Hello Experts,
I need some help with an NAD 208THX that I am repairing. This is one of the more complex designs that I have worked on. When I first got it, it was a mess from previous 'repair attempts'. However, after replacing many damaged components and recapping the entire amp (except for the filter caps) I now have it to the point where where it is stable, idles well, and generates a nice looking sine wave from the outputs.
However... one channel was blown and repaired by me. That channel has MUCH higher gain now than the unrepaired channel. For example, at a given voltage sine wave at input to both channels, the unrepaired channel might have 3 volts point to point at the speaker terminals while the repaired channel has 70! Also, when running at that voltage, the resistor in the input board (R208) between signal ground and main ground smokes! These voltages are with no load at the speaker terminals.
I have played with the resistor values that short the feedback to signal ground (R324, R326) on the amp board, and can get the gain to be pretty close, but it requires quite a bit larger value resistor than is currently in place (original was 62 ohm for R326 and 300 ohm for R324 for a combine value of about 52 ohms in parallel), the new resistor value to get similar gain is 200 ohms for R326, open for R324. All replaced transistors were replace with same number parts except the augmenting power fets where I used IRF350 in place of BUK437. Also, the signal voltage coming from the input boards are nearly identical between the two channels, so I've ruled out an issue with the input board.
So any thoughts on why the gain is so high?
The service manual can be found here:
NAD 208 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
I need some help with an NAD 208THX that I am repairing. This is one of the more complex designs that I have worked on. When I first got it, it was a mess from previous 'repair attempts'. However, after replacing many damaged components and recapping the entire amp (except for the filter caps) I now have it to the point where where it is stable, idles well, and generates a nice looking sine wave from the outputs.
However... one channel was blown and repaired by me. That channel has MUCH higher gain now than the unrepaired channel. For example, at a given voltage sine wave at input to both channels, the unrepaired channel might have 3 volts point to point at the speaker terminals while the repaired channel has 70! Also, when running at that voltage, the resistor in the input board (R208) between signal ground and main ground smokes! These voltages are with no load at the speaker terminals.
I have played with the resistor values that short the feedback to signal ground (R324, R326) on the amp board, and can get the gain to be pretty close, but it requires quite a bit larger value resistor than is currently in place (original was 62 ohm for R326 and 300 ohm for R324 for a combine value of about 52 ohms in parallel), the new resistor value to get similar gain is 200 ohms for R326, open for R324. All replaced transistors were replace with same number parts except the augmenting power fets where I used IRF350 in place of BUK437. Also, the signal voltage coming from the input boards are nearly identical between the two channels, so I've ruled out an issue with the input board.
So any thoughts on why the gain is so high?
The service manual can be found here:
NAD 208 | Owners Manual, Service Manual, Schematics, Free Download | HiFi Engine
Last edited:
Hi!
Nice to see someone else trying to tame this beast!
Could it be that your faulty channel is running with the higher rail at all times? As you know, this is a class G construction and the higher rail should kick in when needed. Perhaps the higher rail is on at all times?
Also, if you want to mess with the feedbacks, why not try to change R325 and/or R327 to see if one of the two feedback loops is faulty. In the schematic it says R324 AOT, without a resistor value. I can't see 300 ohms anywhere.
I hope it's OK if I drop a question of my own?
I clip R393 for the bias adjustment and set it to 12mV. On my R channel it stays at 12mV steady, even when I introduce a sinus 0,5Vpp 1kHz at the input. The L channel, however, changes to 20mV.
What behaviour is the correct one? I can see that the L has been tampered with, because there are small marks of probes at every MOSFET solder pad.
/Bo
Nice to see someone else trying to tame this beast!
Could it be that your faulty channel is running with the higher rail at all times? As you know, this is a class G construction and the higher rail should kick in when needed. Perhaps the higher rail is on at all times?
Also, if you want to mess with the feedbacks, why not try to change R325 and/or R327 to see if one of the two feedback loops is faulty. In the schematic it says R324 AOT, without a resistor value. I can't see 300 ohms anywhere.
I hope it's OK if I drop a question of my own?
I clip R393 for the bias adjustment and set it to 12mV. On my R channel it stays at 12mV steady, even when I introduce a sinus 0,5Vpp 1kHz at the input. The L channel, however, changes to 20mV.
What behaviour is the correct one? I can see that the L has been tampered with, because there are small marks of probes at every MOSFET solder pad.
/Bo
Hi everyone,
Looks like I need help with my 208 amp
It worked like a charm for one month, and today... I powered it up, then, in a minute, there was a not very loud click from left speaker continued with a quiet hissing sound (like an electric spark). I turned amplifier off immediately and disconnected the preamp. Then powered up again - both channels remained quiet. Then I connected preamp again (with amp turned off) and noticed that sound in left channel is much quieter and heavily distorted.
Could this be a relay problem or damage is more severe? Any suggestions are highly appreciated.
Looks like I need help with my 208 amp
It worked like a charm for one month, and today... I powered it up, then, in a minute, there was a not very loud click from left speaker continued with a quiet hissing sound (like an electric spark). I turned amplifier off immediately and disconnected the preamp. Then powered up again - both channels remained quiet. Then I connected preamp again (with amp turned off) and noticed that sound in left channel is much quieter and heavily distorted.
Could this be a relay problem or damage is more severe? Any suggestions are highly appreciated.
I did some investigation of my issue (as far as it is possible without a scope, with broken AC voltmeter and old Stax headphones) - looks like distortion gets worse as signal volume increases. If I'm not wrong, this is most likely caused by a faulty relay RLY301.
Could anyone suggest a model name or replacement for this relay please? Tried searching the forum, but had no luck. I remember someone here had been posting a link to electronic parts distributor selling original relay, but can't find it
Could anyone suggest a model name or replacement for this relay please? Tried searching the forum, but had no luck. I remember someone here had been posting a link to electronic parts distributor selling original relay, but can't find it
The parts list gives RLY301 as a Schrack RP310 relay. It's single pole, 16A, 250VAC rated. Schrack are part of Tyco (now TE Connectivity) so you should find something suitable in their range at Mouser, Digi Key, Newark etc. depending on where in the world you are.
Relay RP310-24 (24VDC 16A/250VAC 1100R 1U) TYCO/SCHRACK, V*RP310-24 - "EVITA"
To be honest, it doesn't sound like just a relay or any kind of output connection problem. Sound that gets worse with volume is more like output stage problems. If you are testing with only a very light headphones load, you may not see the real problem, which a 4-8 ohm loudspeaker will soon expose. Then, you'll probably get none, or very distorted output.
Relay RP310-24 (24VDC 16A/250VAC 1100R 1U) TYCO/SCHRACK, V*RP310-24 - "EVITA"
To be honest, it doesn't sound like just a relay or any kind of output connection problem. Sound that gets worse with volume is more like output stage problems. If you are testing with only a very light headphones load, you may not see the real problem, which a 4-8 ohm loudspeaker will soon expose. Then, you'll probably get none, or very distorted output.
Thank you Ian!
I found these relays in stock at our local supplier. And also found someone who can actually service my amp.
Sound is basically the same in headphones and loudspeakers (though I have disconnected it immediately to avoid tweeter damage), and it is very similar to distortions in my first DIY amp when I fed its output stage with +-5 V instead of +- 30
I found these relays in stock at our local supplier. And also found someone who can actually service my amp.
Sound is basically the same in headphones and loudspeakers (though I have disconnected it immediately to avoid tweeter damage), and it is very similar to distortions in my first DIY amp when I fed its output stage with +-5 V instead of +- 30
Hello again, guys.
I have replaced the speaker relays in the amp, but it didn't help so left it for a while.
Some time ago, I downloaded the 208's schematic diagrams again and spent few nights trying to understand the way it works.
Then, after understanding what "Class H amplifier" is, I supposed that, given the character of sound distortion, problem is somewhere inside the supply commutation circuit.
Finally, after hours of playing with high voltage, oscilloscope, multimeter and other scary stuff I found open P600D diode. Accurately pushed it back and forth few times - and what do you think, it helped!
Listened few hours to this beast driving my old dynaudio speakers. Man, this is... I don't know, this is just awesome. Now I need the same kind of power amplifier for the rest 3 channels of my home theater setup
I have replaced the speaker relays in the amp, but it didn't help so left it for a while.
Some time ago, I downloaded the 208's schematic diagrams again and spent few nights trying to understand the way it works.
Then, after understanding what "Class H amplifier" is, I supposed that, given the character of sound distortion, problem is somewhere inside the supply commutation circuit.
Finally, after hours of playing with high voltage, oscilloscope, multimeter and other scary stuff I found open P600D diode. Accurately pushed it back and forth few times - and what do you think, it helped!
Listened few hours to this beast driving my old dynaudio speakers. Man, this is... I don't know, this is just awesome. Now I need the same kind of power amplifier for the rest 3 channels of my home theater setup
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.