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NAD 3150 Channel distored.

PE9ZZ said:
There are no switches in the power amp except the speaker switches and the relays. Oh, you could tap those relays to see if the symptom changes. If it does you're in luck because you then just have to change the relays for new ones.
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Thanks another piece of great advice!
 
If I were to buy a scope is there an simple scope that would be suitable to diagnose audio issues but not be too expensive?
 
Simple second hand CRT scope of 10 MHz (or more) is sufficient for this task. 50-100 quid.
 
HIFIuk said:
If I were to buy a scope is there an simple scope that would be suitable to diagnose audio issues but not be too expensive?
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I'd ask around too. I just gave away 4 scopes (3 tek and 1 hp). Plus a 2213 last year. I can't keep everything.
 
HIFIuk said:
Ok sure I think its the power amp because I crossed over the pre out / power in jumpers on the back and the issue remained on the same channel. I'm going to clean everything with decent switch cleaner on Wednesday and I'll take it from there.
Thanks for ypur help it's really appreciated -
Dan
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I adjusted the DC offset to as near to 0mv on the left and right channel as I could as in the 3150 service video (part 2) above. I then adjusted the Bias on the right channel to about 8mv. But I'm not getting any voltage from the test points when trying to adjust the Bias on the left channel (that's the distorted one) and turning VR603 has no effect to this voltage. When measuring at the + and - speaker terminals, I get 7mv on the good (right) channel and 7.5mv on the bad (left channel)
 

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I adjusted the DC offset to as near to 0mv on the left and right channel as I could as in the 3150 service video (part 2) above. I then adjusted the Bias on the right channel to about 8mv. But I'm not getting any voltage from the test points when trying to adjust the Bias on the left channel (that's the distorted one). I have also cleaned all the switches and pots. That did get rid of a slight crackle on the balance control but that's all unfortunately.
 

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If your bias voltage on the left channel is stuck on zero the drive of the output transistors is bad. Lacking a scope check V(be) of Q611, -3, -5 and -7. They should all be around 0.6 V. See the schematic (and compare right channel).

Be VERY careful not to short anything with your test leads! Been there, done that and yes, I have the T-shirt...
 

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PE9ZZ said:
If your bias voltage on the left channel is stuck on zero the drive of the output transistors is bad. Lacking a scope check V(be) of Q611, -3, -5 and -7. They should all be around 0.6 V. See the schematic (and compare right channel).

Be VERY careful not to short anything with your test leads! Been there, done that and yes, I have the T-shirt...
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Is there a guide on how to test the transistors? Are these easy to replace? - I'm not very good with schematics!
 
HIFIuk said:
Is there a guide on how to test the transistors? Are these easy to replace? - I'm not very good with schematics!
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Step 1: locate the transistor you want to test. Start with Q611.
Step 2: identify its base and emitter leads. Sometimes these are marked on the PCB. Dunno if NAD does that.
Step 3: carefully place your meter's test leads on the base and emitter. Polarity is not important. Read the meter. Write down the voltage.
Repeat until you have done all four transistors. Make sure not to short anything because the amp is powered on.

Edit: No, NAD does not mark the electrodes on the PCB. Bummer. Look it up in the datasheets. Usually the two outer leads are B and E.
 
PE9ZZ said:
Step 1: locate the transistor you want to test. Start with Q611.
Step 2: identify its base and emitter leads. Sometimes these are marked on the PCB. Dunno if NAD does that.
Step 3: carefully place your meter's test leads on the base and emitter. Polarity is not important. Read the meter. Write down the voltage.
Repeat until you have done all four transistors. Make sure not to short anything because the amp is powered on.

Edit: No, NAD does not mark the electrodes on the PCB. Bummer. Look it up in the datasheets. Usually the two outer leads are B and E.
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I tested the outer pins on all 4 of the big black ones (in the attached pic) and they are all at 0.59v or 0.6v.
But I'm not sure if I'm testing the correct components!
 

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HIFIuk said:
the big black ones
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Those are the output transistors. Q615-8. They are all OK then. The other test you can do is in the same area: the two small grey transistors. They are Q609 and -10. They should have 2.2 V between their outer terminals.
 
0.15v on the right and 0.65v on the left measuring the outside pins- both quite low then...
As this is now upside down I assume the distorted left channel is the one reading 0.15v ?
 

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0.15 V is clearly wrong but my instruction was also wrong, it should be the center pin and the left pin, not the two outer pins. Those are the collector and emitter terminals.
 
HIFIuk said:
Okay
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Ok I've measured the centre and left pin on those. The right hand one is a rather disappointing 0.62v and the left is more healthy at 2.3v. Again, being upside down I guess the right hand one is the distorted channel.
 
HIFIuk said:
being upside down I guess the right hand one is the distorted channel.
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I believe your guess is correct. The top drive for Q609 is Q607. This is an emitter follower from the VAS transistor Q605. The bottom drive is just the bootstrap capacitor C625. Since there is not enough current through Q609 to develop a healthy voltage now the cause for that must be found. I'd start with the voltages across R633 and R635. I expect 1.5 V and 0.5 V respectively.
 

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PE9ZZ said:
I believe your guess is correct. The top drive for Q609 is Q607. This is an emitter follower from the VAS transistor Q605. The bottom drive is just the bootstrap capacitor C625. Since there is not enough current through Q609 to develop a healthy voltage now the cause for that must be found. I'd start with the voltages across R633 and R635. I expect 1.5 V and 0.5 V respectively.
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R633 measures .038v and R605 measures 0v
 
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