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NAD 3150 Integrated amplifier (vintage 1985) - preamp not working when upright

donken

New Member
I have a rather strange issue I'm trying to diagnose. I don't think I necessarily need very specific advice, but I'm wondering what electrical component would cause these symptoms.

First off, I've narrowed down my problem to the preamplifier. The power amp section works fine and is no problem. On this amplifier, there's a physical bridge between the pre-amp and power amp ports, and the power amp works when fed a source directly. The preamplifier however ONLY works when the amplifiers is put upright on either side (left and right) or upside down. If the amplifier is right side up the pre-amp section stops working. At around a 45 degree angle is where it switches from working to not working (45 degrees picking up the left or right side of the amp). I realize this is a strange diagnosis, but I can't seem to narrow it down to any specific components. Tapping on the relay, capacitors, resistors, everything doesn't seem to do anything, but without fail putting it on it's side or over a 45 degree angle makes it work.

I did notice that one of the capacitors was leaking, but Iooked at the repair manual for this and found that the leaking capacitor is a part of the power amp side, which is still working. https://www.hifiengine.com/manual_library/nad/3150.shtml

So my question is, what do I start looking at? What electrical component behaves this way?
 

alec_t

Well-Known Member
Most Helpful Member
Welcome to ETO!
Are there any heavy components (transformer, choke etc) on the pre-amp section? I'd be looking for a dry joint associated with any component which might be significantly affected by gravity in that way.
Regardless, replace that leaky cap and any others which show signs of leaking or bulging.
 

donken

New Member
Thanks!
From what I can see on the dedicated pre-amp board and based on the schematic most of the components in the pre-amp section are resistors. I did try lightly pushing on everything that I knew was a part of the pre-amp section, and then everything else on the board to see if it moved it enough to make a connection. I'll do more investigating when I get home tonight. Good advice on looking for heavy components... also I just placed an order for the replacement capacitor.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
To be honest, such an issue sounds more mechanical than electronic, it would be incredibly rare for a component to fail in that way. However, as it's repeatedly faulty then it should be fairly straightforward to fault find the issue - either signal injecting, or signal tracing, to find where the signal disappears, along with voltage measurements. You have the obvious advantage that it's stereo, so if both channels are going off (as I presume they are?), then there's only a VERY tiny possible number of causes, as almost all of it is channel specific.

I see there is a mute circuit on the output of the preamp, triggered by a switch? - that would be a prime suspect, as it's a mechanical part common to both channels of the preamp. There are two small diodes, D501 and D502, their cathodes join together, measure the voltage at that point, with it working, and with it not working - does it change?.
 
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donken

New Member
Good catch, correct, BOTH channels are going out at the same time when this happens. I've been going through the diagram to see what components might affect both channels. There's pre-amp controls that affect both channels, I did try pushing every button, and turning the volume/balance knob to see if i got anything, but it's just dead when it's upright. I'm guessing you're thinking it's power related? since it likely goes to both channels. Attached is the block diagram and the circuit diagram (appears to be handwritten, and the scan is pretty low quality, also linked in my first post)
 

Attachments

Nigel Goodwin

Super Moderator
Most Helpful Member
As I said before, check the voltage on the cathodes of D501/2. And I never suggested a power problem, although that would be one option, but the mute circuit looks more likely.
 

Ylli

Active Member
I did notice that one of the capacitors was leaking, but Iooked at the repair manual for this and found that the leaking capacitor is a part of the power amp side, which is still working.
Is the leakage wet? Or is it dry, hard and at the base of a largish component? If the latter, it is more likely to be glue the manufacturer used to help hold the bulky part in place and reduce stress on its leads. An actual physically leaking e-cap is fairly rare.
 

donken

New Member
Is the leakage wet? Or is it dry, hard and at the base of a largish component? If the latter, it is more likely to be glue the manufacturer used to help hold the bulky part in place and reduce stress on its leads. An actual physically leaking e-cap is fairly rare.
This is what the cap looks like

I did peel a bit of the black crusty stuff off the top when I was trying to get the specs on the cap
 

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Ylli

Active Member
I'd agree that is leakage. Just seems that many mistake the glue for leakage. Yours is not glue :).
 

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