I'm making the adjustments to finish setting this amp up and want to check my understanding of the procedures before I do (see attached photo for the Luxman procedure)
Firstly I wanted to check the idle current so I pulled the fuses and connected my meter across the fuse holders and switched on, set this on both channels to 200ma as its on a bulb limiter, is this about right?
Now DC offset adjust it says to earth the test point to whichever channel I'm adjusting, I take it just clip a lead between TP and Chassis? Then follow procedure?
And the AVR adjustment, cannot find the test points what am I adjusting here?
Firstly I wanted to check the idle current so I pulled the fuses and connected my meter across the fuse holders and switched on, set this on both channels to 200ma as its on a bulb limiter, is this about right?
Now DC offset adjust it says to earth the test point to whichever channel I'm adjusting, I take it just clip a lead between TP and Chassis? Then follow procedure?
And the AVR adjustment, cannot find the test points what am I adjusting here?
OK so found all adjustment points and all is good apart from possibly the bias setting.
After setting this as the procedure states the heatsink gets quite warm on idle ~50 degrees, would the bias setting benefit from settling longer than the minute it says in the manual then setting it?
After setting this as the procedure states the heatsink gets quite warm on idle ~50 degrees, would the bias setting benefit from settling longer than the minute it says in the manual then setting it?
Well, some amplifiers are designed to run with the output transistors at around 50 degrees C. Its not necessarily too hot, and some designers feel it can sound better to run them a bit on the hot side.
That said, you might check the bias current once its fully warmed up. The term, "only after one minute's lapse," could mean more than one thing. It could mean don't make the adjustment in anything less than a minute after turn-on. It doesn't say how long after one minute, nor does it say to measure at exactly one minute after turn-on. Besides, it doesn't say to start with it at room temperature either. So, some common sense interpretation may be called for.
That said, you might check the bias current once its fully warmed up. The term, "only after one minute's lapse," could mean more than one thing. It could mean don't make the adjustment in anything less than a minute after turn-on. It doesn't say how long after one minute, nor does it say to measure at exactly one minute after turn-on. Besides, it doesn't say to start with it at room temperature either. So, some common sense interpretation may be called for.
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Thanks for that, yes if I leave it idling for around 30 minutes it seems to creep upto 290ma, so maybe leave to settle then adjust.
On an unrelated note on one of the channels R730 15r 5w gets around 10-20 degrees warmer than the other, what's that resistor doing and what could cause that?
Looks like its supposed to be a zobel network to help with stability. You might check C706 to see if its shorted. Also might want to look at the output with a scope to see if any oscillation is present.
Might also measure those two components on the other channel to see if they measure the same for both channels.
If you don't find any explanation at idle, could be some instability that only occurs when playing music. Something like that.
Might also measure those two components on the other channel to see if they measure the same for both channels.
If you don't find any explanation at idle, could be some instability that only occurs when playing music. Something like that.
Thanks for that, the Zobel differences occur at idle, not measured it with a signal yet.
Would swapping the output Fets between channels to see if the would be fault moves be a reasonable idea?
Would swapping the output Fets between channels to see if the would be fault moves be a reasonable idea?
You're getting good advice from Markw4. Swapping parts would put your working channel in jeopardy.
You didn't say whether you have a scope. Confirm that there's 0V DC across R730--- any DC at all would imply leakage through C706 which would have to be quite large to cause noticeable dissipation in the Zobel. Far more likely is oscillation at high frequency. Would be great to have a scope.
Good luck!
You didn't say whether you have a scope. Confirm that there's 0V DC across R730--- any DC at all would imply leakage through C706 which would have to be quite large to cause noticeable dissipation in the Zobel. Far more likely is oscillation at high frequency. Would be great to have a scope.
Good luck!
If you cannot set the bias current as indicated in thé service manual, thé amplifier could be oscillating at a high frequency.
You need and oscilloscope to diagnose this.
Thé L58 is a very good amplifier. Never change of replace components at random, things will only get worse without a proper diagnose.
You need and oscilloscope to diagnose this.
Thé L58 is a very good amplifier. Never change of replace components at random, things will only get worse without a proper diagnose.
Thanks, will check C706.
I do have a scope and generator, any ideas where to start looking?
Readjust thé bias to 250 mA and leave thé amplifier on, no speakers connected and volume at zero.
Observe if they bias changes again.
Observe if they bias changes again.
You can look at the output of each amplifier channel with no input signal (you can ground the inputs just to make sure they are not acting like antennas).
See any RF or other oscillations/noise at the output of either channel?
See any RF or other oscillations/noise at the output of either channel?
Yep, left the bias to settle for 20 mins then adjust to 250ma, checked 10mins later and it was still 250ma
I've let the amp idle for 20minutes then reset the bias to 250ma on both channels, 15 minutes later its still set very close to 250ma so all good there I think.
With the Zobel resistor and possible oscillation I will scope it later today but am sanity checking this would be fault as its a 5watt resistor and gets to 50 degrees no load or signal, other heats to 35 degrees
I've let the amp idle for 20minutes then reset the bias to 250ma on both channels, 15 minutes later its still set very close to 250ma so all good there I think.
With the Zobel resistor and possible oscillation I will scope it later today but am sanity checking this would be fault as its a 5watt resistor and gets to 50 degrees no load or signal, other heats to 35 degrees
Checked R703 pretty much no DC across that, measured 0.008vdc.
Scoped the output on both channels, first video is the cooler zobel channel second the warmer one, tested no input or load connected
Scoped the output on both channels, first video is the cooler zobel channel second the warmer one, tested no input or load connected
Looks like there may be some oscillation bursts on the lower scope video. Maybe try some of the following ideas to get a better pic of what is going on.
Connect the scope ground to the amp, then touch the probe tip to the ground. Whatever you see is some combination of ground noise and ambient EMI/RFI. To further investigate, disconnect the probe ground from the amp. Once again touch the probe tip to the probe ground. What do you see? Its whatever the ground lead is picking up as a small loop antenna. Moving the probe-and-ground-wire around may pick different noise and or signals, just as aiming any antenna may help pick up some signals better.
Also, maybe worth trying to get a stable scope shot of that garbage. To do that set the horizontal mode to Normal, then adjust the trigger level until it just barely triggers on the highest noise peak. Then set it to single sweep and see if you can capture details of the noise peak.
More info on scope grounding in the Advanced Probing section (starting at page 46) of: https://download.tek.com/document/02_ABCs-of-Probes-Primer.pdf
Also worth reading: https://www.tek.com/en/documents/primer/xyzs-oscilloscopes-primer
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Here is one frame of the 2nd video:
Connect the scope ground to the amp, then touch the probe tip to the ground. Whatever you see is some combination of ground noise and ambient EMI/RFI. To further investigate, disconnect the probe ground from the amp. Once again touch the probe tip to the probe ground. What do you see? Its whatever the ground lead is picking up as a small loop antenna. Moving the probe-and-ground-wire around may pick different noise and or signals, just as aiming any antenna may help pick up some signals better.
Also, maybe worth trying to get a stable scope shot of that garbage. To do that set the horizontal mode to Normal, then adjust the trigger level until it just barely triggers on the highest noise peak. Then set it to single sweep and see if you can capture details of the noise peak.
More info on scope grounding in the Advanced Probing section (starting at page 46) of: https://download.tek.com/document/02_ABCs-of-Probes-Primer.pdf
Also worth reading: https://www.tek.com/en/documents/primer/xyzs-oscilloscopes-primer
EDIT:
Here is one frame of the 2nd video:
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