Looking for help with Luxman L-80V

[xoaphexox]

Hi all,

I've spent many hours trying to debug this issue and I am almost ready to throw in the towel. I am generally a parts replacer but am eager to learn and have a good foundation of what the different components do individually and have learned general troubleshooting procedures but am not yet at the point where I can determine what is causing this issue.

I bought this amp knowing that there was a 'scratchy' channel. Lo and behold the deoxit was not the solution - I should be so lucky.

Anyhow, here is the schematic for those interested in helping me out:

https://home.comcast.net/~devanymarino/LUXMAN_L80_L80V.pdf

Symptom: Right channel sounds like crap (very distorted - see image). The left channel sounds wonderful.

Here is what I know:
1. When injecting a 1khz sine wave in the AUX input the output waves looks beautiful through the preamp and tone controls. I checked this by hooking my scope up to pin 1 (which I believe is the input from the pre to power amp - there are no jumpers on this integrated).
2. The output of the left channel wave is beautiful (see photo)
3. The output of the right channel is very distorted (missing peaks)
4. The DC offset of both channels has been dialed down to zero in both channels
5. The voltage taken from the collector of all four output transistors is a solid +/- 46V

Based on what I know of things I figure there is an issue supplying + voltage to some part of the right channel driver board. I am assuming this because the top half of the peaks is missing. This service manual sucks.

Here is what I have done so far trying to fix this issue:
1. Replaced every electrolytic capacitor in the unit (even the ripple filters - there is no AC ripple)
2. Replaced every transistor in the problem (right) channel (even outputs)
3. Replaced the relay and protection circuit transistors
4. Resoldered the joints
5. Replaced both multijunction diodes in the right channel
6. Tested the values of many (but not all) resistors between right and left channels (lifted legs, cannot find discrepancies)

If anyone can tell me a good next step that would me amazing of you.

Thanks!
Rob

ps I attached a 'before' pic to give a view of the insides. I realize that the offset and bias trimpots are reversed in the service manual.

 

[ecluser]

Check R117 and D102. You may replace the VD1221 by two diodes connected in series (1N4148 or similar). It is the current from Q105 that drives the transistors Q108 and Q110. It seems that you have no drive for the positive part of the signal.

 

[dr*audio]

There is a lot more wrong with the right channel waveform than the tops clipping. The bottom should look like half a sine wave and it doesn't.
1. Does the output look un distorted with no load on it?
2. If you turn the level down, is there some level at which it looks clean?
3. If yes to #2, if you apply the same signal to both channels at the lower level, are the outputs the same level?
Here are some things you can check in case I can't get back to you right away:
Check the emitter resistors on the outputs. Check R120, R116, R112, R111.
Check the voltage on the cathode of D101, it should be about 12V. If it is low, check for a short on D101 or C108. This is the supply for the frontend differential pair and if it is missing you will get distortion if you get anything at all.

 

[xoaphexox]

Check R117 and D102. You may replace the VD1221 by two diodes connected in series (1N4148 or similar). It is the current from Q105 that drives the transistors Q108 and Q110. It seems that you have no drive for the positive part of the signal.

I already replaced both VD1221's in the channel with series'd 1N4148s. I will check R117, thanks!

 

[xoaphexox]

There is a lot more wrong with the right channel waveform than the tops clipping. The bottom should look like half a sine wave and it doesn't.
1. Does the output look un distorted with no load on it?
2. If you turn the level down, is there some level at which it looks clean?
3. If yes to #2, if you apply the same signal to both channels at the lower level, are the outputs the same level?
Here are some things you can check in case I can't get back to you right away:
Check the emitter resistors on the outputs. Check R120, R116, R112, R111.
Check the voltage on the cathode of D101, it should be about 12V. If it is low, check for a short on D101 or C108. This is the supply for the frontend differential pair and if it is missing you will get distortion if you get anything at all.

I will look in to the others but I did check the voltage on both channels D101's - 11.63V steady

 

[xoaphexox]

I will be damned! R117 is wide open! And as luck would have it, 33K is a value I don't have on hand. To be continued....

You guys are amazing (and generous)!

 

[ryuuoh]

Make a 33k out of stuff you have, just to check the circuit~

 

[xoaphexox]

Make a 33k out of stuff you have, just to check the circuit~

Good idea. I made a 33.3k resistor out of a 30k and a 3.3k and put it in. Voila, I now have both halves of the sine wave.

I wouldn't say it is back to normal though - the sine wave is a bit 'fuzzy' with what look like tiny deviations from the shape at regular intervals. I thought it might be interference from my bench light but unfortunately it is not.

When I get a chance I will snap a pic and post it here. This distortion also appears in the left (untouched) channel but moreso in the right (rebuilt) channel.

 

[ryuuoh]

Maybe those zeners are next~
How's the sound? Try headphones, to get an idea of low level noise.

 

[xoaphexox]

While I'm at it - maybe the bias has something to do with this. I need to adjust it - is there a simpler way than cutting all the wires that the service manual says? The SM was wrong about the location of the pots so I am not sure whether to trust it here either.
Thanks

 

[ecluser]

"I will be damned! R117 is wide open!" ... No damnation... I've been there before...

Bias? 23mV between point 4 and point 3

35mA X (0.33 Ohm + 0.33 Ohm) = 23mV

 

[xoaphexox]

"I will be damned! R117 is wide open!" ... No damnation... I've been there before...

Bias? 23mV between point 4 and point 3

35mA X (0.33 Ohm + 0.33 Ohm) = 23mV

That seems like a more elegant solution than cutting wires - I wonder why they suggest that in the service manual. I noticed the SM for the Sansui AU-7700 also suggests removing wires for setting bias...

Thanks, I will give this a try tonight.

 

[dr*audio]

The reason they want you to cut wires is it allows you to accurately measure the total current draw of the output stage. Theoretically, you can get some variation in the current through each of the emitter resistors if there are parallel output transistors. In practice, there is not that much difference, and it's fine to just measure the voltage drop across the emitter resistors to set the current.

 

[xoaphexox]

Okay, in the previously good channel I was able to dial in 23mV between the emitters of both outputs (points 3 and 4).

However, with the trimpot in the same place on the right channel, only measuring .8mV. I guess there are more bad resistors in this channel. The reason I say resistors is that I've already replaced the zener, multijunction diodes, transistors, and capacitors. There isn't anything left! Although I do admit since the problem appeared to be with the bias circuitry, I did put the matched input differential pair back in this channel.

BTW the emitter resistors seem OK, I am getting a steady reading of .8 ohms between points 3 and 4 on both channels.

 

[xoaphexox]

I can get the bias in both channels to 23mV but the trimpot in the right channel is turned much more clockwise than the left. I guess if at the end of the day the bias is correct that's all that matters? Monitoring the temp of Q106 and Q108 in both channels they are around 104 degrees F about 5 minutes after turn-on.

 

[dr*audio]

What voltage do you read on Q109 and Q110 base?

 

[xoaphexox]

What voltage do you read on Q109 and Q110 base?

Left Channel: -.588V, .608V
Right Channel: -.542V, .631V

 

[xoaphexox]

Bias and offset are spot-on now but I am still seeing this weirdness in the sine wave at low output voltage.

I have made a (very poor) depiction of what this looks like. It is like tiny straight lines are "riding" the wave.

 

[xoaphexox]

Okay, I did a listening test through headphones. I immediately hear the noise in the right channel superimposed on the signal, but at least the signal actually sounds like a 1khz sine.

Thinking that maybe my choice of replacements for the transistors in the channel may have been the issue I am systematically putting back in the old transistors one at a time and looking at the signal through the scope. So far Q101, Q102, Q103, and Q104 have gone back in and nothing has changed.

 

[dr*audio]

Is the bias now ok on both channels, and if so, how did you fix it?
Do you see the spikes if there is no input signal?