NAD 3020 preamp

[Black Octagon]

Hey guys,

So I've got this NAD 3020. Checked the other threads and come up with nothing so far. Anyway, I've done a rebuild on it and it was working great for awhile. Then this scratching noise started on the left channel and is now a full blown hissing and crackling noise. But this only happens after the amp has been left off over night. Turn it on, it hisses and crackles for about 15minutes to half an hour and then its fine.

Traced it to the preamp section's left channel. Checked the four transistors (Q501, 503, 505, 507) and even swapped them and the problem still persists. Reflowed the solder joints even. According to the schematic I should be getting about +17V between R523 & R525 which I am getting on the right channel but on the left its +13.4V. This is after the amp has warmed up. If left overnight, this voltage drops considerably to about +3.2V and then the transistors I mentioned above start emitting negative voltage on the left channel. What's happening here? I've tested the resistors out of circuit and they measure well. Do resistors fail that they increase in resistance only to be alright once warmed up?

Or could it be a capacitor but which? The electrolytic ones have been changed to Panasonic FC caps. Also at times instead of the hissing and crackling it gives off this sound like you know when a lightsaber is swung around? A bit weird.

The schematics are attached.



Thanks guys! Appreciate it!

[Nrik]

Hi
It's a bit hard to read the schematic, and the different versions of 3020 seems to be quite different ( check this link).

Anyway the two resistor you mention, are those the 2x6.8K voltage divider for the bootstrap? I think so.

The crackling noise and shifting voltage must be because of a faulty component or solder joint. Since you already checked the resistors and transistors and reflowed some of the solders, then I am guessing the capacitor C507 is sometimes short-circuiting. Check the solders on that one and/or replace it with a matching type.

That lightsaber swing sound ( oooh nice - who hasn't played around with a lightsaber as a kid) I have no idea what comes from, but if you fix the above mentioned problem, this one might disappear too.

[Black Octagon]

Originally Posted by Nrik

Anyway the two resistor you mention, are those the 2x6.8K voltage divider for the bootstrap? I think so.

The crackling noise and shifting voltage must be because of a faulty component or solder joint. Since you already checked the resistors and transistors and reflowed some of the solders, then I am guessing the capacitor C507 is sometimes short-circuiting. Check the solders on that one and/or replace it with a matching type.

That lightsaber swing sound ( oooh nice - who hasn't played around with a lightsaber as a kid) I have no idea what comes from, but if you fix the above mentioned problem, this one might disappear too.

Hi, yup they're the two 6.8K ones you mentioned (what's a bootstrap?). I've changed the transistors so many times, all 4. I even replaced C507 and C505 with ceramic ones of the same value and nothing. Why the negative voltage readings when there are supposed to be positve readings? Any idea? Bad caps can cause this? I've also replaced the other ceramic caps in the circuit and checked the diodes. Really am at a lost here and for that channel I've resoldered every joint right up to the switches.

I know about the different versions of the amp but mine comes pretty close to the one in the schematic. Only difference is the schematic has a Mono switch whereas mine has a muting one.

[Ian Finch]

Do you have another amplifier that can be used to substitute for the left channel?
You could use this to first confirm that the noise does come from the preamp and that it is not
the muting JFET Q510 (which I would suspect) by disconnecting speakers and plugging into "pre out".

It seems clear to me that if it takes >15mins to clear the problem, it is heat related and this could be
anywhere but if it is a consistent fault as you describe, it would seem a semiconductor is the culprit.

As a servicing tool, a freezer spray can could be used to check this. Wait 'til the problem clears and
hit the suspect part with freeze spray and bingo, problem returns and suspect confirmed (or not).
I don't offer safety advice, but these sprays were just Freon refrigerant and you should not use
hydrocarbon based ones on powered circuits in case you get ignition and incinerate your amp,
yourself and the place you're in.

If that's all too hard, you could warm the part gently with a mini heater or the tip of your soldering
iron to see if that speeds up the clearing of the noise immediately after switch- on. In in any case,
you don't want to make matters worse by damaging the parts. Just slight heat - enough to raise
it to operating temperature - is all that's needed for a simple check.

You could extend this to other preamp transistors but I haven't found many problems this way with
small signal bipolar audio transistors.

[Black Octagon]

Hi, I suspected the JFET too but ran it with both JFETs bypassed and it showed the same signs. I'm sure its the preamp as I've switched the left and right preamp channels to the power amp and the hiss appears on the other channel. I've also run the power amp direct from a CD player and its fine.

Will try the heating up component method you suggested and see what happens. Thanks!

[mjf]

have you changed all electrolytics (c 513,517,519,525) ?

[Black Octagon]

Originally Posted by mjf

have you changed all electrolytics (c 513,517,519,525) ?

Yup, all electrolytics have been changed to Panasonic caps. Even switched those but problem persists.

I've got the schematics and service manual for the 3020 and the 3020B. I noticed that the caps mentioned earlier C505 and C507 are mylar caps in the amp and in the parts sheet for the 3020 manual they're also mylar caps but in the 3020B manual, they're listed as ceramics. When I switched these out earlier I put ceramics in. Wondering if they're at fault. In fact when I switched the mylars out for ceramics, the noise became worse so I put the old mylars back in.

I know these manuals aren't always correct as when I started this build I got every manual and schematic I could get my hands on and compared notes. To be honest till today I'm not sure if its a 3020 or 3020A! What I do know of the fault right now is:

1. Its the left preamp channel
2. The voltage reaching the transistors vary greatly, sometimes negative and sometimes positive. When its positive voltage its slightly lower than the right channel. The noise is greatly reduced but is still there. The noise gets really loud when the voltage becomes negative. This is all measured with multimeter black probe to chasses.
3. All transistors are fine.
4. Earlier suggested its a heat issue but I just turned the amp on just now after shutting it off for about 5 hours and now the voltage is back to positive but is still too low. Noise is reduced but still there with occassional pops and crackles.
5. Volume control doesn't affect the noise. Balance and tone controls do affect the noise however as increasing the treble will also make the noise erm...also increases in treble.
6. Left and right section of the phono stage is very stable and measurements show exact or very close to each other. Power amp section is very stable with bias and offset voltages remaining stable since set.

[ashok]

Originally Posted by Black Octagon

2. The voltage reaching the transistors vary greatly, sometimes negative and sometimes positive.

What do you mean by this? Is the dc supply voltage ( on the supply rail) to this section of the preamp varying ? Or are the dc conditions around the transistor varying ? ( like say collector to ground voltage etc. )

If it's the supply rail you should back track on it and see where the problem starts. Since the phono stage gets the same supply maybe there is a track or resistor problem. You said the phono stages are working OK on both channels so supply at that point should be OK.

[Black Octagon]

Sorry my bad, wasn't clear with my earlier statement. The dc conditions around the transistor are varying. The supply rail is fine and the phono stage is working fine on both channels.

The dc supply voltage to both sections of the preamp are fine. Its just around some of the transistor base the voltage is far off or sometimes close but not matching the right channel. When its far off it shows negative voltage on the multimeter. For example on one of the bases instead of +3.4V, it shows -17V when its really bad. Sometimes it shows +3.2V but even that difference of +0.2V comes out as static on that channel.

[Ian Finch]

Could you be more specific here with the transistors that you have said show varying DC voltages "around the base".
Taking the lower channel, are you referring to Q502 or 504,506,508?

Are the power supply points you checked as being steady, the same as where the marked voltages of +30 and -25
at the collectors of Q506,8 are shown?

Monitor the bias voltage across D502,504. You haven't mentioned them yet but diodes also have their problems and
these could cause strange problems.