. Think I'll start looking at TR9 and TR10 (the emitter drivers).

 

[trancera]

Very odd that an external PSU should not burn the resistors. Sounds like something is intermittently shorting perhaps when moving it about / connecting the other PSU.

Just a novice thought - the LM317 on the heat sink. Is it screwed down and insulated? The back is normally connected to the output pin in the 317, not ground. I'm assuming that circuit is totally disconnected but if it were to catch...

Shown here with screw http://www.comiston-jones.pwp.blueyonder.co.uk/NAP90-3_internal_modified.JPG

(also quite a nice idea using an external bridge - maybe for later once reliable)

 

[juz400]

Im sure the output trannies need to be insulated aswell!

Have you tried using the nap90 part by itself?
I mean lift the Left/Right/0v links between the boards and feed a known working Preamp into these points. Might be an idea to lift the 24v link too if the 0v isnt connected.

 

[SQR1]

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?

My nait 3 had exactly the same symptoms on the left channel and it turned out to be that driver transistor directly to the left of your negative DMM lead.

Many hours of removing and testing the transistors and it was that one!

Good luck.

 

[SQR1]

...ztx752 driver

 

[nickcase]

Many hours of removing and testing the transistors and it was that one!

Thanks SQR, you've described my Sunday

I've gone backwards. Following the test without the current limiter where the right channel boiled that channel now shows a bias voltage of 40mV across the 0R22 resistors, way too high (No signal or speakers are connected). The output drivers now get rather toasty within a minute of power applied.

I've replaced the three 100R resistors connected to the emitter drivers.

I've also removed and tested the ztx652 and ztx752 emitter drivers and they tested fine.

The left channel is sitting pretty at 4mV and no heat problems.

The rest of the circuit has been compared channel to channel and component to component. No differences could be seen (comparing resistances in circuit).

The only parts not replaced on the output section are the diodes and TR9, 10 and TR 11 and 12.

Something else is dodgy. Possibly my testing is not picking up issues that are present when power is applied.

Any suggestions where excess bias current comes from?

I have spare diodes and output trannies (11 & 12) but should I be looking 'upstream' towards the front end>?

Frustrated of York.

 

[SQR1]

No problem, just sounded like the same component might have blown on yours ..unfortunately no such luck.

I recall the first thing I replaced as per Neil Jadmans (guru member here) advice was the VAS transistors as these are known to degrade with time I believe its ZTX384 situated on the reverse side of the NAP board adjacent to the bias trim pot. The ZTX 384 is no longer available so was advised to use the latest equivalent - BC550c. However as mentioned it was only until I got further down stream is when I found my problem. If the VAS doesn't remedy you'll just have to while away the hours, I think you might be close though.

Once I got mine working I decided to replace all the trannies/ diodes with the latest equivalents (ZTX 653/ 753 and the like) as part of an overall service. ...I later applied some significant modifications to good effect but I digress.

I just wanted to clarify that I am merely a novice hifi tinkerer and all credit ought to go the hugely knowledgeable Martin Clark and Neil Jadman who aided me in my quest to remedy my nait 3. I'm sure if they read this they might be able to shed some more technical light in your direction.

Best

 

[trancera]

Hi Nick,

I agree, I'm an outright rough amateur as well. Advice from the guys with more knowledge would be great but as you are finding it very difficult to measure any faulty components it may still be tricky to debug.

At this stage I would probably go over the board while live. I'd use a crock clip on a 0V where a nudge cannot short it elsewhere and for the red lead I would insulate off the probe except for the very point so that it is very difficult to short on anything, then compare board to board again. You might see something drifting. Run a fan over it to help cool to give you a couple of minutes or more.

One thing in your pictures. I cannot see insulators nor do your retaining blocks appear to have bolts, did you make them are they different? None I google are without bolts. Maybe stupid question.

Also earlier you mentioned changing some transistors,which?. Not sure the rules apply but in the NCC200 there are some to keep correct offset,see note on left:
http://www.users.zetnet.co.uk/avondale/NCC200Notes.jpg

You can do it!

One for later... some benefit to those caps, .... once working!
http://s26.photobucket.com/user/neiljadman/media/NAIT3-1.jpg.html

 

[trancera]

Also, not sure you have found this but some comments re transformer as you are using a CD3 transformer you never know.

http://www.pinkfishmedia.net/forum/showthread.php?t=71710

Also earlier you mention the transformer is 35 0 35. That's rectified DC right? as if AC would be too high surely.

 

[misterc6]

Also, not sure you have found this but some comments re transformer as you are using a CD3 transformer you never know.

http://www.pinkfishmedia.net/forum/showthread.php?t=71710

Also earlier you mention the transformer is 35 0 35. That's rectified DC right? as if AC would be too high surely.

Neil's comments on that thread are correct whilst my original thoughts were not. The power amplifier section of the Nait 3 is the same as the NAP90/3. The transformer used in the NAP90/3 and the Nait3(R) is usually labelled as 'NAP90'.
Although shown differently in the circuit diagram the transformer has one continuous secondary which is centre-tapped and measures at around 28 - 0 - 28v AC. This full secondary winding is full wave regulated with the positive half providing the raw DC for the LM317 and the negative half powering the logo display. The transformer also has taps at 22 - 0 - 22v AC and it is these taps that are full wave bridge rectified to produce rails of +/- 29v DC for the amplifier circuits.

The driver and output transistors are lower power devices than those used in other NAPs and +/- 35v DC rails may be too high for them. If the transformer being used is actually 35 - 0 - 35v AC it will produce roughly 50v DC rails.

If the two channels have identical voltage readings under static conditions but one is running hot the oscillation must be a prime suspect.

 

[nickcase]

Thanks for the input and encouragement chaps, much appreciated. Built and tinkered many projects but troubleshooting old inherited kit with multiple unknown issues and merging it all together hasn't been made any easier by the inevitable hubris induced mistake - all good fun though.

At this stage I would probably go over the board while live.

Yep, I use insulated crocks already but this is a nice tip ...

insulate off the probe except for the very point so that it is very difficult to short on anything

Standard issue from now on

Only transistor changes to date are the original blown output trannies, BD911's for BD743A. (inherited board from years ago).

Thanks by the way for this image, had this on my laptop for a while! Got a OV tap on the star already fitted, caps ready, MMK's for feedback, wimas for signal coupling and ALWSR's (or TeddyReg's) knocking about for the pre-section.

Before that I like the fan idea to buy me some time to fix the problem caused by my overheating the right channel. Oh and then fix the cause of the over heat which ...

 

[nickcase]

... I suspect might be an issue caused by the coupling of two transformers which are not in phase.

Neil's comments on that thread are correct whilst my original thoughts were not. The power amplifier section of the Nait 3 is the same as the NAP90/3. The transformer used in the NAP90/3 and the Nait3(R) is usually labelled as 'NAP90'.

This board is marked NAP90/5. I don't have a Nait 3 transformer, so this

Although shown differently in the circuit diagram the transformer has one continuous secondary which is centre-tapped and measures at around 28 - 0 - 28v AC. This full secondary winding is full wave regulated with the positive half providing the raw DC for the LM317 and the negative half powering the logo display. The transformer also has taps at 22 - 0 - 22v AC and it is these taps that are full wave bridge rectified to produce rails of +/- 29v DC for the amplifier circuits.

is brilliant, thanks! Been looking for that information.

I have a CD3 trafo, which I measure as giving me +/- rails of 24.1vDC.

A second 30VA transformer with both 25 - 0 secondaries (in parallel) are pushing 35VDC to the pre-amp power section after the smoothing cap.

My issue a while back was what if any interaction might there be between the two trafos. It all appeared to work but was getting rail dropouts at modest volume which tripped the mute relay and my current limiter.

Leaving the current limiter out enabled me to drive it harder for about 30 seconds before the output dropped out and the 100R resistors near the emitter drivers started smoking.

If the two channels have identical voltage readings under static conditions but one is running hot the oscillation must be a prime suspect.

I only connect a speaker to one channel at a time; the unloaded channel survived.

I cannot see insulators nor do your retaining blocks appear to have bolts

Don't panic, everything is insulated and clamped during testing. The clamps I made from some alu billet from which I milled a grove, drilled a hole and covered in engineers tape.

 

[nickcase]

Righty ho, found a difference.

TR9 output transistor (ZTX-652) was reading 9.5V at the emitter whereas its 0.5V on the working channel. Base voltage was the same on both.

Replaced the transistor with a ZTX-653 from the parts bin but no change.

So, perhaps TR11 the output tranny is shot. Rather not go there again as the pads are shot and re-work requires component leg yoga to bridge the gaps

Nick

 

[nickcase]

OK, fixed

TR11 output tranny replaced with BD911.

DC offset is 11mV and bias current is now sane, although a little low at 2.7mV (across both 0R22 resistors) perhaps a new trimpot will sort that.

Now, back to sorting out the power sag and tripping issues. Still reckon my trafo arrangement is suss. I'll dig out another psu.

Thanks again chaps.

 

[trancera]

Nick,

Is there a different resistor at R5. Looks like brown green red = 1.5k
Other channel is grey I think ? = 1.8k

... on the NCC200 Les moves this up to 2.2k I think (though the cap goes up as well).

Make the preamp separate - you still have the DC caps in circuit don't you.

 

[nickcase]

Yes, good spot!

They are different values but I don't think this is the R5 resistor.

These hang off the emitter of TR4 and the base of TR9 and terminate at the base of TR5.

The NAP90 certainly varies from the NAP250 schematic!

Nick