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Old 30th November 2012, 08:09 PM   #21
gilwe is offline gilwe  United States
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Hi.

I managed to get a few units of ZTX752 and a Fairchild J13009-2 to replace the burnt one,
but I'm still experiencing issues, will appreciate any help...

So I replaced all burnt parts, which includes a few 1N4148 diodes, a couple of ZTX752, the J13009-2 (the original BD743 have been replaced with a four of these) and a few resistors... I also replaced the broken ZTX384C with a BC550C.

I connected everything and plugged the NAP90 to the speakers and inputs. The replacement BC550C showed a huge spark which actually broke off its plastic case immediately - and made it look just the the original ZTX384C after the first "accident" - I assume this what exactly happened while the amp first burnt out...

Luckily, I powered it off immediately, so only the BC550C and an additional ZTX752 have been burnt.

I now decided to test it without any externals, and to do that in steps, as it is the last ZTX752 I have and I don't want it to get burnt again.

I now used a BC550B and not BC550C (this is what I have, hope its fine).

I first powered the amp with a new BC550B in place of the ZTX384C, but with the ZTX752 out of the circuit -
just to see if the BC550B gets blown, and it didn't.

Then I put the ZTX752 in and powered the NAP90 up - it didn't make a spark but the BC550B went dead (shorts between all 3 legs). The ZTX752 hasn't gone dead.

I now took the dead BC550B and powered it up with all parts in, except for the BC550B -
Now one of the 0.22 fuse resistors has shown some smoke and has burnt out !

I really can't find any reason why it will keep burning parts... just thought you might have a direction...

I can see no shorts and made sure all parts which have been replaced are put correctly etc.
I put the BC550 in reverse to the original orientation of the ZTX384C (to match the C/B/E orientation)

Just a note - when putting a BC550B on the bad channel, I didn't replace the matching ZTX384C on the other channel.
Also, could it be that the J13009-2 doesn't work well with the BC550B in place of the ZTX384C and you need an original ZTX384C there -
or it really needs the original BD743 there ?

Attaching a schematics which shows the two specific parts I referred to:

Click the image to open in full size.

Thanks !

Last edited by gilwe; 30th November 2012 at 08:23 PM.
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Old 1st December 2012, 05:26 AM   #22
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I guess this was coming. Never simply power up under load after any repair. Don't connect any input or speakers.

Always use a current limiting device such as a bulb tester - a standard 60W incandescent lightbulb in series with the mains active lead before the amplifier or its transformer.

Build it in a separare box or wire up a plug with a standard light socket lead wired into it. Obviously, construct this with safety as a prime feature and make sure it's wiring is enclosed and insulated well enough such that a baby would be safe playing near it. Very safe, in case other people touch your gear too.

Read any repair thread and see these same dire warnings over and over again with any DIY amplifier repair, which is 95% certain to be blown output stages. Repeated attempts without these precautions just wind up with more collateral damage and loss of clues as to how it all went so wrong.

With this test gizmo, any serious fault will show as a steady glow with usually no damage and you don't keep filling your bin with silicon $$$ either. You even get the chance to compare channels for voltage differences to pinpoint what is wrong. Since these are DC coupled circuits, the errors accumulate through the circuit and wind up as big DC errors at the output and everywhere, like full rail potential on your speaker! Leave no stone unturned and now check the limiters too, since you say you replaced diodes, presumably those in collectors of the MPSA06/56. If they blew, likely, so did those transistors.

Consider that the last guy substituted the MJE13009s and got the result you are getting. I suggested BD911 as they should be less load on the drivers. Remember, you can just as well use ZTX653/753 as previously advised for drivers and VAS, since they are probably only different by sorting for higher Vceo, which is fine anyway.

The BC550/ZTX384 fails because it is trying to drive a shorted output stage. The type is irrelevant, either will and has failed. It would be good to monitor the voltages when you power up. You need to know if the voltage at the output is not zero and locate the reason before trying anything else. You also need to be sure the bias setting can be set to minimum, or no current flowing in the 0.22R resistors. If this is accidentally set high to start with, it will almost always mean disaster. Look at the good channel ...where is it set?

With the right parts installed and a check that there are no shorts to ground at the output or damage to the heatsink insulating washers on the output transistors, parts orientation is correct (the pinouts are different for most BC, ZTX and MPSA types.) There is much to check and compare before and when powered via the bulb tester.
Only when it all compares well and there is no glow in the bulb can you consider removing the tester.
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Old 1st December 2012, 08:59 AM   #23
djk is offline djk
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I run an amplifier on the bulb for several hours before going direct. This has saved me several times. Just because the amp fires up, goes to zero on the output, and bias adjusts properly, doesn't mean that everything is OK.

I remember a Robertson 4010 that I had rebuilt, outputs, drivers, etc. After about 4 hours on the bulb with moderate program material into a dummy load I was ready to put the case back on.

The bulb started flashing intermittently.

A potted module containing one of the Vas devices went open. After digging out the potting compound, I was able to re-solder a bad joint, re-pot the module, and the amplifier ran fine.

Without the bulb it would have burned up again and I would have had no clue as to the cause.
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Old 2nd December 2012, 09:07 PM   #24
gilwe is offline gilwe  United States
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Thanks for the bulb limiter suggestion. I built one easily, and once getting all parts in their places I can easily determine there's a fault in the circuit. The 75W bulb constantly lights in full power... But I still can't get what's wrong. Could there be bad resistors which have been damaged once the channel was heating ? (nothing visual, anyhow). Also, just wondering what are the chances I got faked ZTX752 and Fairchild 13009-2 from the HK dealer :/

As for using the limiter - is it safe to let the amp run through it as long as I have the bulb there ? so I can check the voltages ? or should I turn the amp off as soon as I realize there's an issue (bulb fully lights) ?

Last edited by gilwe; 2nd December 2012 at 09:37 PM.
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Old 2nd December 2012, 09:46 PM   #25
gilwe is offline gilwe  United States
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The voltages between ground and the various 13009-2 IC's legs are very different on both channels... one the damaged channel they are quite lower than on the healthy one.
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Old 2nd December 2012, 10:36 PM   #26
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Good to have a delicate fuse now, but is it in series with just the active line or have you wired it across A & N?
Measure the current in each branch of the circuit to see what is drawing the current. For example, Measure the voltage across the 68R resistor in the VAS branch to see what goes on through the ZTX 753/ZTX384/ZTX 653 path, which should only be in the order of 8-12 mA, when it is functional.

In the output stage, you can measure what the 100R resistor in the emitter of the upper ZTX652 is doing and the 47R resistors that set the threshold of the limiter transistors. They normally respond to excess current on the output branch, by shunting driver base current to 0V, thus limiting or clamping output power. The MPSA transistors which do this, can be temporarily removed if the circuit operation interferes with testing but this should not be necessary.

Current will be flowing but it must be reduced by locating what is passing so much current that should not. The 0.22R resistors, which you use to measure the bias current, should be only passing 25-30 mA. That equates to only ~10mV across either. Likely they will be passing a lot more.

The chief suspects are the VAS transistors and drivers (both pairs of ZTX 652/752) The output transistors may again be suspect too.

Which diodes did you also replace and have you checked out the limiter transistors?

Generally, when you replace a transistor because it measures bad by a simple diode or continuity test, its complement (as in this circuit) will be damaged to some extent too.
They are low-cost silicon, so replace them both + anything else in that rail-to-rail current path. If you are being frugal and just replacing what you measure as bad, you will likely continue to burn parts until you eliminate every collateral shred of the fault.

Why not mark up the voltages you measure and currents you calculate from measurements across the resistors, that seem to give markedly different readings to those in the good channel. That way, wiser heads here might be able to make wiser suggestions with some facts to go on.
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Old 2nd December 2012, 11:03 PM   #27
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Particularly, what is the voltage at the output (+) which should be within 25 mV of 0?
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Old 6th December 2012, 08:09 PM   #28
gilwe is offline gilwe  United States
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I investigated further and found that the MPSA06 on the bad channel is defected (no continuity at all). When I took it out, the bulb still shown a malfunction.

I think it would be the best to replace just about everything, on both channels. Parts will cost some, but hopefully the amp will be back.

When the accident occurred, there was almost fire inside it. Many parts were fried. I guess there are a few parts which test correctly but might also malfunction.
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Old 7th December 2012, 12:19 AM   #29
gilwe is offline gilwe  United States
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More:

I diagnosed by cutting the -V and +V of each channel at a time, so I could investigate each channel separately, taking out one channel off the circuit at a time.

I found a few things:

1. The "good" channel is healthy and works as should - which means it amplifies. I assume that actually lets me now compare voltages between the bad and good channels, it that correct ?

2. What shorts the bad channel circuit was the ZTX752 which is located at the bottom-right on the NAP90 channel diagram (the one which has a 100R connected between itself and the V-). This was a NEW transistor actually :/

3. Taking that ZTX752 out of the circuit, the bulb limiter is not lighting, and having both channels connected to the power supply, the "good" channel still works and "plays".

4. With that ZTX752 out of the circuit (and the missing MPSA06), the voltage reading between the + and - speaker terminals of the bad channel,
is 25 Volts DC.

This is strange... could the fact that there are no ZTX752 and MPSA06 in place, cause such voltage to be read ?? or is there probably still a problem which is not related to the lack of them which causes this (which also could have also kill the ZTX752) ?

Thanks for any help, much appreciated !


Click the image to open in full size.

Last edited by gilwe; 7th December 2012 at 12:22 AM.
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Old 7th December 2012, 04:04 AM   #30
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Default Please follow the script

Transistors fail when you give them a load that's too close to a short under their operating conditions.
The ZTX752 drives the NA005 or BD743 etc. and then that shares drive with the +ve NA005 to the speaker.
If any transistor is shorted or overloaded, it will frequently, as already said, take out its driving transistor too.
If you said "I found a transistor that doesn't work so I replaced it but it still doesn't work", you'd realise that
without replacing any collateral damaged parts, you get a repeat failure because the original fault is not cleared.
In this case, it is most likely the NA005 (don't neglect the possibility of shorts to the heatsink if you aren't
replacing the washer and bush each time. Measure and check). As you fitted lower gain output transistors,
driver failure is now more likely.

You haven't answered how you test transistors either. There are a few failure modes so it's not just a simple
B-E diode test as you need to test for gain or Hfe, which usually involves removal.

These DC amplifiers are connected like dominoes - they can fail from the output stage right back to the input
with almost all devices burning out in a chain in severe circumstances such as you described, so it's not surprising
there is more damage than you imagined. That's why I persist in asking you to check these parts that you now
discover are also burned. If the diodes burned, what is in line with them? - Yes, the MPSA06/56!

That pile of dead parts will just grow whilst you fumble about with no systematic plan here. If you can't make measurements,
stop and think what you are doing. How about answering some of the questions so that we can help rather than just watch?

Now....
As this is in a half disassembled state, remove both output transistors and their drivers. Remove both limiter transistors
- yes the MPSA06/56. Now there is no output stage to cause problems.
Refit the upper (first or VAS) ZTX 752 and power up with the bulb and measure the voltage across Emitter
and Collector of the BC550. It should read sufficient voltage to bias the output transistors with about 30mA.
That will be about 3 diode potentials or 1.95V. check the voltage to ground too - it should be within 2V of 0.

If that is the case, then adjust the 1k trimpot to give 0V or the lowest voltage possible.
Turn off & check the resistors in the output stage whilst there are no transistors to confuse measurements.
Replace the drivers and output transistors with all new parts and use BD911, 909, or 907 + washers, grease and bushes.
Test that there are absolutely no shorts of transistor tab to heatsink.
It should then be safe to power up and verify that voltage at the output is close to 0V.
Then measure the voltage across both (in series) of the large 0.22R resistors (same points as between the
collector terminals of the lower and emitter of the upper output transistors) which should be zero also.
Adjust this voltage by turning the 1k trimpot to read a tiny 11mV. Don't slip whilst doing this or it will only
mean more parts to buy.

That should be OK to fly, but compare this voltage later with the other channel, after leaving closed for a half hour
to stabilise. (Naim are ultra slow to reach thermal equilibrium) You can then fit new limiter transistors and
power up with the bulb again in case there are still faults you didn't find. These limiters are normally inactive -
they don't save you from shorts or DC on the output, just overdrive.

In case it's not clear, bias current must be reset this way every time components are changed in the output stage.
Otherwise disasters will follow.

Edit: check connections of BC550 are different to ZTX types!
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Last edited by Ian Finch; 7th December 2012 at 04:16 AM.
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