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NAD 3150
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Old 27th September 2021, 09:17 AM   #31
poundy is offline poundy  United Kingdom
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Originally Posted by Mooly View Post
I'm getting really confused with what you are measuring here

There should be no interaction between channels. If you turn the bias preset on one channel up or down and the bulb goes brighter or dimmer then the control is at least doing something that it should.



So have you forced a zero bias condition on both channels? and is the bulb out or at least very dim?

Lets get it to that state first.

so an update on my last post.
in the end i stumped reluctantly to good old youtube and watched a guy doing the adjustments on there


doing it his way, which was to connect leads across the resistor and adjust the pots that way i have both bias currents set at 8mv and the centres at 0v, but the lamp is still lit up a bit, so maybe some other faulty component in the circuit i just have to find?


the lamp should be off right?
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Old 27th September 2021, 10:08 AM   #32
poundy is offline poundy  United Kingdom
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Originally Posted by Mooly View Post
I'm getting really confused with what you are measuring here

There should be no interaction between channels. If you turn the bias preset on one channel up or down and the bulb goes brighter or dimmer then the control is at least doing something that it should.



So have you forced a zero bias condition on both channels? and is the bulb out or at least very dim?

Lets get it to that state first.

im wondering if im actualy chasing nothing now


all the voltages seem ok, the centre is 0v on both and the idle 8mv on both


the lamop is still on a bit though, so this is realy whats bothing me at the moment, or is it not a prob?


there is a buzz on both speakers if you plug them in so ive taken them out for now

Last edited by poundy; 27th September 2021 at 10:33 AM.
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Old 27th September 2021, 11:47 AM   #33
Mooly is offline Mooly  United Kingdom
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The lamp will glow when you have bias current flowing. So not a problem.

Just remember to make sure the adjustment is still correct when on full mains as it can change with the higher supply voltage.

The centre pin of the resistor pack will go to the speaker output and so measuring from speaker + to that centre pin would always show as 0 volt. If the manual says to do it that way then it is wrong. You have to measure across one of the resistors which you seem to be doing.

8mv will about 36 milliamps current

A buzz a loud buzz, a quiet buzz. It needs quantifying and checking. Does it change with the volume control for example.
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Old 27th September 2021, 12:01 PM   #34
poundy is offline poundy  United Kingdom
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its a bit like when you get electrical interference, doesnt change with volume adjustment
a bit like you used to get in car years ago when you needed to fit a supressor on the dynamo or a bit like a noise from a transformer

i also get a noise if i touch the pre amp/power amp jumpers-comes through on the speaker-grounding?

i keep finding odd things


R676 should be a 2w 680ohm, but what was in there, if the colours are right is a 390meg ohm(orange,white violet) yet it reads 490 ohm on the meter


i replaced the 680ohm with one out of another amp

Last edited by poundy; 27th September 2021 at 12:28 PM.
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Old 27th September 2021, 12:42 PM   #35
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if the colours are right is a 390meg ohm

That's very resistory


It may as well not be there
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Old 27th September 2021, 01:09 PM   #36
poundy is offline poundy  United Kingdom
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well i think im almost there(as andy williams used to say )



i have sound coming out, well kind off, the pre amp boards at the back must have poor connections because i get sound out of the right speaker, and when i twist the casing the buzzing sound changes pitch and goes quiter
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Old 27th September 2021, 02:04 PM   #37
Mooly is offline Mooly  United Kingdom
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R676 could have been a 390 ohm as brown and violet can look a bit similar. It feeds the relay so the basic check is to look at the voltage actually across the relay coil and see how it matches up to the marked value on the relay.

390meg would be about the same as 100 yards of string

If physical movement effects the buzz then there is a bad connection/s somewhere.
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Old 27th September 2021, 02:19 PM   #38
poundy is offline poundy  United Kingdom
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bad connections lol i should say
so:
idle is all good plugged in without the lamp-steady at 8mv
centre is all good at 0v +/- 2mv


sounds ok, the pre amp had a plug in board 10 loose connections i found on that! so i reflowed all the joints


all good now, boy that was a marathon, but i still enjoyed doing it, even if it was a bit frustrating at times
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Old 27th September 2021, 02:27 PM   #39
wg_ski is online now wg_ski  United States
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If the resistor is in series with a relay coil, sitting there running hot day in and day out, it could turn brown into violet easily. Discoloration is a sign of long term (over)heating. And if it does read 490 ohms that pretty much confirms that it’s running too hot - values tend to go *up*. Probably using 1/2 watt carbon film when they really needed 2 watt MOX or WW for long term stability.
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Old 27th September 2021, 02:30 PM   #40
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by poundy View Post
sounds ok, the pre amp had a plug in board 10 loose connections i found on that! so i reflowed all the joints


all good now, boy that was a marathon, but i still enjoyed doing it, even if it was a bit frustrating at times
Well it is a NAD

You should be getting to be the sites resident expert on these now

Get something else to play with next time
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